Eccentric exercise-induced delayed onset trunk muscle soreness alters high-density surface EMG- torque relationships and lumbar kinematics

Eccentric exercise-induced delayed onset trunk muscle soreness alters high-density surface EMG- torque relationships and lumbar kinematics | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Eccentric exercise-induced delayed onset trunk muscle soreness alters high-density surface EMG- torque relationships and lumbar kinematics Michail Arvanitidis, David Jiménez-Grande, Nadège Haouidji-Javaux, and 2 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4426332/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 10 Aug, 2024 Read the published version in Scientific Reports → Version 1 posted 10 You are reading this latest preprint version Abstract We aimed to assess high-density surface electromyography (HDsEMG)-torque relationships in the presence of delayed onset trunk muscle soreness (DOMS) and the effect of these relationships on torque steadiness (TS) and lumbar movement during concentric/eccentric submaximal trunk extension contractions. Twenty healthy individuals attended three laboratory sessions (24 hours apart). HDsEMG signals were recorded unilaterally from the thoracolumbar erector spinae with two 64-electrode grids. HDsEMG-torque signal relationships were explored via coherence (0-5Hz) and cross-correlation analyses. Principal component analysis was used for HDsEMG-data dimensionality reduction and improvement of HDsEMG-torque-based estimations. DOMS did not reduce either concentric or eccentric trunk extensor muscle strength. However, in the presence of DOMS, improved TS, alongside an altered HDsEMG-torque relationship and kinematic changes were observed, in a contraction-dependent manner. For eccentric trunk extension, improved TS was observed, with greater lumbar flexion movement and a reduction in δ-band HDsEMG-torque coherence and cross-correlation. For concentric trunk extensions, TS improvements were observed alongside reduced thoracolumbar sagittal movement. DOMS does not seem to impair the ability to control trunk muscle force, however, perceived soreness induced changes in lumbar movement and muscle recruitment strategies, which could alter motor performance if the exposure to pain is maintained in the long term. Biological sciences/Neuroscience/Motor control Biological sciences/Physiology/Neurophysiology delayed onset of muscle soreness exercise-induced muscle damage eccentric concentric high-density surface EMG torque steadiness Figures Figure 1 Figure 2 Figure 3 Figure 4 1. INTRODUCTION Delayed-onset muscle soreness (DOMS) represents a form of ultrastructural muscle injury that typically emerges following intense, unfamiliar exercises, particularly those emphasizing eccentric contractions 1,2 . Clinical signs of DOMS, include reduced force production, painful movement limitations, stiffness, swelling, and dysfunction in adjacent joints 1 . These symptoms can persistently impair muscle function 3 . Although considered a mild form of injury, DOMS frequently undermines athletic performance 1 and may disrupt daily activities 3 , likely due to alterations in electromyography (EMG)-force relationships, during muscle contractions 3 . Numerous studies have investigated the effect of DOMS on the neuromuscular function of peripheral muscles, such as the elbow flexors and hamstrings 3–9 . In contrast, research focusing on trunk muscles is less extensive, with only a few studies inducing DOMS to assess its influence on trunk muscle function. For example some of these investigations have revealed altered activation patterns 10,11 and decreased force accuracy 2 . Given these findings, and considering that people often move differently when experiencing pain 12 , it is hypothesized that DOMS may also lead to changes in trunk movement patterns, yet this aspect remains poorly understood. Additionally, it remains to be investigated whether DOMS alters trunk extensor muscle torque steadiness. Performing contractions with minimal force fluctuations is crucial in everyday life activities, as reduced torque steadiness can affect the precision of voluntary movements and functional ability. During a voluntary contraction, the output from the activated motor unit population leads to force generation, which is not constant but rather fluctuates around an average value 13 . Years of investigation into the neural mechanisms causing variations in muscle force during voluntary contractions have revealed that the neural command's low-frequency component (< 10Hz) is the most relevant for force generation 14 . This component mirrors the common synaptic input received by the motor unit population 15 . It has previously been demonstrated that eccentric exercise can cause increased elbow flexor muscle force fluctuations, likely due to an increased common neural drive to the muscles 16 . Given these findings, it is relevant to explore the effect of DOMS on torque steadiness, trunk muscle activation, and spine kinematics during dynamic contractions. To understand the relationship between muscle activity and the force output, surface EMG (sEMG)/force relationships have been extensively examined. Such investigations 17,18 have identified an association between low-frequency force fluctuations and the corresponding low-frequency component of the rectified interference sEMG. For certain muscles such as the erector spinae (ES), where HDsEMG decomposition is challenging, this methodology offers an alternative, especially when establishing the correlation between motor unit discharge times and force (the gold standard) is difficult. Staudenmann et al. 19,20 have indicated that employing high-density sEMG (HDsEMG), known for its superior spatial sampling resolution, alongside the application of Principal Component Analysis (PCA), enhances the force estimations based on sEMG. PCA acts as a dimensionality-reduction tool that can identify and select a subset of principal components (PCs) accountable for the majority of data variation, thereby explaining most of the variance in the exerted torque 19 . This method has been applied previously to quantify both magnitude and regional alterations in HDsEMG-torque relationships in individuals with and without chronic low back pain (CLBP) during isometric 21 and dynamic trunk extension 22 . However, how these relationships are altered in the presence of thoracolumbar ES DOMS remains unclear. Given the frequent use of trunk muscles in daily activities involving eccentric contractions, understanding the effects of DOMS on these muscles is crucial. Specifically, changes in trunk neuromuscular function, such as increased force fluctuations and altered recruitment strategies, may predispose individuals to more severe muscle injuries under certain conditions 23 . This study aimed to evaluate the influence of DOMS on various motor performance measures and HDsEMG parameters measured from the thoracolumbar ES during both concentric and eccentric trunk extension contractions. Specifically, the objectives were to (i) quantify changes in the relationship between HDsEMG oscillations and torque oscillations in both time and frequency domains, (ii) assess and compare regional differences in HDsEMG amplitude and HDsEMG–torque cross-correlation and coherence of the ES, and (iii) investigate differences in torque steadiness and associated kinematic data from the lumbar spine following the induction of DOMS in asymptomatic individuals. It was hypothesized that individuals would exhibit reduced torque steadiness under the influence of DOMS, accompanied by alterations in EMG-torque relationships and lumbar kinematics. 2. METHODS 2.1. Study design and setting This observational, cross-sectional study with a repeated measures design received approval from the Ethical Review Committee at the University of Birmingham, United Kingdom (approval number: ERN 19-1148) and adhered to the Declaration of Helsinki. The manuscript follows the STROBE guidelines for reporting. Data collection spanned from April 2019 to July 2022 at a laboratory within the Centre of Precision Rehabilitation for Spinal Pain, University of Birmingham, United Kingdom. The study was conducted over three sessions on consecutive days, and each were separated by approximately 24 h (baseline, post 24h, post 48h). All participants gave their written consent before involvement. 2.2. Participants Twenty asymptomatic controls (ten males, ten females) were recruited from the local Birmingham community, including the University of Birmingham's student and staff populations, through social media announcements and distributed information leaflets. The sample size was based on previous studies 10,11 that used a similar number of participants but also on a moderate effect size of f = 0.36, an α of 0.05, a β power of 0.9 and a 10% data loss due to signal quality of participant withdrawal, for a repeated measures analysis of variance (ANOVA) using a within factors design consisting of three measurement points. The effect size was calculated from the mean ± SD of lumbar ES HDsEMG reflex amplitude values before and after the induction of DOMS (averaged effect sizes for both sides and first and last trials) reported by Abboud et al., (2021) 10 . Participants were eligible for the study if they were men or women aged 18 to 55 years, asymptomatic, with no prior back or lower limb pain requiring medical attention. Exclusion criteria included cardiovascular diseases, pregnancy, spinal deformities or surgeries, systemic or inflammatory conditions, rheumatic and neuromuscular disorders, neurological conditions, and lumbar radiculopathy. During the three days, as well as the day preceding the experiment, participants were advised against engaging in any high-intensity or atypical exercises and refraining from taking medications intended to alleviate pain or soreness ( 4 ). 2.3. Questionnaires At the start of the session, to assess the level of physical activity, participants were asked to complete the full version of the International Physical Activity Questionnaire (IPAQ). The reliability and validity of the complete IPAQ have been previously established 24 . Lumbar muscle soreness was also verbally assessed at the beginning and end (i.e., immediately after the eccentric exercise protocol) of the first session. Subsequent evaluations were made at the beginning of the following sessions at 24 and 48 hours, respectively. Participants were instructed to evaluate the subjective intensity of movement-related lumbar muscle soreness using an adapted 0–10 visual analogue scale (VAS), where 0 represented "no soreness at all" and 10 signified "extreme soreness" 6,25 . During the eccentric exercise protocol (detailed below), participants were prompted to indicate their perceived exertion levels (after each set of 5 repetitions) using a modified Borg scale (Category-Ratio-10 Scale; Borg, 1998). The scale spans from "0" (no perceived exertion) to "10" (extreme exertion). 2.4. Pain sensitivity assessment Pressure pain threshold (PPT) testing was performed to quantitatively assess changes in thoracolumbar ES sensitivity across consecutive days after the induction of DOMS. Following a familiarisation phase, PPTs were measured with the participants lying on a plinth in a prone position, using an electronic algometer (probe tip: 1 cm 2 , 30kPa/s; NOD, OT Bioelettronica, Italy). Participants were guided to indicate to the researcher when the sensation of pressure transitioned to pain, at which moment the pressure application ceased. PPTs were performed only on the right side for all participants over ten testing sites (spanning approximately from L5 to T10) as described previously 22 . At each location, two PPT measurements were performed (randomised order), and the average of the two measurements at each site was used for subsequent analysis (mean thoracolumbar ES sensitivity). PPTs were assessed at the beginning and end of session 1 (immediately after the eccentric exercise protocol), as well as at the start of session 2 (24h later) and session 3 (48h later). Topographical maps of the PPT were also generated using the mean values. The centroid (x-axis and y-axis coordinates) was computed to identify the region of increased sensitivity (i.e., areas with the lowest PPTs). This approach facilitated comparisons of sensitivity areas across different days. The same researcher performed all PPT measurements, ensuring the reduction of inter-experimenter variability. 2.5. Electromyography Surface HDsEMG recordings were acquired in monopolar configuration using four 2D electrode grids, each arranged in a 13 x 5 pattern. These grids had evenly spaced electrodes (diameter: 1mm, spacing: 8mm; GR08MM1305, OT Bioelettronica, Italy), with one electrode missing in the upper left corner. Each HDsEMG electrode was attached with a double-sided adhesive foam (FOA08MM1305, OT Bioelettronica, Italy). Conductive gel was then applied to the electrode sections of the grids to ensure proper skin contact (AC-CREAM, SPES Medica, Genoa, Italy). Participant skin preparation included shaving (if required), mild abrasion using an abrasive gel (Nuprep Skin Prep Gel, Weaver and Company, Aurora, Colorado), followed by water rinse and drying. To monitor thoracolumbar ES activity, two of the HDsEMG grids were placed vertically on one side. The remaining two grids were positioned on the same side: one over the rectus abdominis (RA) and the other on the external oblique (EO). Reference electrodes (WhiteSensor WS, Ambu A/S, Ballerup, Denmark) were also affixed to the participant's sacrum, the anterior superior iliac spine (ASIS), and wrist. For a comprehensive description and visual representation of the HDsEMG electrode placement, we direct readers to our previous work 22 . Both torque and HDsEMG signals were sampled at a rate of 2048Hz. These signals were digitised using a 16-bit A/D converter (Quattrocento, 400-channel EMG amplifier, OT Bioelettronica, Torino, Italy, amplification: 150, frequency range 10-500Hz, primary order, 3dB). The OTBiolab + software platform was used for data acquisition. 2.6. Lumbar kinematics Lumbar movements during the contractions were quantified using Noraxon's myoMOTION system and two wearable Inertial Measurement Units (Research PRO IMUs, Noraxon USA) with a sampling rate of 100 Hz. The sensors were attached via double-sided tape on the lower thoracic and lumbar spine (T12 and L5, respectively). Using Noraxon's myoRESEARCH software (version 3.14), custom angles were created based on the difference between these two sensors. This allowed lumbar flexion/extension, lateral flexion and rotation angles to be captured during all contractions. Before any measurement commenced, participants were asked to adopt a natural upright position while sitting on the dynamometer's chair, and the sensors were calibrated. The myoMOTION software and hardware allowed the synchronization of the myoMOTION receiver with other systems. 2.7. Isokinetic dynamometry An isokinetic dynamometer (System 3 Pro, Biodex Medical Systems, New York) was utilised to assess the torque produced by participants during concentric and eccentric trunk extension maximal voluntary contractions (MVCs) and the torque steadiness tasks performed at submaximal levels. To focus on the lumbar spine, participants were positioned on the Biodex Dual Position Back Extension/Flexion Attachment, ensuring their hips and knees were at a 90° angle and feet spaced at shoulder width 21,26 . The chair's seat was tilted upwards approximately 15°, and its height was adjusted so the dynamometer's rotational axis aligned with the ASIS bilaterally 26 . To prevent compensatory movements, participants' upper trunk, pelvis, and thighs were securely strapped to the chair. A specific Biodex attachment was also in place to reduce knee muscle engagement. For all contractions, the dynamometer operated in isokinetic mode. The range of motion (ROM) for both concentric/eccentric extension contractions was 50°, spanning from 20° extension to 30° flexion, emphasising lumbar movement and minimising compensatory actions from the legs 26 . A consistent angular speed of 5°/s was maintained. When returning to the starting position, the concentric contraction mode was consistently set at an angular velocity of 90°/s. For the eccentric mode, the return velocity was adjusted to 20°/s to ensure optimised comfort for the participant. Nonetheless, participants were assisted back to the initial position by the researcher, who manually readjusted the chair for every repetition. 2.8. Eccentric exercise protocol (DOMS) The eccentric exercise protocol consisted of three sets of 15 eccentric contractions of the trunk extensors, moving from 20° of trunk extension to 30° of trunk flexion. The protocol was performed only at the end of session 1. Resistance was set at 50% of their MVC (the %MVC was chosen after pilot testing to induce mild-moderate muscle soreness), with an angular speed of 5°/s. Participants were given a 30-second rest between each set. Participants were asked to perform an MVC immediately after the eccentric contractions to further assess the trunk extensors and validate the effectiveness of the eccentric exercise to induce fatigue (i.e., reduction in maximal torque output). 2.9. Experimental protocol Participants were given time to familiarise themselves with the dynamometer, by practising all contractions and warming up their trunk extensor muscles via sustained and dynamic submaximal contractions. After a brief rest, they were asked to perform two concentric trunk extension MVCs, moving through a 50° ROM. The starting position for the concentric trunk extension, MVC, was 30° of trunk flexion, and the final position was 20 o of trunk extension. Between these contractions, 1-minute rest was provided. After a 5-minute rest, participants were instructed to perform submaximal concentric trunk contractions, four times at 25% and three times at 50% MVC, in a randomised order. During these contractions, they were asked to match the target torque lines at 25% and 50% MVC, keeping the contraction for 10 seconds. They were allowed to practice each submaximal contraction once before the actual recordings. The highest peak torque observed during the MVC was used to set the submaximal torque targets. After these, participants repeated the same procedures for the eccentric trunk contractions. The only difference for the eccentric trunk extension contractions was that the starting position was at 20° of trunk extension, and the final position at 30°of trunk flexion (i.e., the opposite from the concentric contractions). Throughout all submaximal contractions, real-time visual feedback of their torque output and a line representing the desired contraction level (%MVC) was visible on a computer screen positioned 1.5 meters in front of them. This real-time torque data was superimposed over the template for visual feedback. Participants aimed to swiftly and accurately match the %MVC target. Once they achieved the set target (either 25% or 50% MVC), they were advised to keep their torque output as steady as possible for the whole contraction. Essentially, they were asked to produce consistent torque output (25% or 50%MVC) across the entire 50° ROM. The overall study procedure is depicted schematically in Fig. 1 . [PLEASE INSERT FIGURE 1 HERE] 2.10. Analysis of the torque signal The highest peak torque during the MVCs was used to assess each individual's maximum trunk extension concentric and eccentric strength. Torque steadiness was evaluated by calculating the absolute and relative amplitude of the torque fluctuations, determined by the standard deviation and the CoV of torque, respectively (CoV, standard deviation of the torque/mean of torque × 100) 13 . A custom MATLAB script was employed to allow the computation of the SD and CoV of torque within the same time window used for the HDsEMG analysis, during the steady phase of the contraction. These values were determined for each repetition and then averaged to provide a single value for each torque level for each contraction. The time windows selected for analysis were approximately 8 seconds for both submaximal torque levels. This approach aimed to exclude the first and last second of the steady part of the contraction, periods during which participants typically overestimated or underestimated the requested torque level. 2.11. EMG amplitude and topographical map computations During the offline analysis, the 64-monopolar HDsEMG channels from the electrodes positioned over the RA and EO muscles, as well as the 128-monopolar channels from the merged electrodes over the thoracolumbar ES, were processed to obtain 59 and 124 bipolar channels, respectively. Before the root mean square (RMS) amplitude calculations, the HDsEMG signals were filtered with a bandpass zero-lag Butterworth filter (10-350Hz, 2nd order) and visually inspected, to exclude channels with low-quality signals due to electrical interference and/or artifacts. Less than 15% of the channels were discarded. One RMS value was calculated for each of the 59 bipolar channels for the abdominal muscles (RA and EO) during each trunk eccentric/concentric extension contraction. Similarly, the RMS amplitude was also determined for each of the 124 bipolar channels for the thoracolumbar ES during all trunk eccentric/concentric extension contractions. By determining the RMS for each channel, a HDsEMG amplitude map was generated for the (agonist) thoracolumbar ES and the centroid (x-,y- coordinates) of this map was calculated during all muscle contractions. This allowed us monitor regional changes in thoracolumbar ES activation. Additionally, a global measure of myoelectric activity was calculated for each muscle, by calculating the average of RMS values from all channels for each muscle, which formed a single value (HDsEMG amplitude; RMSmean). The HDsEMG amplitude from the abdominal muscles was only used to assess the level of co-activation. The level of trunk flexor and extensor co-activation was measured during all muscle contractions. The co-activation level was calculated using the raw (i.e., non-normalised) RMSmean values, calculated as: antagonist muscle activity/primary muscle activity x 100. For this, we added the RMSmean values of the abdominal muscles (RA + EO). In this study, EMG signals were not normalized due to the established reliability of HDsEMG-derived measures in within-subject designs, particularly during voluntary trunk movements 27 . All HDsEMG and torque data was assessed offline using a custom MATLAB 2020b script (The MathWorks Inc., USA). Concentric and eccentric muscle contractions were recorded separately. 2.12. HDsEMG data pre-processing for PCA analysis PCA is a dimensionality reduction technique that effectively identifies redundant information in HDsEMG data. This approach can also enhance the accuracy of HDsEMG-based force estimations 19,20 . By transforming complex, multivariate data into a series of linearly independent PCs through an orthogonal transformation, PCA reduces dimensionality while retaining most of the data’s variance. In this study, these components, which were linear combinations of the original 124 channels, were organized in descending order based on the variance they accounted for, with the first few components representing the majority of the data's variance. PCs that collectively accounted for a cumulative 85% of the total variance were retained 21,22,28 to keep only the most informative components and improve the signal-to-noise ratio. This process, which marked the initial phase of dimensionality reduction, generated a new matrix containing the eigenvectors of these selected PCs. The original dataset, therefore, remained intact until the transformation in the final phase of PCA. In this phase, the dataset was reorientated from its original axes to those delineated by the selected PCs. This was achieved by transforming the original dataset into a lower-dimensional space (i.e., < 124) precisely defined by these PCs that explained 85% of the variance in the data. Consequently, the original 124-channel dataset was transformed into a new dataset with reduced dimensionality that explained most of the variance in the data. For further details on the PCA methodology, please refer to earlier work 21,22 . Before performing the cross-correlation and coherence analyses, the formerly offline-referenced 124 bipolar HDsEMG channels (from the combined electrodes placed over ES) underwent the following pre-processing for PCA computations: 1) 10 Hz high-pass filtering, 2) selection of the most informative subset of PCs using PCA on the 124 (for ES) differential HDsEMG signals in temporal domain, 3) full-wave rectification and averaging the selected PCs to generate a single time signal for the thoracolumbar ES muscle, 4) low-pass filtering at 10 Hz (supplementing the initial filtering), aiming to detect slow-frequency variations in motor unit activation/recruitment 14 , 5) application of a first-order Savitzky-Golay filter for smoothing, and 6) removal of DC (zero frequency) components 19–22 . These processing procedures resulted in a final signal envelope that was obtained by applying PCA to the HDsEMG grid and contained the low-frequency components of the HDsEMG data. Coherence and cross-correlation analyses were then conducted to estimate the similarity between this final EMG signal envelope and torque signals. Please note that the PCA and subsequent coherence and cross-correlation analyses were exclusively conducted for the thoracolumbar ES, which functioned as the primary agonist muscle during the eccentric and concentric contractions. 2.13. Cross-correlation and coherence analyses Cross-correlation analysis quantified the similarity (cross-correlation coefficient) between the final signal envelope (specifically, the low-pass filtered average PCA-selected signal derived from the first PCs accounting for 85% variance) and torque signals in the time domain. The relationship between torque and EMG was further investigated through coherence analysis, as done previously 21 . Coherence analysis, aiming to measure the intensity and frequency of synchronous synaptic inputs across the motor unit population in relation to torque, was executed using the magnitude squared coherence (MSC) method with a 1-s Hamming window and 50% overlap, as detailed earlier 29 . Given the contraction's brief duration, this method was chosen to optimize analysis resolution while minimizing data loss. The same sEMG envelope (Final Signal Envelope) was evaluated against the torque signal in the frequency domain. MSC, a frequent measure to evaluate signal similarity in the frequency domain, was computed using MATLAB's mscohere function. This function applies Welch's overlapped averaged periodogram method with a 50% overlap across N sub-windows. Further MSC computation insights can be found in previous studies 21,30 . The Fisher's z-transformation was utilised on coherence estimates (C) to enable statistical comparisons, as these transformed values (FZ) follow a normal distribution. Given the potential crosstalk affecting sEMG recordings, bias was determined as the coherence profile's peak value at 250 Hz, an area with no significant correlated activity 29 . The utilised equation was the following: $$FZ=\text{atanh}\left(\sqrt{C}\right)-bias$$ This study's coherence analysis focused on the δ band (0-5Hz), which is most relevant to muscle torque generation 15 . Additionally, topographical coherence maps, as described in previous research 21 , were generated. In this process, each of the 124 HDsEMG signals from the thoracolumbar ES was individually assessed for coherence in frequency with the filtered torque signal. This analysis resulted in 124 distinct coherence values, one for each HDsEMG signal. These values were then utilized to construct topographical coherence maps. The maps were then normalized to the maximum coherence value at each torque level. The coherence's centroid provided an estimate of the centre of ES muscle δ-band coherence along both medial-lateral (x-axis) and cranial-caudal (y-axis) directions. This analysis enabled us to examine if certain areas of the thoracolumbar ES have greater influence on torque generation and to determine whether the regions with predominant influence shifted across days due to muscle soreness, by statistically comparing x- and y-axis centroid values across days. 2.14. Analysis of lumbar kinematics Custom anatomical angles (flexion/extension, rotation, and lateral flexion) were generated based on the differences between the two IMU sensors (T12 and L5). The raw data of custom angles was exported in MATLAB (R2020b, MathWorks) for further offline analysis. A low-pass Butterworth filter with a cut-off frequency of 10Hz was applied to all kinematic data 31 . Kinematic angles were calculated for all contractions, for all days, during each repetition (25%: 4 repetitions, 50%: 3 repetitions) and then averaged. Based on the averaged data (average of all repetitions), eight epochs were created. The eight epochs were normalized by subtracting epoch 1 from all epochs and were converted to positive (absolute) values. This was necessary to allow the calculation of the area under the curve (AUC) for all movement planes (i.e., sagittal, frontal and transverse), which was used to assess differences in the magnitude of movement across days. Please note that HDsEMG, torque, cross-correlation, coherence and kinematic analyses were performed separately for each type of contraction (concentric, eccentric). Additionally, for every repetition, a single value was computed, i.e., four values for 25%MVC and three values for 50%MVC. These were then averaged to form one value for each submaximal level. This analysis was consistently applied to the data from all three sessions and was used for the statistical analysis. 2.15. Statistical procedures Statistical analyses were performed on SPSS Statistics, version 29 (IBM, USA). The Shapiro–Wilk and the Levene tests were used to confirm data normality and ensure homogeneity of variance, respectively. Given that these conditions were satisfied, parametric tests were used. Data are presented as mean and standard deviation (SD), unless otherwise specified. For the pre-and-post assessments of muscle soreness, PPTs and MVCs, paired samples t -Tests were employed. For HDsEMG, torque and kinematic variables (i.e., RMS, centroid values of the RMS amplitude map in both axes, level of co-activation between the ES and abdominal muscles, δ band sEMG-torque coherence, sEMG-torque cross-correlation, centroid values for sEMG-torque coherence in both axes mean torque, torque CoV, torque SD, AUC), a two-way repeated measures ANOVA was performed separately for each type of contraction (eccentric, concentric). The torque target (20%MVC and 50%MVC) and session (baseline, 24h, 48h) were used as within-subject factors. One-way repeated measures ANOVA was also used to monitor changes in muscle soreness, PPTs and MVCs across the three testing days. In instances where the ANOVA indicated significant interactions, a Bonferroni correction was applied for detailed pairwise comparisons. The threshold for statistical significance was set at p = 0.05 . 3. RESULTS 3.1. Muscle soreness All 20 participants (10 males, 10 females) completed the study, with their characteristics detailed in Table 1 . The eccentric exercise protocol performed at the end of session one induced a moderate level of movement-related back muscle soreness immediately post-exercise (4.1/10) ( t=-8.305, p < 0.001 ). Participants reported mild muscle soreness 24h and 48h compared to baseline (session effect: F = 28.222 , p < 0.001, ηp 2 = 0.744 ). Muscle soreness immediately post-exercise was higher compared to that observed at 24h and was similar to that observed at 48h (session effect: F = 5.760 , p = 0.012 , ηp 2 = 0.233 ). Mean ± SD values of muscle soreness are reported in Table 1 . Table 1 Demographic characteristics and descriptive data of all 20 asymptomatic individuals that participated in the study. Data are presented across different time points (baseline, after the eccentric exercise protocol, after 24 and 48h respectively). Characteristic Mean ± SD Age (years) 28.7 ± 3.9 Height (cm) 171.9 ± 7.4 Body mass (Kg) 70.6 ± 13.4 Body mass Index (kg/m 2 ) 23.9 ± 4.2 PPT Session 1 – Baseline (kPa) 397.8 ± 95.0 PPT Session 1 – Post 370 ± 110.74 PPT 24h 361.0 ± 113.7 PPT 48h 357.9 ± 106.6 VAS Session 1 – Baseline (0–10) 0.0 ± 0.0 VAS Session 1 – Post 4.1 ± 2.2 VAS 24h 2.6 ± 2.1 VAS 48h 2.9 ± 1.8 IPAQ (Total METs score) 4325.1 ± 3578.2 ECC MVC Session 1 – Baseline (N⋅m) 203.4 ± 48.9 ECC MVC Session 1 – Post 174.2 ± 53.3 ECC MVC 24h 194.8 ± 51.6 ECC MVC 48h 191.4 ± 54.9 CON MVC Session 1 (N⋅m) 211.8 ± 58.7 CON MVC 24h 200.2 ± 43.1 CON MVC 48h 197.0 ± 48.1 Abbreviations: Pressure pain threshold (PPT), visual analogue scale (VAS) and International Physical Activity Questionnaire (IPAQ), eccentric (ECC), maximum voluntary contractions (MVC), metabolic equivalents (METs), concentric (CON). 3.2. Mean thoracolumbar PPTs During the first session, after the eccentric exercise protocol, PPTs assessed over the lumbar region decreased significantly ( t = 2.195 , p = 0.041 ). This heightened mean thoracolumbar sensitivity (i.e., lower values of PPTs) persisted 24h and 48h post-baseline (session effect: F = 6.355, p = 0.004 ). No significant differences in thoracolumbar sensitivity were observed between measurements taken immediately after the eccentric exercise protocol, and those taken at 24 hours and 48 hours later ( F = 0.744 , p = 0.482 ). Mean ± SD PPT values are reported in Table 1 . 3.3. PPT maps PPT maps were generated to determine if regional pain sensitivity at 24h and 48h differed significantly from baseline in specific thoracolumbar areas due to the eccentric exercise protocol. There were no differences for both the y- and x-axes compared to baseline (session effect: F = 1.054, p = 0.358, ηp 2 = 0.053 and F = 1.340, p = 0.274, ηp 2 = 0.066 respectively). The PPT maps are illustrated in Fig. 2 . [PLEASE INSERT FIGURE 2 HERE] 3.4. Rate of perceived exertion At the end of session 1, during the eccentric exercise protocol, the participants were asked to report their rate of perceived exertion on a modified Borg scale (10 point scale), every 5 repetitions. An increase in the rate of perceived exertion was observed, with the average Borg value at the end of the protocol being 7.4/10, suggesting that the task was interpreted as “very hard” by the individuals (time effect: F = 44.082, p < 0.001, ηp 2 = 0.699 ). The results presented below are reported per outcome variable for the eccentric and concentric contractions respectively. 3.5. Muscle strength In session one, following the eccentric exercise protocol, there was a decrease in eccentric torque, equivalent to a 29.2 N⋅m or 14.35% decrease ( t = 3.032 , p = 0.007 ; Fig. 3 A). No decline in eccentric torque was observed 24h and 48h compared to baseline at the beginning of the session (session effect: F = 0.939, p = 0.379, ηp 2 = 0.047 ; Fig. 3 B). No decline in concentric torque was observed 24h and 48h compared to baseline, suggesting that individuals were able to produce the same amount of torque during all three sessions irrespective of their muscle soreness and increased mean thoracolumbar muscle sensitivity (i.e., reduced PTT values) (session effect: F = 1.896, p = 0.164, ηp 2 = 0.091 ; Fig. 4 A). Mean ± SD peak torque values for both contractions are reported in Table 1 . 3.6. Torque steadiness Eccentric contractions. A significant lower torque SD was observed 24h and 48h compared to baseline, suggesting that individuals had better torque steadiness in sessions two and three (session effect: F = 5.952, p = 0.006, ηp 2 = 0.239 ; Fig. 3 C; baseline: 1.1 ± 0.3%; 24h: 0.9 ± 0.2%; 48h: 0.9 ± 0.2%). No differences were observed across sessions for torque CoV (session effect: F = 1.295, p = 0.280, ηp 2 = 0.263 ; baseline: 3.2 ± 0.9%; 24h: 2.6 ± 0.7%; 48h: 3.0 ± 1.6%). Concentric contractions. The torque steadiness performance of the participants was better during session 3 compared to baseline as shown from both the torque CoV and SD variables (session effect: F = 4.327, p = 0.020, ηp 2 = 0.185 ; baseline: 3.5 ± 0.9%; 24h: 3.2 ± 0.9%; 48h: 3.0 ± 0.5% and F = 5.452, p = 0.008, ηp 2 = 0.223 ; baseline: 1.3 ± 0.3%; 24h: 1.1 ± 0.3%; 48h: 1.1 ± 0.2%, respectively; Figs. 4 B, 3 C). 3.7. Electromyographic activity Eccentric contractions. No differences in the level of activation of the thoracolumbar ES across the three sessions (session effect: F = 1.250, p = 0.289, ηp 2 = 0.062 ; baseline: 41.8 ± 21.5 µV; 24h: 40.6 ± 21.7 µV; 48h: 38.6 ± 19.7 µV). The regional activation of the thoracolumbar ES along the y- and x-axes was similar during all sessions (session effect: F = 0.479, p = 0.539, ηp 2 = 0.025 ; baseline: 93.7 ± 11.1 mm; 24h: 95.9 ± 5.5 mm; 48h: 95.6 ± 6.8 mm and F = 1.945, p = 0.157, ηp 2 = 0.093 ; baseline: 16.2 ± 1.2 mm; 24h: 16.2 ± 0.9 mm; 48h: 15.8 ± 1.0 mm). The level of co-activation between the RA, EO and ES was similar across days (session effect: F = 0.649, p = 0.491, ηp 2 = 0.033 ; baseline: 151.7 ± 123.2%; 24h: 153.3 ± 117.2%; 48h: 142.3 ± 101.0%). Concentric contractions. No differences in the level of thoracolumbar ES activity were observed across days ( F = 0.021, p = 0.923, ηp 2 = 0.001 ; baseline: 44.7 ± 25.1 µV; 24h: 44.2 ± 21.8 µV; 48h: 44.2 ± 21.2 µV). The regional activation of the thoracolumbar ES along the y- and x-axes was similar for all three days ( F = 0.281, p = 0.656, ηp 2 = 0.015 ; baseline: 96.4 ± 9.5 mm; 24h: 97.1 ± 8.1 mm; 48h: 95.2 ± 9.1 mm and F = 1.120, p = 0.337, ηp 2 = 0.056 ; baseline: 16.3 ± 1.0 mm; 24h: 16.1 ± 0.8 mm; 48h: 15.9 ± 0.8 mm). The level of co-activation between the abdominal (RA, EO) and lumbar ES was similar during all three sessions ( F = 1.474, p = 0.243, ηp 2 = 0.076 ; baseline: 153.3 ± 99.2%; 24h: 136.8 ± 93.5%; 48h: 132.0 ± 114.9%). 3.8. EMG-torque relationships Eccentric contractions. A session effect was observed for the magnitude of δ band coherence, showing that δ band coherence at 48h was lower compared to baseline (session 1) and 24h (session 2) ( F = 6.685, p = 0.003, ηp 2 = 0.260 ; Z-coherence in the δ band, baseline: 0.8 ± 0.2; 24h: 0.7 ± 0.1; 48h: 0.7 ± 0.1; Fig. 3 D). In terms of differences in the topographical representation of δ band coherence along the y- and x-axes, no differences were observed (session effect: F = 1.306, p = 0.276, ηp 2 = 0.064 ; baseline: 94.2 ± 8.1 mm; 24h: 96.6 ± 2.3 mm; 48h: 96.1 ± 3.0 mm and F = 0.958, p = 0.393, ηp 2 = 0.048 ; baseline: 16.7 ± 0.5 mm; 24h: 16.6 ± 0.4 mm; 48h: 16.5 ± 0.3 mm). Similarly, a session effect was observed for the magnitude of HDsEMG-torque cross-correlation, showing a reduction in the contribution of the thoracolumbar ES at session 3, compared to the other two sessions ( F = 7.416, p = 0.002, ηp 2 = 0.281 ; cross-correlation coefficient, baseline: 0.4 ± 0.0; 24h: 0.4 ± 0.1; 48h: 0.37 ± 0.0; Fig. 3 E). No differences in the topographical representation of HDsEMG-torque cross-correlation along the y- and x-axes, were observed (session effect: F = 0.096, p = 0.847, ηp 2 = 0.005 ; baseline: 94.3 ± 7.9 mm; 24h: 94.8 ± 3.2 mm; 48h: 95.0 ± 4.2 mm and F = 1.541, p = 0.227, ηp 2 = 0.075 ; baseline: 16.7 ± 0.5 mm; 24h: 16.6 ± 0.4 mm; 48h: 16.5 ± 0.3 mm). Concentric contractions. No differences were observed for the magnitude of δ band coherence, across days ( F = 0.373, p = 0.691, ηp 2 = 0.019 ; Z-coherence in the δ band, baseline: 0.8 ± 0.1; 24h: 0.8 ± 0.1; 48h: 0.8 ± 0.1). Similarly, no differences in the topographical representation of δ band coherence along the y- and x-axes, were observed (session effect: F = 1.216, p = 0.297, ηp 2 = 0.060 ; baseline: 94.7 ± 10.4 mm; 24h: 98.3 ± 5.7 mm; 48h: 96.8 ± 3.2 mm and F = 0.270, p = 0.765, ηp 2 = 0.014 ; baseline: 16.6 ± 0.4 mm; 24h: 16.6 ± 0.3 mm; 48h: 16.5 ± 0.3 mm). The magnitude of HDsEMG-torque cross-correlation was similar across sessions ( F = 0.343, p = 0.712, ηp 2 = 0.018 ; cross-correlation coefficient, baseline: 0.4 ± 0.1; 24h: 0.4 ± 0.1; 48h: 0.4 ± 0.1). No differences in the topographical representation of HDsEMG-torque cross-correlation along the y- and x-axes, were observed (session effect: F = 1.461, p = 0.245, ηp 2 = 0.071 ; baseline: 93.7 ± 10.1 mm; 24h: 97.1 ± 4.8 mm; 48h: 96.5 ± 3.1 mm and F = 0.742, p = 0.483, ηp 2 = 0.038 ; baseline: 16.6 ± 0.4 mm; 24h: 16.6 ± 0.3 mm; 48h: 16.5 ± 0.4 mm). 3.9. Thoracolumbar kinematics Eccentric contractions. A session × torque interaction was observed for the AUC variable in the sagittal plane, suggesting that in sessions two and three, as the task was more demanding (i.e., higher %MVC), individuals increased their trunk flexion ROM ( F = 6.479, p = 0.004, ηp 2 = 0.254 ; 25%MVC-24h: 50.6 ± 28.3 o ⋅ epoch, 50%MVC-24h: 69.8 ± 32.2 o ⋅ epoch; 25%MVC-48h: 46.7 ± 18.8 o ⋅ epoch, 50%MVC-48h: 65.1 ± 25.9 o ⋅ epoch ;Figure 3 F). Additionally, this interaction showed that at high forces there was a significant difference between sessions one and two for this variable (50%MVC-baseline: 54.0 ± 22.9 o ⋅ epoch, 50%MVC-24h: 69.8 ± 32.2 o ⋅ epoch). No significant differences were observed for this kinematic variable nor for the frontal or transverse planes ( p > 0.05 ). Concentric contractions. In Session 3, there was a reduction in the AUC in the sagittal plane compared to the baseline, suggesting decreased thoracolumbar ROM (session effect: F = 5.723, p = 0.007, ηp 2 = 0.23 ; baseline: 48.7 ± 20.0 o ⋅ epoch; 24h: 36.4 ± 18.7 o ⋅ epoch; 48h: 33.4 ± 16.7 o ⋅ epoch; Fig. 4 D). This might indicate that participants were aiming to maintain a more neutral or stable lumbar spine position. No significant differences were observed for movement in the frontal or transverse planes ( p > 0.05 ). 4. DISCUSSION This study examined the influence of DOMS on torque steadiness, HDsEMG-torque relationships from the thoracolumbar ES, and kinematic data from the thoracolumbar spine in asymptomatic individuals during submaximal concentric and eccentric trunk extension contractions at 25% and 50%MVC. No significant changes were observed for eccentric and concentric trunk extension muscle strength across sessions. The participants demonstrated improved torque steadiness during submaximal concentric and eccentric trunk extension contractions in the presence of DOMS. However, HDsEMG-torque relationships and kinematics were altered in a contraction-dependent manner. During eccentric contractions, a decrease in the thoracolumbar ES contribution to the resultant torque was observed, and individuals showed increased lumbar flexion during the more demanding contractions (50%MVC). During concentric contractions, a reduction in thoracolumbar ROM in the sagittal plane was observed, suggesting the maintenance of a more neutral lumbar spine posture, but no alterations in HDsEMG-torque relationships were observed in the presence of DOMS. 4.1. Muscle soreness and sensitivity The increased soreness and mean thoracolumbar pressure pain sensitivity observed after 24h and 48h support the occurrence of DOMS following the eccentric exercise protocol. This aligns with the observations of previous studies 10,11,32 . Notably, the peak muscle soreness observed in this study immediately after the eccentric exercise protocol, averaging 4.1 ± 2.2 on the VAS, mirrors the intensity (VAS: 3.80 ± 2.35) reported in another study 2 . Furthermore, the mild soreness levels we observed at 24h and 48h (VAS scores of 2.6 ± 2.1 and 2.9 ± 1.8, respectively) fall within the range of peak soreness levels (2 to 2.9/10 on the VAS) documented in similar timeframes by other researchers 10 . The observed decrease in mean PPT values aligns with findings from Abboud et al., 2019 32 , who also reported reduced PPT values over the L2-L5 region with the presence of DOMS, suggesting a link to peripheral sensitization driven by inflammatory processes or tissue damage associated with DOMS. The mechanisms behind DOMS are multifaceted and not fully understood 33 . Traditionally, eccentric exercise-induced microstructural muscle damage was believed to trigger inflammation followed by biochemical, thermal, and mechanical changes, sensitizing muscle afferents and causing soreness and mechanical hyperalgesia 33 . However, recent studies highlight bradykinin, nerve growth factor (NGF), and COX-2-glial cell line-derived neurotrophic factor (GDNF) as key contributors, suggesting that myofiber micro-damage may not be necessary for initiating inflammation or DOMS 33–35 . These molecules could stimulate muscle nociceptors or extracellular receptor binding, indicating their role in mechanical hyperalgesia and inflammation in the extracellular matrix, even without apparent muscle damage 36 . Interestingly, an ultrasound study also suggested an involvement of the paraspinal extramuscular connective tissue (ECT) of trunk extensors in the genesis of DOMS 37 . However, it is important to note that research on these mechanisms has predominantly focused on limb muscles, leaving other areas, such as the trunk, less explored. 4.2. Trunk muscle strength DOMS typically induces a temporary decrease in muscle force. As expected, following the eccentric exercise protocol, we observed an immediate reduction in eccentric extension trunk strength. However, contrary to some prior studies 6,10,25,32 , this decrease in trunk extension MVC was not observed at 24h and 48h post-exercise. Reductions in muscle force following eccentric exercise are well-documented for upper and lower limb muscles 6 , however, this trend does not appear to apply as consistently to trunk muscles, particularly in the context of dynamic trunk MVCs. This could be because trunk extension is a multi-joint movement involving the lumbar spine, pelvis, and hips, where potential compensation by lower limb muscles, such as the gluteus maximus and hamstrings, can play a significant role. These muscles can influence both the hip and pelvis, thereby minimizing the engagement of the thoracolumbar extensors. Despite stabilization efforts, such as using straps to secure the thighs and pelvis, it is challenging to prevent these compensatory strategies completely. This suggests that participants might have executed the MVCs by compensating with their hip extensor muscles, particularly if DOMS influenced the use of the thoracolumbar extensors. Additionally, contrasting findings may result from protocol differences: (i) previous studies focused on isometric strength, whereas we assessed eccentric/concentric strength, which could increase the likelihood of compensatory torque exertion as mentioned above, (ii) variations in exercise protocol speed (to induce DOMS), load and the use of an isokinetic dynamometer, considering that fast velocity eccentric exercise causes more muscle fibre damage than slow contractions 5 and because muscle loading level impacts damage extent and recovery rates 8 . Lastly, as mentioned above, individuals might exhibit DOMS without damaging contractile structures, suggesting the involvement of neurotrophic factors and/or connective tissue damage in the increased soreness experienced after exercise 34,35 . These factors likely explain how individuals’ trunk strength was maintained 24h and 48h post-exercise. 4.3. Torque steadiness Considering that DOMS can impair motor function and that torque steadiness is commonly reduced in individuals with CLBP 21,22 , we anticipated a reduction in both eccentric and concentric trunk extension torque steadiness due to DOMS. However, contrary to our hypothesis, torque steadiness improved in both concentric and eccentric contractions 48h after the eccentric exercise protocol that was used to induce DOMS. According to previous studies there is conflicting evidence on how DOMS influences the control of muscle force. Some studies 2,38,39 have observed worse motor control in the presence of DOMS, while others have observed no changes 40 . However, no previous study has assessed trunk extensor torque steadiness in the presence of DOMS. To date, only one study has investigated this aspect using an alternative model of acute experimental pain. Specifically, this study found no significant changes in trunk extensor force variability following intramuscular injection of hypertonic saline into the longissimus muscle 41 . In contrast, investigations into lower/upper limb muscles 7 , observed significant increases in force fluctuations, but only immediately and 1h after eccentric exercise, with no differences observed at 24h and 48h. Similarly, a study investigating the knee extensors 39 observed that force steadiness deficits were more pronounced immediately post-exercise and primarily at lower %MVC levels (2.5, 5 and 10%), with less impact at 20 or 30% MVC, and no data available for periods beyond 24h or for higher loads. Collectively, the findings from these studies support our observation that DOMS does not necessarily lead to reduced force steadiness. Additionally, it is important to consider that immediate reductions in force steadiness, observed by some studies 7,39 , are mainly attributed to acute muscular fatigue following eccentric exercise rather than DOMS which occurs later. The lack of reductions in force steadiness in the presence of trunk extensor DOMS can be attributed to some factors, including: (i) compensatory mechanisms inherent in multi-joint movements, such as trunk extension, which allow for significant compensatory strategies when experiencing DOMS. The activation of the hamstrings and gluteals to compensate for decreased functionality in the sore thoracolumbar ES and (ii) a learning effect, possibly linked to the improved force steadiness after 48h. This effect, likely arising from frequent task repetition with visual feedback during the eccentric exercise protocol, demonstrates that despite the presence of DOMS or acute pain, individuals can enhance their performance, suggesting that pain does not necessarily hinder the learning and adaptation process. This notion is further supported by a previous study 42 using an experimental pain model, where individuals with acute shoulder pain showed an improvement level in movement accuracy during fast arm-reaching movements and force field perturbations comparable to that of pain-free controls. Additionally, the use of HDsEMG and kinematic data in the current study helped explore these improvements further. 4.4. HDsEMG amplitude and HDsEMG regional activation During both eccentric and concentric contractions, we observed no changes in the magnitude of thoracolumbar ES activation, regional activity or level of co-activation in the presence of DOMS. These findings align with similar previous research on the trunk extensors 10,25 and medial gastrocnemius 43 , further supporting the theory related to the involvement of muscle-associated connective tissue 33,44 instead of changes in the muscle itself. A review 3 has previously aimed to examine the course of EMG changes and alterations at the motor unit level. However, the most common muscle assessed was the biceps brachii, and most differences were reported immediately after an eccentric exercise protocol, 2h and 24h. Importantly, it highlighted an inconsistency of change in agonistic and antagonistic EMG amplitude at 24h, and no data were presented at 48h. Interestingly, many changes can happen at the motor unit level. However, capturing these changes using HDsEMG in the thoracolumbar ES poses significant challenges. By examining the HDsEMG-torque relationship, we gained additional insights into why individuals show improved torque steadiness in the presence of DOMS. Nevertheless, it is important to note that DOMS is not always indicative of muscle damage, which might explain some of the inconsistency in the findings and the absence of differences in EMG variables when DOMS is present. It is important to mention that most of these changes at a motor unit level were reported immediately after eccentric exercise, thereby not excluding, or underestimating the confounding factor of fatigue in these observations. 4.5. HDsEMG-torque relationships The magnitude of HDsEMG cross-correlation and δ band coherence at 48h post-exercise was reduced during eccentric trunk extensions when compared to the previous two sessions. However, no regional changes in δ band coherence maps were observed during either eccentric or concentric contractions at 24h and 48h post-exercise. This uniformity in δ band coherence, reinforced by the PPT maps, which revealed no regional sensitivity differences in the thoracolumbar area, suggests that the changes were primarily in magnitude and indicates a highly localised nature of DOMS. Another noteworthy observation was the absence of differences in the magnitude of these variables during concentric trunk extension contractions across days. This is not surprising, considering the inherent differences between eccentric and concentric contractions. Eccentric contractions involve greater passive muscle element contribution, distinct neural control, higher force production with lower metabolic energy consumption, and increased mechanical stress, resulting in more microtrauma 34,45,46 . Consequently, due to the increased stress on connective tissue and muscle fibres during DOMS, we can anticipate more pronounced changes in these patterns, aiding in task adaptation and performance enhancement. Furthermore, during eccentric contractions, individuals are likely to leverage this additional component effectively for torque production. These insights could be further elucidated by examining thoracolumbar kinematics, as detailed in the following section. 4.6. Thoracolumbar kinematics Distinct differences were observed in the sagittal plane during both eccentric and concentric contractions. Notably, in the presence of DOMS, individuals demonstrated increased lumbar flexion during eccentric contractions at both 24h and 48h, particularly at higher loads (i.e., 50%MVC). This pattern was not observed at baseline, suggesting that the individuals had to alter their movement pattern to be able to perform the more demanding trunk extension eccentric contractions. In contrast, during the concentric contractions, different adaptations were observed. At 48h, individuals used a more neutral or stable lumbar spine position during the concentric contractions compared to baseline. The lack of notable differences in other planes of movement could be attributed to the individual's position on the chair, which was stabilised by straps, and the attachment system that limits movements to the frontal and transverse planes. Even though distinct, the adaptations observed in movement patterns during both eccentric and concentric contractions could likely serve two functions. Firstly, these adaptations might be a self-protective mechanism for the tissues involved, potentially reducing the risk of further damage. Secondly, they may be related to a learning effect, particularly considering the exercise protocol's similarity with the torque steadiness tasks, enabling more efficient and effective task performance. For example, in the case of eccentric extension, an increased lumbar flexion can be explained in two ways, (i) it might be related to a more efficient utilisation of the passive muscle elements or of the extramuscular connective tissue and/or (ii) it could represent a protective strategy to reduce strain on these tissues by engaging compensatory muscle groups such as the hamstrings and gluteals. This explanation is supported by recent findings showing that eccentric exercise can lead to increases in lumbar extramuscular connective tissue thickness 37 and immediate changes in the optimum length for force generation in the hamstring muscles 4 . Conversely, the controlled back extension observed during concentric movements could contribute to minimising unnecessary thoracolumbar motions. This controlled motion might optimise the length-tension relationship in the trunk extensor muscles, avoiding overly stretched or contracted positions. Consequently, this approach could reduce the risk of further damage to the connective or muscle tissue and likely enhance force steadiness by maintaining muscle efficiency and stability. 4.7. Methodological considerations A potential limitation of the study relates to the generalizability of the results, considering the sample primarily consisted of young and highly active individuals. Such a group may exhibit a faster recovery from DOMS compared to older individuals or those with lower levels of physical activity. Despite this, the presence of mechanical hyperalgesia and mild muscle soreness did confirm the presence of DOMS in our sample. Moreover, while the positioning of individuals could have allowed for compensation using hip and gluteal muscles during the task, we aimed to minimise such compensatory movements in our protocol. This was achieved by restricting movement with straps over the pelvis and thighs and by instructing participants to rely mainly on their trunk extensor muscles as they performed the task. Another limitation of the study is the use of rectified sEMG to estimate neural drive to muscles. While sEMG signals can be influenced by various factors, the rationale behind choosing this method has been previously detailed 22 . Another important consideration is the specific load, speed, and measurement time points chosen for this study. Although changes were observed and DOMS was induced by the eccentric exercise protocol, employing an eccentric protocol with maximal effort and faster contractions, along with incorporating immediate and 2-hour post-task measurements, might have revealed additional insights, particularly in terms of HDsEMG measures related to thoracolumbar ES activity, as indicated in previous research for limb muscles 3 . Lastly, it would be beneficial to confirm our speculation regarding the contribution of lower limb musculature to the resultant torque and gain insights into the behaviour of synergistic muscles. The positioning of the electrodes, coupled with the seated posture of the participants, restricted our ability to place additional electrodes on these lower limb muscles. 5. CONCLUSION This study uniquely demonstrates that in the presence of DOMS, individuals exhibit improved torque steadiness during trunk concentric and eccentric extension contractions, likely due to adaptations of movement and muscle recruitment strategies, influenced by a learning effect from initial training exposure. This also shows that contrary to individuals with CLBP who may lack the motor resources to compensate for motor control impairments, resulting in reduced torque steadiness, pain-free individuals can make adjustments that lead to improvements in torque steadiness. Importantly, the study also reveals that movement patterns differ significantly between contractions in the presence of DOMS. Increased lumbar flexion was observed in the more challenging eccentric contractions, whereas in concentric contractions, there was a noticeable reduction in thoracolumbar (sagittal) movement. This variation in behaviour could suggest a strategy of protecting the involved tissues and learned efficiency to optimise torque steadiness performance. Collectively, the findings of this work underscore the significance of adaptive strategies in response to DOMS and their influence on muscular performance. Declarations ACKNOWLEDGEMENTS We extend our gratitude to our colleague Ignacio Contreras-Hernandez for his assistance in data collection for this research. Our appreciation also goes to every participant involved in the study. This research was undertaken without the support of any external funding. AUTHOR CONTRIBUTIONS MA, DF, and E.M.-V. conceived and designed research; M.A. and N.H.-J. performed experiments; M.A., D.J.-G., and E.M.-V. analysed data; MA, DF, and E.M.-V. interpreted results of experiments; M.A. prepared figures: M.A. drafted manuscript; M.A., D.J.-G., N.H.-J., D.F., and E.M.-V. edited and revised manuscript; M.A., D.J.-G., N.H.-J., D.F., and E.M.-V. approved final version of manuscript. FUNDING INFORMATION This study was conducted with no external funding sources. CONFLICT OF INTEREST STATEMENT The authors confirm that there are no conflicts of interest regarding the publication of this paper, financial or otherwise. DATA AVAILABILITY STATEMENT Research data can be shared upon reasonable request. 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Spatial distribution of lumbar erector spinae muscle activity in individuals with and without chronic low back pain during a dynamic isokinetic fatiguing task. Clin Biomech 81, doi: 10.1016/j.clinbiomech.2020.105214 (2021). van Helden, J. F. L., Martinez-Valdes, E., Strutton, P. H., Falla, D. & Chiou, S. Y. Reliability of high-density surface electromyography for assessing characteristics of the thoracic erector spinae during static and dynamic tasks. J Electromyogr Kinesiol 67, 102703, doi: 10.1016/j.jelekin.2022.102703 (2022). Naik, G. R., Selvan, S. E., Gobbo, M., Acharyya, A. & Nguyen, H. T. Principal Component Analysis Applied to Surface Electromyography: A Comprehensive Review. IEEE Access 4, 4025–4037, doi: 10.1109/ACCESS.2016.2593013 (2016). Castronovo, A. M., Negro, F., Conforto, S. & Farina, D. The proportion of common synaptic input to motor neurons increases with an increase in net excitatory input. J Appl Physiol (1985) 119, 1337–1346, doi: 10.1152/japplphysiol.00255.2015 (2015). Jiménez-Grande, D., Atashzar, S. F., Martinez-Valdes, E. & Falla, D. Muscle network topology analysis for the classification of chronic neck pain based on EMG biomarkers extracted during walking. PLoS ONE 16, e0252657, doi: 10.1371/journal.pone.0252657 (2021). Devecchi, V., Alalawi, A., Liew, B. & Falla, D. A network analysis reveals the interaction between fear and physical features in people with neck pain. Sci 12, 11304, doi: 10.1038/s41598-022-14696-8 (2022). Abboud, J., Lessard, A., Piché, M. & Descarreaux, M. Paraspinal muscle function and pain sensitivity following exercise-induced delayed-onset muscle soreness. Eur J Appl Physiol 119, 1305–1311, doi: 10.1007/s00421-019-04117-6 (2019). Hyldahl, R. D. & Hubal, M. J. Lengthening our perspective: morphological, cellular, and molecular responses to eccentric exercise. Muscle Nerve 49, 155–170, doi: 10.1002/mus.24077 (2014). Hody, S., Croisier, J.-L., Bury, T., Rogister, B. & Leprince, P. Eccentric Muscle Contractions: Risks and Benefits. Frontiers in Physiology 10, doi: 10.3389/fphys.2019.00536 (2019). Mizumura, K. & Taguchi, T. Delayed onset muscle soreness: Involvement of neurotrophic factors. The Journal of Physiological Sciences 66, 43–52, doi: 10.1007/s12576-015-0397-0 (2016). Peake, J. M., Neubauer, O., Della Gatta, P. A. & Nosaka, K. Muscle damage and inflammation during recovery from exercise. J Appl Physiol (1985) 122, 559–570, doi: 10.1152/japplphysiol.00971.2016 (2017). Brandl, A., Wilke, J., Egner, C., Schmidt, T. & Schleip, R. Effects of Maximal Eccentric Trunk Extensor Exercise on Lumbar Extramuscular Connective Tissue: A Matched-Pairs Ultrasound Study. J Sports Sci Med 22, 447–454, doi: 10.52082/jssm.2023.447 (2023). Koutris, M., Türker, K. S., van Selms, M. K. A. & Lobbezoo, F. Delayed-onset muscle soreness in human masticatory muscles increases inhibitory jaw reflex responses. J Oral Rehabil 45, 430–435, doi: 10.1111/joor.12635 (2018). Vila-Chã, C., Hassanlouei, H., Farina, D. & Falla, D. Eccentric exercise and delayed onset muscle soreness of the quadriceps induce adjustments in agonist-antagonist activity, which are dependent on the motor task. Exp Brain Res 216, 385–395, doi: 10.1007/s00221-011-2942-2 (2012). Look, M. C., Iyengar, Y., Barcellona, M. & Shortland, A. Does delayed onset muscle soreness affect the biomechanical variables of the drop vertical jump that have been associated with increased ACL injury risk? A randomised control trial. Hum Mov Sci 76, 102772, doi: 10.1016/j.humov.2021.102772 (2021). Hirata, R. P., Salomoni, S. E., Christensen, S. W. & Graven-Nielsen, T. Reorganised motor control strategies of trunk muscles due to acute low back pain. Hum Mov Sci 41, 282–294, doi: 10.1016/j.humov.2015.04.001 (2015). Salomoni, S. E., Marinovic, W., Carroll, T. J. & Hodges, P. W. Motor Strategies Learned during Pain Are Sustained upon Pain-free Reexposure to Task. Med Sci Sports Exerc 51 (2019). Pincheira, P. A. et al. Regional changes in muscle activity do not underlie the repeated bout effect in the human gastrocnemius muscle. Scand J Med Sci Sports 31, 799–812, doi: https://doi.org/10.1111/sms.13912 (2021). Wilke, J. & Behringer, M. Is “Delayed Onset Muscle Soreness” a False Friend? The Potential Implication of the Fascial Connective Tissue in Post-Exercise Discomfort. International Journal of Molecular Sciences 22, 9482 (2021). Herzog, W. Why are muscles strong, and why do they require little energy in eccentric action? J Sport Health Sci 7, 255–264, doi: 10.1016/j.jshs.2018.05.005 (2018). Herzog, W., Schappacher, G., DuVall, M., Leonard, T. R. & Herzog, J. A. Residual Force Enhancement Following Eccentric Contractions: A New Mechanism Involving Titin. Physiology 31, 300–312, doi: 10.1152/physiol.00049.2014 (2016). Additional Declarations No competing interests reported. Cite Share Download PDF Status: Published Journal Publication published 10 Aug, 2024 Read the published version in Scientific Reports → Version 1 posted Editorial decision: Revision requested 17 Jun, 2024 Reviews received at journal 14 Jun, 2024 Reviews received at journal 12 Jun, 2024 Reviewers agreed at journal 05 Jun, 2024 Reviewers agreed at journal 05 Jun, 2024 Reviewers invited by journal 04 Jun, 2024 Editor assigned by journal 04 Jun, 2024 Editor invited by journal 17 May, 2024 Submission checks completed at journal 17 May, 2024 First submitted to journal 15 May, 2024 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-4426332","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":307363930,"identity":"430c1b9d-9b76-4483-a9bf-dc3d77e3867c","order_by":0,"name":"Michail Arvanitidis","email":"","orcid":"","institution":"University of Birmingham","correspondingAuthor":false,"prefix":"","firstName":"Michail","middleName":"","lastName":"Arvanitidis","suffix":""},{"id":307363931,"identity":"d089e59a-3413-42dc-9f1b-606be32e98f2","order_by":1,"name":"David Jiménez-Grande","email":"","orcid":"","institution":"University of Birmingham","correspondingAuthor":false,"prefix":"","firstName":"David","middleName":"","lastName":"Jiménez-Grande","suffix":""},{"id":307363933,"identity":"0651ea49-4294-4534-adc1-914dbe5e3433","order_by":2,"name":"Nadège Haouidji-Javaux","email":"","orcid":"","institution":"University of Birmingham","correspondingAuthor":false,"prefix":"","firstName":"Nadège","middleName":"","lastName":"Haouidji-Javaux","suffix":""},{"id":307363934,"identity":"67ee1ff4-64ca-4116-8a09-76f2bdfef78c","order_by":3,"name":"Deborah Falla","email":"","orcid":"","institution":"University of Birmingham","correspondingAuthor":false,"prefix":"","firstName":"Deborah","middleName":"","lastName":"Falla","suffix":""},{"id":307363936,"identity":"866f4c70-3ec9-46cf-9db4-f7e042c31703","order_by":4,"name":"Eduardo Martinez-Valdes","email":"data:image/png;base64,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","orcid":"","institution":"University of Birmingham","correspondingAuthor":true,"prefix":"","firstName":"Eduardo","middleName":"","lastName":"Martinez-Valdes","suffix":""}],"badges":[],"createdAt":"2024-05-15 15:56:01","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4426332/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4426332/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1038/s41598-024-69050-x","type":"published","date":"2024-08-10T15:58:09+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":57628476,"identity":"3cb713ec-0437-4610-b6da-2cc7f5060051","added_by":"auto","created_at":"2024-06-03 14:29:52","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":239351,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic representation of the overall study procedure.\u003cstrong\u003e \u003c/strong\u003eParticipants performed the same concentric and eccentric contractions 24h and 48h after the induction of delayed onset trunk extensor muscle soreness (part within the red dashed line).\u003c/p\u003e","description":"","filename":"Figure1.png","url":"https://assets-eu.researchsquare.com/files/rs-4426332/v1/bd1dcf2a194a90551b1bb8e5.png"},{"id":57628474,"identity":"01c5f7f8-e298-4fe1-921e-a8ed0c68f649","added_by":"auto","created_at":"2024-06-03 14:29:52","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":858050,"visible":true,"origin":"","legend":"\u003cp\u003eTopographical maps of PPT are presented across three consecutive days: baseline, after 24h, and 48h respectively. The red coloration indicates areas of heightened sensitivity, while blue colours represent areas of less sensitivity, denoting higher PPT values. Notably, there is increased sensitivity observed at 24h and 48h compared to baseline. All PPT maps utilise the same scale and have been normalised according to the maximum and minimum of all values. A white circle with an 'X' inside represents the center of sensitivity, indicating the location of overall heightened sensitivity, which was similar during all days.\u003c/p\u003e","description":"","filename":"OnlineFigure2.png","url":"https://assets-eu.researchsquare.com/files/rs-4426332/v1/341f89b40cf3060c0fdc5ea1.png"},{"id":57628475,"identity":"cdb3889b-8bc6-4d89-b0b3-979fcaa1cb48","added_by":"auto","created_at":"2024-06-03 14:29:52","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":949203,"visible":true,"origin":"","legend":"\u003cp\u003eA visual representation of the key results obtained from the eccentric trunk extensions. Eccentric strength during the first session, before and after the exercise protocol to induce delayed onset of trunk muscle soreness (A), eccentric strength (B), torque SD (C), coherence z-scores in the δ band (D), HDsEMG-torque cross correlation (E) across all three measurement points, area under the curve in the sagittal plane across all measurement points and for both force levels (F). The results for graph F are presented as mean ± SD and for the rest as mean and individual values. For graphs B, C, D and E data is pooled and presented across force levels (i.e., 25% and 50%). Main effect of session, \u003cem\u003ep\u0026lt;0.01\u003c/em\u003e: **; session × torque interaction, \u003cem\u003ep\u0026lt;0.01\u003c/em\u003e: ##; \u003cem\u003eη, θ\u003c/em\u003e: post hoc pairwise comparisons with Bonferroni correction.\u003c/p\u003e","description":"","filename":"OnlineFigure3.png","url":"https://assets-eu.researchsquare.com/files/rs-4426332/v1/38a04f7522cb894a993ff6aa.png"},{"id":57628477,"identity":"65d6f1fc-f046-4f69-9163-e4a9a7d4d82a","added_by":"auto","created_at":"2024-06-03 14:29:52","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":680169,"visible":true,"origin":"","legend":"\u003cp\u003eA visual representation of the key results obtained from the concentric trunk extension contractions. Concentric strength (A), torque CoV (B), torque SD (C) over the three measurement points, area under the curve in the sagittal plane across all measurement points and for both force levels (D). The results for all graphs are presented as mean and individual values. For all graphs data is pooled and presented across force levels (i.e., 25% and 50%). Main effect of session, p\u0026lt;0.05: *, p\u0026lt;0.01: **; η: post hoc pairwise comparisons with Bonferroni correction.\u003c/p\u003e","description":"","filename":"OnlineFigure4.png","url":"https://assets-eu.researchsquare.com/files/rs-4426332/v1/c0ef177183d5652e9a098c41.png"},{"id":62298571,"identity":"58d79c58-3818-4b7d-b868-d594fc05f658","added_by":"auto","created_at":"2024-08-12 16:14:45","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":6156236,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4426332/v1/95094a8c-60a1-4618-a699-f4f2dad6017b.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Eccentric exercise-induced delayed onset trunk muscle soreness alters high-density surface EMG- torque relationships and lumbar kinematics","fulltext":[{"header":"1. INTRODUCTION","content":"\u003cp\u003eDelayed-onset muscle soreness (DOMS) represents a form of ultrastructural muscle injury that typically emerges following intense, unfamiliar exercises, particularly those emphasizing eccentric contractions \u003csup\u003e1,2\u003c/sup\u003e. Clinical signs of DOMS, include reduced force production, painful movement limitations, stiffness, swelling, and dysfunction in adjacent joints \u003csup\u003e1\u003c/sup\u003e. These symptoms can persistently impair muscle function \u003csup\u003e3\u003c/sup\u003e. Although considered a mild form of injury, DOMS frequently undermines athletic performance \u003csup\u003e1\u003c/sup\u003e and may disrupt daily activities \u003csup\u003e3\u003c/sup\u003e, likely due to alterations in electromyography (EMG)-force relationships, during muscle contractions \u003csup\u003e3\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eNumerous studies have investigated the effect of DOMS on the neuromuscular function of peripheral muscles, such as the elbow flexors and hamstrings \u003csup\u003e3\u0026ndash;9\u003c/sup\u003e. In contrast, research focusing on trunk muscles is less extensive, with only a few studies inducing DOMS to assess its influence on trunk muscle function. For example some of these investigations have revealed altered activation patterns \u003csup\u003e10,11\u003c/sup\u003e and decreased force accuracy \u003csup\u003e2\u003c/sup\u003e. Given these findings, and considering that people often move differently when experiencing pain \u003csup\u003e12\u003c/sup\u003e, it is hypothesized that DOMS may also lead to changes in trunk movement patterns, yet this aspect remains poorly understood. Additionally, it remains to be investigated whether DOMS alters trunk extensor muscle torque steadiness.\u003c/p\u003e \u003cp\u003ePerforming contractions with minimal force fluctuations is crucial in everyday life activities, as reduced torque steadiness can affect the precision of voluntary movements and functional ability. During a voluntary contraction, the output from the activated motor unit population leads to force generation, which is not constant but rather fluctuates around an average value \u003csup\u003e13\u003c/sup\u003e. Years of investigation into the neural mechanisms causing variations in muscle force during voluntary contractions have revealed that the neural command's low-frequency component (\u0026lt;\u0026thinsp;10Hz) is the most relevant for force generation \u003csup\u003e14\u003c/sup\u003e. This component mirrors the common synaptic input received by the motor unit population \u003csup\u003e15\u003c/sup\u003e. It has previously been demonstrated that eccentric exercise can cause increased elbow flexor muscle force fluctuations, likely due to an increased common neural drive to the muscles \u003csup\u003e16\u003c/sup\u003e. Given these findings, it is relevant to explore the effect of DOMS on torque steadiness, trunk muscle activation, and spine kinematics during dynamic contractions.\u003c/p\u003e \u003cp\u003eTo understand the relationship between muscle activity and the force output, surface EMG (sEMG)/force relationships have been extensively examined. Such investigations \u003csup\u003e17,18\u003c/sup\u003e have identified an association between low-frequency force fluctuations and the corresponding low-frequency component of the rectified interference sEMG. For certain muscles such as the erector spinae (ES), where HDsEMG decomposition is challenging, this methodology offers an alternative, especially when establishing the correlation between motor unit discharge times and force (the gold standard) is difficult. Staudenmann et al. \u003csup\u003e19,20\u003c/sup\u003e have indicated that employing high-density sEMG (HDsEMG), known for its superior spatial sampling resolution, alongside the application of Principal Component Analysis (PCA), enhances the force estimations based on sEMG. PCA acts as a dimensionality-reduction tool that can identify and select a subset of principal components (PCs) accountable for the majority of data variation, thereby explaining most of the variance in the exerted torque \u003csup\u003e19\u003c/sup\u003e. This method has been applied previously to quantify both magnitude and regional alterations in HDsEMG-torque relationships in individuals with and without chronic low back pain (CLBP) during isometric \u003csup\u003e21\u003c/sup\u003e and dynamic trunk extension \u003csup\u003e22\u003c/sup\u003e. However, how these relationships are altered in the presence of thoracolumbar ES DOMS remains unclear. Given the frequent use of trunk muscles in daily activities involving eccentric contractions, understanding the effects of DOMS on these muscles is crucial. Specifically, changes in trunk neuromuscular function, such as increased force fluctuations and altered recruitment strategies, may predispose individuals to more severe muscle injuries under certain conditions \u003csup\u003e23\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eThis study aimed to evaluate the influence of DOMS on various motor performance measures and HDsEMG parameters measured from the thoracolumbar ES during both concentric and eccentric trunk extension contractions. Specifically, the objectives were to (i) quantify changes in the relationship between HDsEMG oscillations and torque oscillations in both time and frequency domains, (ii) assess and compare regional differences in HDsEMG amplitude and HDsEMG\u0026ndash;torque cross-correlation and coherence of the ES, and (iii) investigate differences in torque steadiness and associated kinematic data from the lumbar spine following the induction of DOMS in asymptomatic individuals. It was hypothesized that individuals would exhibit reduced torque steadiness under the influence of DOMS, accompanied by alterations in EMG-torque relationships and lumbar kinematics.\u003c/p\u003e"},{"header":"2. METHODS","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1. Study design and setting\u003c/h2\u003e \u003cp\u003e This observational, cross-sectional study with a repeated measures design received approval from the Ethical Review Committee at the University of Birmingham, United Kingdom (approval number: ERN 19-1148) and adhered to the Declaration of Helsinki. The manuscript follows the STROBE guidelines for reporting. Data collection spanned from April 2019 to July 2022 at a laboratory within the Centre of Precision Rehabilitation for Spinal Pain, University of Birmingham, United Kingdom. The study was conducted over three sessions on consecutive days, and each were separated by approximately 24 h (baseline, post 24h, post 48h). All participants gave their written consent before involvement.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2. Participants\u003c/h2\u003e \u003cp\u003eTwenty asymptomatic controls (ten males, ten females) were recruited from the local Birmingham community, including the University of Birmingham's student and staff populations, through social media announcements and distributed information leaflets. The sample size was based on previous studies \u003csup\u003e10,11\u003c/sup\u003e that used a similar number of participants but also on a moderate effect size of \u003cem\u003ef\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.36, an \u003cem\u003eα\u003c/em\u003e of 0.05, a \u003cem\u003eβ\u003c/em\u003e power of 0.9 and a 10% data loss due to signal quality of participant withdrawal, for a repeated measures analysis of variance (ANOVA) using a within factors design consisting of three measurement points. The effect size was calculated from the mean\u0026thinsp;\u0026plusmn;\u0026thinsp;SD of lumbar ES HDsEMG reflex amplitude values before and after the induction of DOMS (averaged effect sizes for both sides and first and last trials) reported by Abboud et al., (2021) \u003csup\u003e10\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eParticipants were eligible for the study if they were men or women aged 18 to 55 years, asymptomatic, with no prior back or lower limb pain requiring medical attention. Exclusion criteria included cardiovascular diseases, pregnancy, spinal deformities or surgeries, systemic or inflammatory conditions, rheumatic and neuromuscular disorders, neurological conditions, and lumbar radiculopathy. During the three days, as well as the day preceding the experiment, participants were advised against engaging in any high-intensity or atypical exercises and refraining from taking medications intended to alleviate pain or soreness (\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3. Questionnaires\u003c/h2\u003e \u003cp\u003eAt the start of the session, to assess the level of physical activity, participants were asked to complete the full version of the International Physical Activity Questionnaire (IPAQ). The reliability and validity of the complete IPAQ have been previously established \u003csup\u003e24\u003c/sup\u003e. Lumbar muscle soreness was also verbally assessed at the beginning and end (i.e., immediately after the eccentric exercise protocol) of the first session. Subsequent evaluations were made at the beginning of the following sessions at 24 and 48 hours, respectively. Participants were instructed to evaluate the subjective intensity of movement-related lumbar muscle soreness using an adapted 0\u0026ndash;10 visual analogue scale (VAS), where 0 represented \"no soreness at all\" and 10 signified \"extreme soreness\"\u003csup\u003e6,25\u003c/sup\u003e. During the eccentric exercise protocol (detailed below), participants were prompted to indicate their perceived exertion levels (after each set of 5 repetitions) using a modified Borg scale (Category-Ratio-10 Scale; Borg, 1998). The scale spans from \"0\" (no perceived exertion) to \"10\" (extreme exertion).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e2.4. Pain sensitivity assessment\u003c/h2\u003e \u003cp\u003ePressure pain threshold (PPT) testing was performed to quantitatively assess changes in thoracolumbar ES sensitivity across consecutive days after the induction of DOMS. Following a familiarisation phase, PPTs were measured with the participants lying on a plinth in a prone position, using an electronic algometer (probe tip: 1 cm\u003csup\u003e2\u003c/sup\u003e, 30kPa/s; NOD, OT Bioelettronica, Italy). Participants were guided to indicate to the researcher when the sensation of pressure transitioned to pain, at which moment the pressure application ceased.\u003c/p\u003e \u003cp\u003ePPTs were performed only on the right side for all participants over ten testing sites (spanning approximately from L5 to T10) as described previously \u003csup\u003e22\u003c/sup\u003e. At each location, two PPT measurements were performed (randomised order), and the average of the two measurements at each site was used for subsequent analysis (mean thoracolumbar ES sensitivity). PPTs were assessed at the beginning and end of session 1 (immediately after the eccentric exercise protocol), as well as at the start of session 2 (24h later) and session 3 (48h later). Topographical maps of the PPT were also generated using the mean values. The centroid (x-axis and y-axis coordinates) was computed to identify the region of increased sensitivity (i.e., areas with the lowest PPTs). This approach facilitated comparisons of sensitivity areas across different days. The same researcher performed all PPT measurements, ensuring the reduction of inter-experimenter variability.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e2.5. Electromyography\u003c/h2\u003e \u003cp\u003eSurface HDsEMG recordings were acquired in monopolar configuration using four 2D electrode grids, each arranged in a 13 x 5 pattern. These grids had evenly spaced electrodes (diameter: 1mm, spacing: 8mm; GR08MM1305, OT Bioelettronica, Italy), with one electrode missing in the upper left corner. Each HDsEMG electrode was attached with a double-sided adhesive foam (FOA08MM1305, OT Bioelettronica, Italy). Conductive gel was then applied to the electrode sections of the grids to ensure proper skin contact (AC-CREAM, SPES Medica, Genoa, Italy). Participant skin preparation included shaving (if required), mild abrasion using an abrasive gel (Nuprep Skin Prep Gel, Weaver and Company, Aurora, Colorado), followed by water rinse and drying. To monitor thoracolumbar ES activity, two of the HDsEMG grids were placed vertically on one side. The remaining two grids were positioned on the same side: one over the rectus abdominis (RA) and the other on the external oblique (EO). Reference electrodes (WhiteSensor WS, Ambu A/S, Ballerup, Denmark) were also affixed to the participant's sacrum, the anterior superior iliac spine (ASIS), and wrist. For a comprehensive description and visual representation of the HDsEMG electrode placement, we direct readers to our previous work \u003csup\u003e22\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eBoth torque and HDsEMG signals were sampled at a rate of 2048Hz. These signals were digitised using a 16-bit A/D converter (Quattrocento, 400-channel EMG amplifier, OT Bioelettronica, Torino, Italy, amplification: 150, frequency range 10-500Hz, primary order, 3dB). The OTBiolab\u0026thinsp;+\u0026thinsp;software platform was used for data acquisition.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e2.6. Lumbar kinematics\u003c/h2\u003e \u003cp\u003eLumbar movements during the contractions were quantified using Noraxon's myoMOTION system and two wearable Inertial Measurement Units (Research PRO IMUs, Noraxon USA) with a sampling rate of 100 Hz. The sensors were attached via double-sided tape on the lower thoracic and lumbar spine (T12 and L5, respectively). Using Noraxon's myoRESEARCH software (version 3.14), custom angles were created based on the difference between these two sensors. This allowed lumbar flexion/extension, lateral flexion and rotation angles to be captured during all contractions. Before any measurement commenced, participants were asked to adopt a natural upright position while sitting on the dynamometer's chair, and the sensors were calibrated. The myoMOTION software and hardware allowed the synchronization of the myoMOTION receiver with other systems.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e2.7. Isokinetic dynamometry\u003c/h2\u003e \u003cp\u003eAn isokinetic dynamometer (System 3 Pro, Biodex Medical Systems, New York) was utilised to assess the torque produced by participants during concentric and eccentric trunk extension maximal voluntary contractions (MVCs) and the torque steadiness tasks performed at submaximal levels. To focus on the lumbar spine, participants were positioned on the Biodex Dual Position Back Extension/Flexion Attachment, ensuring their hips and knees were at a 90\u0026deg; angle and feet spaced at shoulder width \u003csup\u003e21,26\u003c/sup\u003e. The chair's seat was tilted upwards approximately 15\u0026deg;, and its height was adjusted so the dynamometer's rotational axis aligned with the ASIS bilaterally \u003csup\u003e26\u003c/sup\u003e. To prevent compensatory movements, participants' upper trunk, pelvis, and thighs were securely strapped to the chair. A specific Biodex attachment was also in place to reduce knee muscle engagement.\u003c/p\u003e \u003cp\u003eFor all contractions, the dynamometer operated in isokinetic mode. The range of motion (ROM) for both concentric/eccentric extension contractions was 50\u0026deg;, spanning from 20\u0026deg; extension to 30\u0026deg; flexion, emphasising lumbar movement and minimising compensatory actions from the legs \u003csup\u003e26\u003c/sup\u003e. A consistent angular speed of 5\u0026deg;/s was maintained. When returning to the starting position, the concentric contraction mode was consistently set at an angular velocity of 90\u0026deg;/s. For the eccentric mode, the return velocity was adjusted to 20\u0026deg;/s to ensure optimised comfort for the participant. Nonetheless, participants were assisted back to the initial position by the researcher, who manually readjusted the chair for every repetition.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003e2.8. Eccentric exercise protocol (DOMS)\u003c/h2\u003e \u003cp\u003eThe eccentric exercise protocol consisted of three sets of 15 eccentric contractions of the trunk extensors, moving from 20\u0026deg; of trunk extension to 30\u0026deg; of trunk flexion. The protocol was performed only at the end of session 1. Resistance was set at 50% of their MVC (the %MVC was chosen after pilot testing to induce mild-moderate muscle soreness), with an angular speed of 5\u0026deg;/s. Participants were given a 30-second rest between each set. Participants were asked to perform an MVC immediately after the eccentric contractions to further assess the trunk extensors and validate the effectiveness of the eccentric exercise to induce fatigue (i.e., reduction in maximal torque output).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e2.9. Experimental protocol\u003c/h2\u003e \u003cp\u003e Participants were given time to familiarise themselves with the dynamometer, by practising all contractions and warming up their trunk extensor muscles via sustained and dynamic submaximal contractions.\u003c/p\u003e \u003cp\u003eAfter a brief rest, they were asked to perform two concentric trunk extension MVCs, moving through a 50\u0026deg; ROM. The starting position for the concentric trunk extension, MVC, was 30\u0026deg; of trunk flexion, and the final position was 20\u003csup\u003eo\u003c/sup\u003e of trunk extension. Between these contractions, 1-minute rest was provided. After a 5-minute rest, participants were instructed to perform submaximal concentric trunk contractions, four times at 25% and three times at 50% MVC, in a randomised order. During these contractions, they were asked to match the target torque lines at 25% and 50% MVC, keeping the contraction for 10 seconds. They were allowed to practice each submaximal contraction once before the actual recordings. The highest peak torque observed during the MVC was used to set the submaximal torque targets. After these, participants repeated the same procedures for the eccentric trunk contractions. The only difference for the eccentric trunk extension contractions was that the starting position was at 20\u0026deg; of trunk extension, and the final position at 30\u0026deg;of trunk flexion (i.e., the opposite from the concentric contractions).\u003c/p\u003e \u003cp\u003eThroughout all submaximal contractions, real-time visual feedback of their torque output and a line representing the desired contraction level (%MVC) was visible on a computer screen positioned 1.5 meters in front of them. This real-time torque data was superimposed over the template for visual feedback. Participants aimed to swiftly and accurately match the %MVC target. Once they achieved the set target (either 25% or 50% MVC), they were advised to keep their torque output as steady as possible for the whole contraction. Essentially, they were asked to produce consistent torque output (25% or 50%MVC) across the entire 50\u0026deg; ROM. The overall study procedure is depicted schematically in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cb\u003e[PLEASE INSERT\u003c/b\u003e FIGURE \u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e \u003cb\u003eHERE]\u003c/b\u003e\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003e2.10. Analysis of the torque signal\u003c/h2\u003e \u003cp\u003eThe highest peak torque during the MVCs was used to assess each individual's maximum trunk extension concentric and eccentric strength. Torque steadiness was evaluated by calculating the absolute and relative amplitude of the torque fluctuations, determined by the standard deviation and the CoV of torque, respectively (CoV, standard deviation of the torque/mean of torque \u0026times; 100) \u003csup\u003e13\u003c/sup\u003e. A custom MATLAB script was employed to allow the computation of the SD and CoV of torque within the same time window used for the HDsEMG analysis, during the steady phase of the contraction. These values were determined for each repetition and then averaged to provide a single value for each torque level for each contraction. The time windows selected for analysis were approximately 8 seconds for both submaximal torque levels. This approach aimed to exclude the first and last second of the steady part of the contraction, periods during which participants typically overestimated or underestimated the requested torque level.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section2\"\u003e \u003ch2\u003e2.11. EMG amplitude and topographical map computations\u003c/h2\u003e \u003cp\u003eDuring the offline analysis, the 64-monopolar HDsEMG channels from the electrodes positioned over the RA and EO muscles, as well as the 128-monopolar channels from the merged electrodes over the thoracolumbar ES, were processed to obtain 59 and 124 bipolar channels, respectively. Before the root mean square (RMS) amplitude calculations, the HDsEMG signals were filtered with a bandpass zero-lag Butterworth filter (10-350Hz, 2nd order) and visually inspected, to exclude channels with low-quality signals due to electrical interference and/or artifacts. Less than 15% of the channels were discarded.\u003c/p\u003e \u003cp\u003eOne RMS value was calculated for each of the 59 bipolar channels for the abdominal muscles (RA and EO) during each trunk eccentric/concentric extension contraction. Similarly, the RMS amplitude was also determined for each of the 124 bipolar channels for the thoracolumbar ES during all trunk eccentric/concentric extension contractions. By determining the RMS for each channel, a HDsEMG amplitude map was generated for the (agonist) thoracolumbar ES and the centroid (x-,y- coordinates) of this map was calculated during all muscle contractions. This allowed us monitor regional changes in thoracolumbar ES activation. Additionally, a global measure of myoelectric activity was calculated for each muscle, by calculating the average of RMS values from all channels for each muscle, which formed a single value (HDsEMG amplitude; RMSmean). The HDsEMG amplitude from the abdominal muscles was only used to assess the level of co-activation.\u003c/p\u003e \u003cp\u003eThe level of trunk flexor and extensor co-activation was measured during all muscle contractions. The co-activation level was calculated using the raw (i.e., non-normalised) RMSmean values, calculated as: antagonist muscle activity/primary muscle activity x 100. For this, we added the RMSmean values of the abdominal muscles (RA\u0026thinsp;+\u0026thinsp;EO). In this study, EMG signals were not normalized due to the established reliability of HDsEMG-derived measures in within-subject designs, particularly during voluntary trunk movements \u003csup\u003e27\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eAll HDsEMG and torque data was assessed offline using a custom MATLAB 2020b script (The MathWorks Inc., USA). Concentric and eccentric muscle contractions were recorded separately.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec14\" class=\"Section2\"\u003e \u003ch2\u003e2.12. HDsEMG data pre-processing for PCA analysis\u003c/h2\u003e \u003cp\u003ePCA is a dimensionality reduction technique that effectively identifies redundant information in HDsEMG data. This approach can also enhance the accuracy of HDsEMG-based force estimations \u003csup\u003e19,20\u003c/sup\u003e. By transforming complex, multivariate data into a series of linearly independent PCs through an orthogonal transformation, PCA reduces dimensionality while retaining most of the data\u0026rsquo;s variance. In this study, these components, which were linear combinations of the original 124 channels, were organized in descending order based on the variance they accounted for, with the first few components representing the majority of the data's variance. PCs that collectively accounted for a cumulative 85% of the total variance were retained \u003csup\u003e21,22,28\u003c/sup\u003e to keep only the most informative components and improve the signal-to-noise ratio. This process, which marked the initial phase of dimensionality reduction, generated a new matrix containing the eigenvectors of these selected PCs. The original dataset, therefore, remained intact until the transformation in the final phase of PCA. In this phase, the dataset was reorientated from its original axes to those delineated by the selected PCs. This was achieved by transforming the original dataset into a lower-dimensional space (i.e., \u0026lt;\u0026thinsp;124) precisely defined by these PCs that explained 85% of the variance in the data. Consequently, the original 124-channel dataset was transformed into a new dataset with reduced dimensionality that explained most of the variance in the data. For further details on the PCA methodology, please refer to earlier work \u003csup\u003e21,22\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eBefore performing the cross-correlation and coherence analyses, the formerly offline-referenced 124 bipolar HDsEMG channels (from the combined electrodes placed over ES) underwent the following pre-processing for PCA computations: 1) 10 Hz high-pass filtering, 2) selection of the most informative subset of PCs using PCA on the 124 (for ES) differential HDsEMG signals in temporal domain, 3) full-wave rectification and averaging the selected PCs to generate a single time signal for the thoracolumbar ES muscle, 4) low-pass filtering at 10 Hz (supplementing the initial filtering), aiming to detect slow-frequency variations in motor unit activation/recruitment \u003csup\u003e14\u003c/sup\u003e, 5) application of a first-order Savitzky-Golay filter for smoothing, and 6) removal of DC (zero frequency) components \u003csup\u003e19\u0026ndash;22\u003c/sup\u003e. These processing procedures resulted in a final signal envelope that was obtained by applying PCA to the HDsEMG grid and contained the low-frequency components of the HDsEMG data.\u003c/p\u003e \u003cp\u003eCoherence and cross-correlation analyses were then conducted to estimate the similarity between this final EMG signal envelope and torque signals. Please note that the PCA and subsequent coherence and cross-correlation analyses were exclusively conducted for the thoracolumbar ES, which functioned as the primary agonist muscle during the eccentric and concentric contractions.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec15\" class=\"Section2\"\u003e \u003ch2\u003e2.13. Cross-correlation and coherence analyses\u003c/h2\u003e \u003cp\u003eCross-correlation analysis quantified the similarity (cross-correlation coefficient) between the final signal envelope (specifically, the low-pass filtered average PCA-selected signal derived from the first PCs accounting for 85% variance) and torque signals in the time domain. The relationship between torque and EMG was further investigated through coherence analysis, as done previously \u003csup\u003e21\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eCoherence analysis, aiming to measure the intensity and frequency of synchronous synaptic inputs across the motor unit population in relation to torque, was executed using the magnitude squared coherence (MSC) method with a 1-s Hamming window and 50% overlap, as detailed earlier \u003csup\u003e29\u003c/sup\u003e. Given the contraction's brief duration, this method was chosen to optimize analysis resolution while minimizing data loss. The same sEMG envelope (Final Signal Envelope) was evaluated against the torque signal in the frequency domain. MSC, a frequent measure to evaluate signal similarity in the frequency domain, was computed using MATLAB's \u003cem\u003emscohere\u003c/em\u003e function. This function applies Welch's overlapped averaged periodogram method with a 50% overlap across N sub-windows. Further MSC computation insights can be found in previous studies \u003csup\u003e21,30\u003c/sup\u003e. The Fisher's z-transformation was utilised on coherence estimates (C) to enable statistical comparisons, as these transformed values (FZ) follow a normal distribution. Given the potential crosstalk affecting sEMG recordings, bias was determined as the coherence profile's peak value at 250 Hz, an area with no significant correlated activity \u003csup\u003e29\u003c/sup\u003e. The utilised equation was the following:\u003cdiv id=\"Equa\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equa\" name=\"EquationSource\"\u003e\n$$FZ=\\text{atanh}\\left(\\sqrt{C}\\right)-bias$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eThis study's coherence analysis focused on the δ band (0-5Hz), which is most relevant to muscle torque generation \u003csup\u003e15\u003c/sup\u003e. Additionally, topographical coherence maps, as described in previous research \u003csup\u003e21\u003c/sup\u003e, were generated. In this process, each of the 124 HDsEMG signals from the thoracolumbar ES was individually assessed for coherence in frequency with the filtered torque signal. This analysis resulted in 124 distinct coherence values, one for each HDsEMG signal. These values were then utilized to construct topographical coherence maps. The maps were then normalized to the maximum coherence value at each torque level. The coherence's centroid provided an estimate of the centre of ES muscle δ-band coherence along both medial-lateral (x-axis) and cranial-caudal (y-axis) directions. This analysis enabled us to examine if certain areas of the thoracolumbar ES have greater influence on torque generation and to determine whether the regions with predominant influence shifted across days due to muscle soreness, by statistically comparing x- and y-axis centroid values across days.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec16\" class=\"Section2\"\u003e \u003ch2\u003e2.14. Analysis of lumbar kinematics\u003c/h2\u003e \u003cp\u003eCustom anatomical angles (flexion/extension, rotation, and lateral flexion) were generated based on the differences between the two IMU sensors (T12 and L5). The raw data of custom angles was exported in MATLAB (R2020b, MathWorks) for further offline analysis. A low-pass Butterworth filter with a cut-off frequency of 10Hz was applied to all kinematic data \u003csup\u003e31\u003c/sup\u003e. Kinematic angles were calculated for all contractions, for all days, during each repetition (25%: 4 repetitions, 50%: 3 repetitions) and then averaged. Based on the averaged data (average of all repetitions), eight epochs were created. The eight epochs were normalized by subtracting epoch 1 from all epochs and were converted to positive (absolute) values. This was necessary to allow the calculation of the area under the curve (AUC) for all movement planes (i.e., sagittal, frontal and transverse), which was used to assess differences in the magnitude of movement across days.\u003c/p\u003e \u003cp\u003ePlease note that HDsEMG, torque, cross-correlation, coherence and kinematic analyses were performed separately for each type of contraction (concentric, eccentric). Additionally, for every repetition, a single value was computed, i.e., four values for 25%MVC and three values for 50%MVC. These were then averaged to form one value for each submaximal level. This analysis was consistently applied to the data from all three sessions and was used for the statistical analysis.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec17\" class=\"Section2\"\u003e \u003ch2\u003e2.15. Statistical procedures\u003c/h2\u003e \u003cp\u003eStatistical analyses were performed on SPSS Statistics, version 29 (IBM, USA). The Shapiro\u0026ndash;Wilk and the Levene tests were used to confirm data normality and ensure homogeneity of variance, respectively. Given that these conditions were satisfied, parametric tests were used. Data are presented as mean and standard deviation (SD), unless otherwise specified. For the pre-and-post assessments of muscle soreness, PPTs and MVCs, paired samples \u003cem\u003et\u003c/em\u003e-Tests were employed.\u003c/p\u003e \u003cp\u003eFor HDsEMG, torque and kinematic variables (i.e., RMS, centroid values of the RMS amplitude map in both axes, level of co-activation between the ES and abdominal muscles, δ band sEMG-torque coherence, sEMG-torque cross-correlation, centroid values for sEMG-torque coherence in both axes mean torque, torque CoV, torque SD, AUC), a two-way repeated measures ANOVA was performed separately for each type of contraction (eccentric, concentric). The torque target (20%MVC and 50%MVC) and session (baseline, 24h, 48h) were used as within-subject factors. One-way repeated measures ANOVA was also used to monitor changes in muscle soreness, PPTs and MVCs across the three testing days.\u003c/p\u003e \u003cp\u003eIn instances where the ANOVA indicated significant interactions, a Bonferroni correction was applied for detailed pairwise comparisons. The threshold for statistical significance was set at \u003cem\u003ep\u0026thinsp;=\u0026thinsp;0.05\u003c/em\u003e.\u003c/p\u003e \u003c/div\u003e"},{"header":"3. RESULTS","content":"\u003cdiv id=\"Sec19\" class=\"Section2\"\u003e\n\u003ch2\u003e3.1. Muscle soreness\u003c/h2\u003e\n\u003cp\u003eAll 20 participants (10 males, 10 females) completed the study, with their characteristics detailed in Table\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e. The eccentric exercise protocol performed at the end of session one induced a moderate level of movement-related back muscle soreness immediately post-exercise (4.1/10) (\u003cem\u003et=-8.305, p\u0026thinsp;\u0026lt;\u0026thinsp;0.001\u003c/em\u003e). Participants reported mild muscle soreness 24h and 48h compared to baseline (session effect: \u003cem\u003eF\u0026thinsp;=\u0026thinsp;28.222\u003c/em\u003e, \u003cem\u003ep\u0026thinsp;\u0026lt;\u0026thinsp;0.001, \u0026eta;p\u003c/em\u003e\u003csup\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sup\u003e\u0026thinsp;\u003cem\u003e=\u0026thinsp;0.744\u003c/em\u003e). Muscle soreness immediately post-exercise was higher compared to that observed at 24h and was similar to that observed at 48h (session effect: \u003cem\u003eF\u0026thinsp;=\u0026thinsp;5.760\u003c/em\u003e, \u003cem\u003ep\u0026thinsp;=\u0026thinsp;0.012\u003c/em\u003e, \u003cem\u003e\u0026eta;p\u003c/em\u003e\u003csup\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sup\u003e\u0026thinsp;\u003cem\u003e=\u0026thinsp;0.233\u003c/em\u003e). Mean\u0026thinsp;\u0026plusmn;\u0026thinsp;SD values of muscle soreness are reported in Table\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e\n\u003cdiv class=\"gridtable\"\u003e\n\u003cdiv class=\"colspec\" align=\"left\"\u003e\u0026nbsp;\u003c/div\u003e\n\u003cdiv class=\"colspec\" align=\"char\"\u003e\u0026nbsp;\u003c/div\u003e\n\u003ctable id=\"Tab1\" border=\"1\"\u003e\u003ccaption\u003e\n\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\n\u003cdiv class=\"CaptionContent\"\u003e\n\u003cp\u003eDemographic characteristics and descriptive data of all 20 asymptomatic individuals that participated in the study. Data are presented across different time points (baseline, after the eccentric exercise protocol, after 24 and 48h respectively).\u003c/p\u003e\n\u003c/div\u003e\n\u003c/caption\u003e\n\u003cthead\u003e\n\u003ctr\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eCharacteristic\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eMean\u0026thinsp;\u0026plusmn;\u0026thinsp;SD\u003c/p\u003e\n\u003c/th\u003e\n\u003c/tr\u003e\n\u003c/thead\u003e\n\u003ctbody\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eAge (years)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e28.7\u0026thinsp;\u0026plusmn;\u0026thinsp;3.9\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eHeight (cm)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e171.9\u0026thinsp;\u0026plusmn;\u0026thinsp;7.4\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eBody mass (Kg)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e70.6\u0026thinsp;\u0026plusmn;\u0026thinsp;13.4\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eBody mass Index (kg/m\u003csup\u003e2\u003c/sup\u003e)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e23.9\u0026thinsp;\u0026plusmn;\u0026thinsp;4.2\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003ePPT Session 1 \u0026ndash; Baseline (kPa)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e397.8\u0026thinsp;\u0026plusmn;\u0026thinsp;95.0\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003ePPT Session 1 \u0026ndash; Post\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e370\u0026thinsp;\u0026plusmn;\u0026thinsp;110.74\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003ePPT 24h\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e361.0\u0026thinsp;\u0026plusmn;\u0026thinsp;113.7\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003ePPT 48h\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e357.9\u0026thinsp;\u0026plusmn;\u0026thinsp;106.6\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eVAS Session 1 \u0026ndash; Baseline (0\u0026ndash;10)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e0.0\u0026thinsp;\u0026plusmn;\u0026thinsp;0.0\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eVAS Session 1 \u0026ndash; Post\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e4.1\u0026thinsp;\u0026plusmn;\u0026thinsp;2.2\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eVAS 24h\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e2.6\u0026thinsp;\u0026plusmn;\u0026thinsp;2.1\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eVAS 48h\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e2.9\u0026thinsp;\u0026plusmn;\u0026thinsp;1.8\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eIPAQ (Total METs score)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e4325.1\u0026thinsp;\u0026plusmn;\u0026thinsp;3578.2\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eECC MVC Session 1 \u0026ndash; Baseline (N\u0026sdot;m)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e203.4\u0026thinsp;\u0026plusmn;\u0026thinsp;48.9\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eECC MVC Session 1 \u0026ndash; Post\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e174.2\u0026thinsp;\u0026plusmn;\u0026thinsp;53.3\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eECC MVC 24h\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e194.8\u0026thinsp;\u0026plusmn;\u0026thinsp;51.6\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eECC MVC 48h\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e191.4\u0026thinsp;\u0026plusmn;\u0026thinsp;54.9\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eCON MVC Session 1 (N\u0026sdot;m)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e211.8\u0026thinsp;\u0026plusmn;\u0026thinsp;58.7\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eCON MVC 24h\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e200.2\u0026thinsp;\u0026plusmn;\u0026thinsp;43.1\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eCON MVC 48h\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e197.0\u0026thinsp;\u0026plusmn;\u0026thinsp;48.1\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003c/tbody\u003e\n\u003c/table\u003e\n\u003c/div\u003e\n\u003cp\u003e\u003cstrong\u003eAbbreviations:\u003c/strong\u003e Pressure pain threshold (PPT), visual analogue scale (VAS) and International Physical Activity Questionnaire (IPAQ), eccentric (ECC), maximum voluntary contractions (MVC), metabolic equivalents (METs), concentric (CON).\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec20\" class=\"Section2\"\u003e\n\u003ch2\u003e3.2. Mean thoracolumbar PPTs\u003c/h2\u003e\n\u003cp\u003eDuring the first session, after the eccentric exercise protocol, PPTs assessed over the lumbar region decreased significantly (\u003cem\u003et\u0026thinsp;=\u0026thinsp;2.195\u003c/em\u003e, \u003cem\u003ep\u0026thinsp;=\u0026thinsp;0.041\u003c/em\u003e). This heightened mean thoracolumbar sensitivity (i.e., lower values of PPTs) persisted 24h and 48h post-baseline (session effect: \u003cem\u003eF\u0026thinsp;=\u0026thinsp;6.355, p\u0026thinsp;=\u0026thinsp;0.004\u003c/em\u003e). No significant differences in thoracolumbar sensitivity were observed between measurements taken immediately after the eccentric exercise protocol, and those taken at 24 hours and 48 hours later (\u003cem\u003eF\u0026thinsp;=\u0026thinsp;0.744\u003c/em\u003e, \u003cem\u003ep\u0026thinsp;=\u0026thinsp;0.482\u003c/em\u003e). Mean\u0026thinsp;\u0026plusmn;\u0026thinsp;SD PPT values are reported in Table\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec21\" class=\"Section2\"\u003e\n\u003ch2\u003e3.3. PPT maps\u003c/h2\u003e\n\u003cp\u003ePPT maps were generated to determine if regional pain sensitivity at 24h and 48h differed significantly from baseline in specific thoracolumbar areas due to the eccentric exercise protocol. There were no differences for both the y- and x-axes compared to baseline (session effect: \u003cem\u003eF\u0026thinsp;=\u0026thinsp;1.054, p\u0026thinsp;=\u0026thinsp;0.358, \u0026eta;p\u003c/em\u003e\u003csup\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sup\u003e\u0026thinsp;\u003cem\u003e=\u0026thinsp;0.053\u003c/em\u003e and \u003cem\u003eF\u0026thinsp;=\u0026thinsp;1.340, p\u0026thinsp;=\u0026thinsp;0.274, \u0026eta;p\u003c/em\u003e\u003csup\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sup\u003e\u0026thinsp;\u003cem\u003e=\u0026thinsp;0.066\u003c/em\u003e respectively). The PPT maps are illustrated in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e[PLEASE INSERT\u003c/strong\u003e FIGURE \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e \u003cstrong\u003eHERE]\u003c/strong\u003e\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec22\" class=\"Section2\"\u003e\n\u003ch2\u003e3.4. Rate of perceived exertion\u003c/h2\u003e\n\u003cp\u003eAt the end of session 1, during the eccentric exercise protocol, the participants were asked to report their rate of perceived exertion on a modified Borg scale (10 point scale), every 5 repetitions. An increase in the rate of perceived exertion was observed, with the average Borg value at the end of the protocol being 7.4/10, suggesting that the task was interpreted as \u0026ldquo;very hard\u0026rdquo; by the individuals (time effect: \u003cem\u003eF\u0026thinsp;=\u0026thinsp;44.082, p\u0026thinsp;\u0026lt;\u0026thinsp;0.001, \u0026eta;p\u003c/em\u003e\u003csup\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sup\u003e\u0026thinsp;\u003cem\u003e=\u0026thinsp;0.699\u003c/em\u003e).\u003c/p\u003e\n\u003cp\u003eThe results presented below are reported per outcome variable for the eccentric and concentric contractions respectively.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec23\" class=\"Section2\"\u003e\n\u003ch2\u003e3.5. Muscle strength\u003c/h2\u003e\n\u003cp\u003eIn session one, following the eccentric exercise protocol, there was a decrease in eccentric torque, equivalent to a 29.2 N\u0026sdot;m or 14.35% decrease (\u003cem\u003et\u0026thinsp;=\u0026thinsp;3.032\u003c/em\u003e, \u003cem\u003ep\u0026thinsp;=\u0026thinsp;0.007\u003c/em\u003e; Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003eA). No decline in eccentric torque was observed 24h and 48h compared to baseline at the beginning of the session (session effect: \u003cem\u003eF\u0026thinsp;=\u0026thinsp;0.939, p\u0026thinsp;=\u0026thinsp;0.379, \u0026eta;p\u003c/em\u003e\u003csup\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sup\u003e\u0026thinsp;\u003cem\u003e=\u0026thinsp;0.047\u003c/em\u003e; Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003eB).\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eNo decline in concentric torque was observed 24h and 48h compared to baseline, suggesting that individuals were able to produce the same amount of torque during all three sessions irrespective of their muscle soreness and increased mean thoracolumbar muscle sensitivity (i.e., reduced PTT values) (session effect: \u003cem\u003eF\u0026thinsp;=\u0026thinsp;1.896, p\u0026thinsp;=\u0026thinsp;0.164, \u0026eta;p\u003c/em\u003e\u003csup\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sup\u003e\u0026thinsp;\u003cem\u003e=\u0026thinsp;0.091\u003c/em\u003e; Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003eA). Mean\u0026thinsp;\u0026plusmn;\u0026thinsp;SD peak torque values for both contractions are reported in Table\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec24\" class=\"Section2\"\u003e\n\u003ch2\u003e3.6. Torque steadiness\u003c/h2\u003e\n\u003cp\u003e\u003cstrong\u003eEccentric contractions.\u003c/strong\u003e A significant lower torque SD was observed 24h and 48h compared to baseline, suggesting that individuals had better torque steadiness in sessions two and three (session effect: \u003cem\u003eF\u0026thinsp;=\u0026thinsp;5.952, p\u0026thinsp;=\u0026thinsp;0.006, \u0026eta;p\u003c/em\u003e\u003csup\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sup\u003e\u0026thinsp;\u003cem\u003e=\u0026thinsp;0.239\u003c/em\u003e; Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003eC; baseline: 1.1\u0026thinsp;\u0026plusmn;\u0026thinsp;0.3%; 24h: 0.9\u0026thinsp;\u0026plusmn;\u0026thinsp;0.2%; 48h: 0.9\u0026thinsp;\u0026plusmn;\u0026thinsp;0.2%). No differences were observed across sessions for torque CoV (session effect: \u003cem\u003eF\u0026thinsp;=\u0026thinsp;1.295, p\u0026thinsp;=\u0026thinsp;0.280, \u0026eta;p\u003c/em\u003e\u003csup\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sup\u003e\u0026thinsp;\u003cem\u003e=\u0026thinsp;0.263\u003c/em\u003e; baseline: 3.2\u0026thinsp;\u0026plusmn;\u0026thinsp;0.9%; 24h: 2.6\u0026thinsp;\u0026plusmn;\u0026thinsp;0.7%; 48h: 3.0\u0026thinsp;\u0026plusmn;\u0026thinsp;1.6%).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConcentric contractions.\u003c/strong\u003e The torque steadiness performance of the participants was better during session 3 compared to baseline as shown from both the torque CoV and SD variables (session effect: \u003cem\u003eF\u0026thinsp;=\u0026thinsp;4.327, p\u0026thinsp;=\u0026thinsp;0.020, \u0026eta;p\u003c/em\u003e\u003csup\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sup\u003e\u0026thinsp;\u003cem\u003e=\u0026thinsp;0.185\u003c/em\u003e; baseline: 3.5\u0026thinsp;\u0026plusmn;\u0026thinsp;0.9%; 24h: 3.2\u0026thinsp;\u0026plusmn;\u0026thinsp;0.9%; 48h: 3.0\u0026thinsp;\u0026plusmn;\u0026thinsp;0.5% and \u003cem\u003eF\u0026thinsp;=\u0026thinsp;5.452, p\u0026thinsp;=\u0026thinsp;0.008, \u0026eta;p\u003c/em\u003e\u003csup\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sup\u003e\u0026thinsp;\u003cem\u003e=\u0026thinsp;0.223\u003c/em\u003e; baseline: 1.3\u0026thinsp;\u0026plusmn;\u0026thinsp;0.3%; 24h: 1.1\u0026thinsp;\u0026plusmn;\u0026thinsp;0.3%; 48h: 1.1\u0026thinsp;\u0026plusmn;\u0026thinsp;0.2%, respectively; Figs.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003eB, \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003eC).\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec25\" class=\"Section2\"\u003e\n\u003ch2\u003e3.7. Electromyographic activity\u003c/h2\u003e\n\u003cp\u003e\u003cstrong\u003eEccentric contractions.\u003c/strong\u003e No differences in the level of activation of the thoracolumbar ES across the three sessions (session effect: \u003cem\u003eF\u0026thinsp;=\u0026thinsp;1.250, p\u0026thinsp;=\u0026thinsp;0.289, \u0026eta;p\u003c/em\u003e\u003csup\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sup\u003e\u0026thinsp;\u003cem\u003e=\u0026thinsp;0.062\u003c/em\u003e; baseline: 41.8\u0026thinsp;\u0026plusmn;\u0026thinsp;21.5 \u0026micro;V; 24h: 40.6\u0026thinsp;\u0026plusmn;\u0026thinsp;21.7 \u0026micro;V; 48h: 38.6\u0026thinsp;\u0026plusmn;\u0026thinsp;19.7 \u0026micro;V). The regional activation of the thoracolumbar ES along the y- and x-axes was similar during all sessions (session effect: \u003cem\u003eF\u0026thinsp;=\u0026thinsp;0.479, p\u0026thinsp;=\u0026thinsp;0.539, \u0026eta;p\u003c/em\u003e\u003csup\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sup\u003e\u0026thinsp;\u003cem\u003e=\u0026thinsp;0.025\u003c/em\u003e; baseline: 93.7\u0026thinsp;\u0026plusmn;\u0026thinsp;11.1 mm; 24h: 95.9\u0026thinsp;\u0026plusmn;\u0026thinsp;5.5 mm; 48h: 95.6\u0026thinsp;\u0026plusmn;\u0026thinsp;6.8 mm and \u003cem\u003eF\u0026thinsp;=\u0026thinsp;1.945, p\u0026thinsp;=\u0026thinsp;0.157, \u0026eta;p\u003c/em\u003e\u003csup\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sup\u003e\u0026thinsp;\u003cem\u003e=\u0026thinsp;0.093\u003c/em\u003e; baseline: 16.2\u0026thinsp;\u0026plusmn;\u0026thinsp;1.2 mm; 24h: 16.2\u0026thinsp;\u0026plusmn;\u0026thinsp;0.9 mm; 48h: 15.8\u0026thinsp;\u0026plusmn;\u0026thinsp;1.0 mm). The level of co-activation between the RA, EO and ES was similar across days (session effect: \u003cem\u003eF\u0026thinsp;=\u0026thinsp;0.649, p\u0026thinsp;=\u0026thinsp;0.491, \u0026eta;p\u003c/em\u003e\u003csup\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sup\u003e\u0026thinsp;\u003cem\u003e=\u0026thinsp;0.033\u003c/em\u003e; baseline: 151.7\u0026thinsp;\u0026plusmn;\u0026thinsp;123.2%; 24h: 153.3\u0026thinsp;\u0026plusmn;\u0026thinsp;117.2%; 48h: 142.3\u0026thinsp;\u0026plusmn;\u0026thinsp;101.0%).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConcentric contractions.\u003c/strong\u003e No differences in the level of thoracolumbar ES activity were observed across days (\u003cem\u003eF\u0026thinsp;=\u0026thinsp;0.021, p\u0026thinsp;=\u0026thinsp;0.923, \u0026eta;p\u003c/em\u003e\u003csup\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sup\u003e\u0026thinsp;\u003cem\u003e=\u0026thinsp;0.001\u003c/em\u003e; baseline: 44.7\u0026thinsp;\u0026plusmn;\u0026thinsp;25.1 \u0026micro;V; 24h: 44.2\u0026thinsp;\u0026plusmn;\u0026thinsp;21.8 \u0026micro;V; 48h: 44.2\u0026thinsp;\u0026plusmn;\u0026thinsp;21.2 \u0026micro;V). The regional activation of the thoracolumbar ES along the y- and x-axes was similar for all three days (\u003cem\u003eF\u0026thinsp;=\u0026thinsp;0.281, p\u0026thinsp;=\u0026thinsp;0.656, \u0026eta;p\u003c/em\u003e\u003csup\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sup\u003e\u0026thinsp;\u003cem\u003e=\u0026thinsp;0.015\u003c/em\u003e; baseline: 96.4\u0026thinsp;\u0026plusmn;\u0026thinsp;9.5 mm; 24h: 97.1\u0026thinsp;\u0026plusmn;\u0026thinsp;8.1 mm; 48h: 95.2\u0026thinsp;\u0026plusmn;\u0026thinsp;9.1 mm and \u003cem\u003eF\u0026thinsp;=\u0026thinsp;1.120, p\u0026thinsp;=\u0026thinsp;0.337, \u0026eta;p\u003c/em\u003e\u003csup\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sup\u003e\u0026thinsp;\u003cem\u003e=\u0026thinsp;0.056\u003c/em\u003e; baseline: 16.3\u0026thinsp;\u0026plusmn;\u0026thinsp;1.0 mm; 24h: 16.1\u0026thinsp;\u0026plusmn;\u0026thinsp;0.8 mm; 48h: 15.9\u0026thinsp;\u0026plusmn;\u0026thinsp;0.8 mm). The level of co-activation between the abdominal (RA, EO) and lumbar ES was similar during all three sessions (\u003cem\u003eF\u0026thinsp;=\u0026thinsp;1.474, p\u0026thinsp;=\u0026thinsp;0.243, \u0026eta;p\u003c/em\u003e\u003csup\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sup\u003e\u0026thinsp;\u003cem\u003e=\u0026thinsp;0.076\u003c/em\u003e; baseline: 153.3\u0026thinsp;\u0026plusmn;\u0026thinsp;99.2%; 24h: 136.8\u0026thinsp;\u0026plusmn;\u0026thinsp;93.5%; 48h: 132.0\u0026thinsp;\u0026plusmn;\u0026thinsp;114.9%).\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec26\" class=\"Section2\"\u003e\n\u003ch2\u003e3.8. EMG-torque relationships\u003c/h2\u003e\n\u003cp\u003e\u003cstrong\u003eEccentric contractions.\u003c/strong\u003e A session effect was observed for the magnitude of \u0026delta; band coherence, showing that \u0026delta; band coherence at 48h was lower compared to baseline (session 1) and 24h (session 2) (\u003cem\u003eF\u0026thinsp;=\u0026thinsp;6.685, p\u0026thinsp;=\u0026thinsp;0.003, \u0026eta;p\u003c/em\u003e\u003csup\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sup\u003e\u0026thinsp;\u003cem\u003e=\u0026thinsp;0.260\u003c/em\u003e; Z-coherence in the \u0026delta; band, baseline: 0.8\u0026thinsp;\u0026plusmn;\u0026thinsp;0.2; 24h: 0.7\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1; 48h: 0.7\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1; Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003eD). In terms of differences in the topographical representation of \u0026delta; band coherence along the y- and x-axes, no differences were observed (session effect: \u003cem\u003eF\u0026thinsp;=\u0026thinsp;1.306, p\u0026thinsp;=\u0026thinsp;0.276, \u0026eta;p\u003c/em\u003e\u003csup\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sup\u003e\u0026thinsp;\u003cem\u003e=\u0026thinsp;0.064\u003c/em\u003e; baseline: 94.2\u0026thinsp;\u0026plusmn;\u0026thinsp;8.1 mm; 24h: 96.6\u0026thinsp;\u0026plusmn;\u0026thinsp;2.3 mm; 48h: 96.1\u0026thinsp;\u0026plusmn;\u0026thinsp;3.0 mm \u003cem\u003eand F\u0026thinsp;=\u0026thinsp;0.958, p\u0026thinsp;=\u0026thinsp;0.393, \u0026eta;p\u003c/em\u003e\u003csup\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sup\u003e\u0026thinsp;\u003cem\u003e=\u0026thinsp;0.048\u003c/em\u003e; baseline: 16.7\u0026thinsp;\u0026plusmn;\u0026thinsp;0.5 mm; 24h: 16.6\u0026thinsp;\u0026plusmn;\u0026thinsp;0.4 mm; 48h: 16.5\u0026thinsp;\u0026plusmn;\u0026thinsp;0.3 mm).\u003c/p\u003e\n\u003cp\u003eSimilarly, a session effect was observed for the magnitude of HDsEMG-torque cross-correlation, showing a reduction in the contribution of the thoracolumbar ES at session 3, compared to the other two sessions (\u003cem\u003eF\u0026thinsp;=\u0026thinsp;7.416, p\u0026thinsp;=\u0026thinsp;0.002, \u0026eta;p\u003c/em\u003e\u003csup\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sup\u003e\u0026thinsp;\u003cem\u003e=\u0026thinsp;0.281\u003c/em\u003e; cross-correlation coefficient, baseline: 0.4\u0026thinsp;\u0026plusmn;\u0026thinsp;0.0; 24h: 0.4\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1; 48h: 0.37\u0026thinsp;\u0026plusmn;\u0026thinsp;0.0; Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003eE). No differences in the topographical representation of HDsEMG-torque cross-correlation along the y- and x-axes, were observed (session effect: \u003cem\u003eF\u0026thinsp;=\u0026thinsp;0.096, p\u0026thinsp;=\u0026thinsp;0.847, \u0026eta;p\u003c/em\u003e\u003csup\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sup\u003e\u0026thinsp;\u003cem\u003e=\u0026thinsp;0.005\u003c/em\u003e; baseline: 94.3\u0026thinsp;\u0026plusmn;\u0026thinsp;7.9 mm; 24h: 94.8\u0026thinsp;\u0026plusmn;\u0026thinsp;3.2 mm; 48h: 95.0\u0026thinsp;\u0026plusmn;\u0026thinsp;4.2 mm \u003cem\u003eand F\u0026thinsp;=\u0026thinsp;1.541, p\u0026thinsp;=\u0026thinsp;0.227, \u0026eta;p\u003c/em\u003e\u003csup\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sup\u003e\u0026thinsp;\u003cem\u003e=\u0026thinsp;0.075\u003c/em\u003e; baseline: 16.7\u0026thinsp;\u0026plusmn;\u0026thinsp;0.5 mm; 24h: 16.6\u0026thinsp;\u0026plusmn;\u0026thinsp;0.4 mm; 48h: 16.5\u0026thinsp;\u0026plusmn;\u0026thinsp;0.3 mm).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConcentric contractions.\u003c/strong\u003e No differences were observed for the magnitude of \u0026delta; band coherence, across days (\u003cem\u003eF\u0026thinsp;=\u0026thinsp;0.373, p\u0026thinsp;=\u0026thinsp;0.691, \u0026eta;p\u003c/em\u003e\u003csup\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sup\u003e\u0026thinsp;\u003cem\u003e=\u0026thinsp;0.019\u003c/em\u003e; Z-coherence in the \u0026delta; band, baseline: 0.8\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1; 24h: 0.8\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1; 48h: 0.8\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1). Similarly, no differences in the topographical representation of \u0026delta; band coherence along the y- and x-axes, were observed (session effect: \u003cem\u003eF\u0026thinsp;=\u0026thinsp;1.216, p\u0026thinsp;=\u0026thinsp;0.297, \u0026eta;p\u003c/em\u003e\u003csup\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sup\u003e\u0026thinsp;\u003cem\u003e=\u0026thinsp;0.060\u003c/em\u003e; baseline: 94.7\u0026thinsp;\u0026plusmn;\u0026thinsp;10.4 mm; 24h: 98.3\u0026thinsp;\u0026plusmn;\u0026thinsp;5.7 mm; 48h: 96.8\u0026thinsp;\u0026plusmn;\u0026thinsp;3.2 mm and \u003cem\u003eF\u0026thinsp;=\u0026thinsp;0.270, p\u0026thinsp;=\u0026thinsp;0.765, \u0026eta;p\u003c/em\u003e\u003csup\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sup\u003e\u0026thinsp;\u003cem\u003e=\u0026thinsp;0.014\u003c/em\u003e; baseline: 16.6\u0026thinsp;\u0026plusmn;\u0026thinsp;0.4 mm; 24h: 16.6\u0026thinsp;\u0026plusmn;\u0026thinsp;0.3 mm; 48h: 16.5\u0026thinsp;\u0026plusmn;\u0026thinsp;0.3 mm).\u003c/p\u003e\n\u003cp\u003eThe magnitude of HDsEMG-torque cross-correlation was similar across sessions (\u003cem\u003eF\u0026thinsp;=\u0026thinsp;0.343, p\u0026thinsp;=\u0026thinsp;0.712, \u0026eta;p\u003c/em\u003e\u003csup\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sup\u003e\u0026thinsp;\u003cem\u003e=\u0026thinsp;0.018\u003c/em\u003e; cross-correlation coefficient, baseline: 0.4\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1; 24h: 0.4\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1; 48h: 0.4\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1). No differences in the topographical representation of HDsEMG-torque cross-correlation along the y- and x-axes, were observed (session effect: \u003cem\u003eF\u0026thinsp;=\u0026thinsp;1.461, p\u0026thinsp;=\u0026thinsp;0.245, \u0026eta;p\u003c/em\u003e\u003csup\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sup\u003e\u0026thinsp;\u003cem\u003e=\u0026thinsp;0.071\u003c/em\u003e; baseline: 93.7\u0026thinsp;\u0026plusmn;\u0026thinsp;10.1 mm; 24h: 97.1\u0026thinsp;\u0026plusmn;\u0026thinsp;4.8 mm; 48h: 96.5\u0026thinsp;\u0026plusmn;\u0026thinsp;3.1 mm \u003cem\u003eand F\u0026thinsp;=\u0026thinsp;0.742, p\u0026thinsp;=\u0026thinsp;0.483, \u0026eta;p\u003c/em\u003e\u003csup\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sup\u003e\u0026thinsp;\u003cem\u003e=\u0026thinsp;0.038\u003c/em\u003e; baseline: 16.6\u0026thinsp;\u0026plusmn;\u0026thinsp;0.4 mm; 24h: 16.6\u0026thinsp;\u0026plusmn;\u0026thinsp;0.3 mm; 48h: 16.5\u0026thinsp;\u0026plusmn;\u0026thinsp;0.4 mm).\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec27\" class=\"Section2\"\u003e\n\u003ch2\u003e3.9. Thoracolumbar kinematics\u003c/h2\u003e\n\u003cp\u003e\u003cstrong\u003eEccentric contractions.\u003c/strong\u003e A session \u0026times; torque interaction was observed for the AUC variable in the sagittal plane, suggesting that in sessions two and three, as the task was more demanding (i.e., higher %MVC), individuals increased their trunk flexion ROM (\u003cem\u003eF\u0026thinsp;=\u0026thinsp;6.479, p\u0026thinsp;=\u0026thinsp;0.004, \u0026eta;p\u003c/em\u003e\u003csup\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sup\u003e\u0026thinsp;\u003cem\u003e=\u0026thinsp;0.254\u003c/em\u003e; 25%MVC-24h: 50.6\u0026thinsp;\u0026plusmn;\u0026thinsp;28.3 \u003csup\u003eo\u003c/sup\u003e\u0026sdot; epoch, 50%MVC-24h: 69.8\u0026thinsp;\u0026plusmn;\u0026thinsp;32.2 \u003csup\u003eo\u003c/sup\u003e\u0026sdot; epoch; 25%MVC-48h: 46.7\u0026thinsp;\u0026plusmn;\u0026thinsp;18.8 \u003csup\u003eo\u003c/sup\u003e\u0026sdot; epoch, 50%MVC-48h: 65.1\u0026thinsp;\u0026plusmn;\u0026thinsp;25.9 \u003csup\u003eo\u003c/sup\u003e\u0026sdot; epoch ;Figure \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003eF). Additionally, this interaction showed that at high forces there was a significant difference between sessions one and two for this variable (50%MVC-baseline: 54.0\u0026thinsp;\u0026plusmn;\u0026thinsp;22.9 \u003csup\u003eo\u003c/sup\u003e\u0026sdot; epoch, 50%MVC-24h: 69.8\u0026thinsp;\u0026plusmn;\u0026thinsp;32.2 \u003csup\u003eo\u003c/sup\u003e\u0026sdot; epoch). No significant differences were observed for this kinematic variable nor for the frontal or transverse planes (\u003cem\u003ep\u0026thinsp;\u0026gt;\u0026thinsp;0.05\u003c/em\u003e).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConcentric contractions.\u003c/strong\u003e In Session 3, there was a reduction in the AUC in the sagittal plane compared to the baseline, suggesting decreased thoracolumbar ROM (session effect: \u003cem\u003eF\u0026thinsp;=\u0026thinsp;5.723, p\u0026thinsp;=\u0026thinsp;0.007, \u0026eta;p\u003c/em\u003e\u003csup\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sup\u003e\u0026thinsp;\u003cem\u003e=\u0026thinsp;0.23\u003c/em\u003e; baseline: 48.7\u0026thinsp;\u0026plusmn;\u0026thinsp;20.0 \u003csup\u003eo\u003c/sup\u003e\u0026sdot; epoch; 24h: 36.4\u0026thinsp;\u0026plusmn;\u0026thinsp;18.7 \u003csup\u003eo\u003c/sup\u003e\u0026sdot; epoch; 48h: 33.4\u0026thinsp;\u0026plusmn;\u0026thinsp;16.7 \u003csup\u003eo\u003c/sup\u003e\u0026sdot; epoch; Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003eD). This might indicate that participants were aiming to maintain a more neutral or stable lumbar spine position. No significant differences were observed for movement in the frontal or transverse planes (\u003cem\u003ep\u0026thinsp;\u0026gt;\u0026thinsp;0.05\u003c/em\u003e).\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003c/div\u003e"},{"header":"4. DISCUSSION","content":"\u003cp\u003eThis study examined the influence of DOMS on torque steadiness, HDsEMG-torque relationships from the thoracolumbar ES, and kinematic data from the thoracolumbar spine in asymptomatic individuals during submaximal concentric and eccentric trunk extension contractions at 25% and 50%MVC. No significant changes were observed for eccentric and concentric trunk extension muscle strength across sessions. The participants demonstrated improved torque steadiness during submaximal concentric and eccentric trunk extension contractions in the presence of DOMS. However, HDsEMG-torque relationships and kinematics were altered in a contraction-dependent manner. During eccentric contractions, a decrease in the thoracolumbar ES contribution to the resultant torque was observed, and individuals showed increased lumbar flexion during the more demanding contractions (50%MVC). During concentric contractions, a reduction in thoracolumbar ROM in the sagittal plane was observed, suggesting the maintenance of a more neutral lumbar spine posture, but no alterations in HDsEMG-torque relationships were observed in the presence of DOMS.\u003c/p\u003e \u003cdiv id=\"Sec29\" class=\"Section2\"\u003e \u003ch2\u003e4.1. Muscle soreness and sensitivity\u003c/h2\u003e \u003cp\u003eThe increased soreness and mean thoracolumbar pressure pain sensitivity observed after 24h and 48h support the occurrence of DOMS following the eccentric exercise protocol. This aligns with the observations of previous studies \u003csup\u003e10,11,32\u003c/sup\u003e. Notably, the peak muscle soreness observed in this study immediately after the eccentric exercise protocol, averaging 4.1\u0026thinsp;\u0026plusmn;\u0026thinsp;2.2 on the VAS, mirrors the intensity (VAS: 3.80\u0026thinsp;\u0026plusmn;\u0026thinsp;2.35) reported in another study \u003csup\u003e2\u003c/sup\u003e. Furthermore, the mild soreness levels we observed at 24h and 48h (VAS scores of 2.6\u0026thinsp;\u0026plusmn;\u0026thinsp;2.1 and 2.9\u0026thinsp;\u0026plusmn;\u0026thinsp;1.8, respectively) fall within the range of peak soreness levels (2 to 2.9/10 on the VAS) documented in similar timeframes by other researchers \u003csup\u003e10\u003c/sup\u003e. The observed decrease in mean PPT values aligns with findings from Abboud et al., 2019 \u003csup\u003e32\u003c/sup\u003e, who also reported reduced PPT values over the L2-L5 region with the presence of DOMS, suggesting a link to peripheral sensitization driven by inflammatory processes or tissue damage associated with DOMS.\u003c/p\u003e \u003cp\u003eThe mechanisms behind DOMS are multifaceted and not fully understood \u003csup\u003e33\u003c/sup\u003e. Traditionally, eccentric exercise-induced microstructural muscle damage was believed to trigger inflammation followed by biochemical, thermal, and mechanical changes, sensitizing muscle afferents and causing soreness and mechanical hyperalgesia\u003csup\u003e33\u003c/sup\u003e. However, recent studies highlight bradykinin, nerve growth factor (NGF), and COX-2-glial cell line-derived neurotrophic factor (GDNF) as key contributors, suggesting that myofiber micro-damage may not be necessary for initiating inflammation or DOMS \u003csup\u003e33\u0026ndash;35\u003c/sup\u003e. These molecules could stimulate muscle nociceptors or extracellular receptor binding, indicating their role in mechanical hyperalgesia and inflammation in the extracellular matrix, even without apparent muscle damage \u003csup\u003e36\u003c/sup\u003e. Interestingly, an ultrasound study also suggested an involvement of the paraspinal extramuscular connective tissue (ECT) of trunk extensors in the genesis of DOMS \u003csup\u003e37\u003c/sup\u003e. However, it is important to note that research on these mechanisms has predominantly focused on limb muscles, leaving other areas, such as the trunk, less explored.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec30\" class=\"Section2\"\u003e \u003ch2\u003e4.2. Trunk muscle strength\u003c/h2\u003e \u003cp\u003eDOMS typically induces a temporary decrease in muscle force. As expected, following the eccentric exercise protocol, we observed an immediate reduction in eccentric extension trunk strength. However, contrary to some prior studies \u003csup\u003e6,10,25,32\u003c/sup\u003e, this decrease in trunk extension MVC was not observed at 24h and 48h post-exercise. Reductions in muscle force following eccentric exercise are well-documented for upper and lower limb muscles \u003csup\u003e6\u003c/sup\u003e, however, this trend does not appear to apply as consistently to trunk muscles, particularly in the context of dynamic trunk MVCs. This could be because trunk extension is a multi-joint movement involving the lumbar spine, pelvis, and hips, where potential compensation by lower limb muscles, such as the gluteus maximus and hamstrings, can play a significant role. These muscles can influence both the hip and pelvis, thereby minimizing the engagement of the thoracolumbar extensors. Despite stabilization efforts, such as using straps to secure the thighs and pelvis, it is challenging to prevent these compensatory strategies completely. This suggests that participants might have executed the MVCs by compensating with their hip extensor muscles, particularly if DOMS influenced the use of the thoracolumbar extensors. Additionally, contrasting findings may result from protocol differences: (i) previous studies focused on isometric strength, whereas we assessed eccentric/concentric strength, which could increase the likelihood of compensatory torque exertion as mentioned above, (ii) variations in exercise protocol speed (to induce DOMS), load and the use of an isokinetic dynamometer, considering that fast velocity eccentric exercise causes more muscle fibre damage than slow contractions \u003csup\u003e5\u003c/sup\u003e and because muscle loading level impacts damage extent and recovery rates \u003csup\u003e8\u003c/sup\u003e. Lastly, as mentioned above, individuals might exhibit DOMS without damaging contractile structures, suggesting the involvement of neurotrophic factors and/or connective tissue damage in the increased soreness experienced after exercise \u003csup\u003e34,35\u003c/sup\u003e. These factors likely explain how individuals\u0026rsquo; trunk strength was maintained 24h and 48h post-exercise.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec31\" class=\"Section2\"\u003e \u003ch2\u003e4.3. Torque steadiness\u003c/h2\u003e \u003cp\u003eConsidering that DOMS can impair motor function and that torque steadiness is commonly reduced in individuals with CLBP \u003csup\u003e21,22\u003c/sup\u003e, we anticipated a reduction in both eccentric and concentric trunk extension torque steadiness due to DOMS. However, contrary to our hypothesis, torque steadiness improved in both concentric and eccentric contractions 48h after the eccentric exercise protocol that was used to induce DOMS.\u003c/p\u003e \u003cp\u003eAccording to previous studies there is conflicting evidence on how DOMS influences the control of muscle force. Some studies \u003csup\u003e2,38,39\u003c/sup\u003e have observed worse motor control in the presence of DOMS, while others have observed no changes \u003csup\u003e40\u003c/sup\u003e. However, no previous study has assessed trunk extensor torque steadiness in the presence of DOMS. To date, only one study has investigated this aspect using an alternative model of acute experimental pain. Specifically, this study found no significant changes in trunk extensor force variability following intramuscular injection of hypertonic saline into the longissimus muscle \u003csup\u003e41\u003c/sup\u003e. In contrast, investigations into lower/upper limb muscles \u003csup\u003e7\u003c/sup\u003e, observed significant increases in force fluctuations, but only immediately and 1h after eccentric exercise, with no differences observed at 24h and 48h. Similarly, a study investigating the knee extensors \u003csup\u003e39\u003c/sup\u003e observed that force steadiness deficits were more pronounced immediately post-exercise and primarily at lower %MVC levels (2.5, 5 and 10%), with less impact at 20 or 30% MVC, and no data available for periods beyond 24h or for higher loads.\u003c/p\u003e \u003cp\u003eCollectively, the findings from these studies support our observation that DOMS does not necessarily lead to reduced force steadiness. Additionally, it is important to consider that immediate reductions in force steadiness, observed by some studies \u003csup\u003e7,39\u003c/sup\u003e, are mainly attributed to acute muscular fatigue following eccentric exercise rather than DOMS which occurs later. The lack of reductions in force steadiness in the presence of trunk extensor DOMS can be attributed to some factors, including: (i) compensatory mechanisms inherent in multi-joint movements, such as trunk extension, which allow for significant compensatory strategies when experiencing DOMS. The activation of the hamstrings and gluteals to compensate for decreased functionality in the sore thoracolumbar ES and (ii) a learning effect, possibly linked to the improved force steadiness after 48h. This effect, likely arising from frequent task repetition with visual feedback during the eccentric exercise protocol, demonstrates that despite the presence of DOMS or acute pain, individuals can enhance their performance, suggesting that pain does not necessarily hinder the learning and adaptation process. This notion is further supported by a previous study \u003csup\u003e42\u003c/sup\u003e using an experimental pain model, where individuals with acute shoulder pain showed an improvement level in movement accuracy during fast arm-reaching movements and force field perturbations comparable to that of pain-free controls. Additionally, the use of HDsEMG and kinematic data in the current study helped explore these improvements further.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec32\" class=\"Section2\"\u003e \u003ch2\u003e4.4. HDsEMG amplitude and HDsEMG regional activation\u003c/h2\u003e \u003cp\u003eDuring both eccentric and concentric contractions, we observed no changes in the magnitude of thoracolumbar ES activation, regional activity or level of co-activation in the presence of DOMS. These findings align with similar previous research on the trunk extensors \u003csup\u003e10,25\u003c/sup\u003e and medial gastrocnemius \u003csup\u003e43\u003c/sup\u003e, further supporting the theory related to the involvement of muscle-associated connective tissue \u003csup\u003e33,44\u003c/sup\u003e instead of changes in the muscle itself. A review \u003csup\u003e3\u003c/sup\u003e has previously aimed to examine the course of EMG changes and alterations at the motor unit level. However, the most common muscle assessed was the biceps brachii, and most differences were reported immediately after an eccentric exercise protocol, 2h and 24h. Importantly, it highlighted an inconsistency of change in agonistic and antagonistic EMG amplitude at 24h, and no data were presented at 48h. Interestingly, many changes can happen at the motor unit level. However, capturing these changes using HDsEMG in the thoracolumbar ES poses significant challenges. By examining the HDsEMG-torque relationship, we gained additional insights into why individuals show improved torque steadiness in the presence of DOMS. Nevertheless, it is important to note that DOMS is not always indicative of muscle damage, which might explain some of the inconsistency in the findings and the absence of differences in EMG variables when DOMS is present. It is important to mention that most of these changes at a motor unit level were reported immediately after eccentric exercise, thereby not excluding, or underestimating the confounding factor of fatigue in these observations.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec33\" class=\"Section2\"\u003e \u003ch2\u003e4.5. HDsEMG-torque relationships\u003c/h2\u003e \u003cp\u003eThe magnitude of HDsEMG cross-correlation and δ band coherence at 48h post-exercise was reduced during eccentric trunk extensions when compared to the previous two sessions. However, no regional changes in δ band coherence maps were observed during either eccentric or concentric contractions at 24h and 48h post-exercise. This uniformity in δ band coherence, reinforced by the PPT maps, which revealed no regional sensitivity differences in the thoracolumbar area, suggests that the changes were primarily in magnitude and indicates a highly localised nature of DOMS. Another noteworthy observation was the absence of differences in the magnitude of these variables during concentric trunk extension contractions across days. This is not surprising, considering the inherent differences between eccentric and concentric contractions. Eccentric contractions involve greater passive muscle element contribution, distinct neural control, higher force production with lower metabolic energy consumption, and increased mechanical stress, resulting in more microtrauma \u003csup\u003e34,45,46\u003c/sup\u003e. Consequently, due to the increased stress on connective tissue and muscle fibres during DOMS, we can anticipate more pronounced changes in these patterns, aiding in task adaptation and performance enhancement. Furthermore, during eccentric contractions, individuals are likely to leverage this additional component effectively for torque production. These insights could be further elucidated by examining thoracolumbar kinematics, as detailed in the following section.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec34\" class=\"Section2\"\u003e \u003ch2\u003e4.6. Thoracolumbar kinematics\u003c/h2\u003e \u003cp\u003eDistinct differences were observed in the sagittal plane during both eccentric and concentric contractions. Notably, in the presence of DOMS, individuals demonstrated increased lumbar flexion during eccentric contractions at both 24h and 48h, particularly at higher loads (i.e., 50%MVC). This pattern was not observed at baseline, suggesting that the individuals had to alter their movement pattern to be able to perform the more demanding trunk extension eccentric contractions. In contrast, during the concentric contractions, different adaptations were observed. At 48h, individuals used a more neutral or stable lumbar spine position during the concentric contractions compared to baseline. The lack of notable differences in other planes of movement could be attributed to the individual's position on the chair, which was stabilised by straps, and the attachment system that limits movements to the frontal and transverse planes.\u003c/p\u003e \u003cp\u003eEven though distinct, the adaptations observed in movement patterns during both eccentric and concentric contractions could likely serve two functions. Firstly, these adaptations might be a self-protective mechanism for the tissues involved, potentially reducing the risk of further damage. Secondly, they may be related to a learning effect, particularly considering the exercise protocol's similarity with the torque steadiness tasks, enabling more efficient and effective task performance. For example, in the case of eccentric extension, an increased lumbar flexion can be explained in two ways, (i) it might be related to a more efficient utilisation of the passive muscle elements or of the extramuscular connective tissue and/or (ii) it could represent a protective strategy to reduce strain on these tissues by engaging compensatory muscle groups such as the hamstrings and gluteals. This explanation is supported by recent findings showing that eccentric exercise can lead to increases in lumbar extramuscular connective tissue thickness \u003csup\u003e37\u003c/sup\u003e and immediate changes in the optimum length for force generation in the hamstring muscles \u003csup\u003e4\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eConversely, the controlled back extension observed during concentric movements could contribute to minimising unnecessary thoracolumbar motions. This controlled motion might optimise the length-tension relationship in the trunk extensor muscles, avoiding overly stretched or contracted positions. Consequently, this approach could reduce the risk of further damage to the connective or muscle tissue and likely enhance force steadiness by maintaining muscle efficiency and stability.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec35\" class=\"Section2\"\u003e \u003ch2\u003e4.7. Methodological considerations\u003c/h2\u003e \u003cp\u003eA potential limitation of the study relates to the generalizability of the results, considering the sample primarily consisted of young and highly active individuals. Such a group may exhibit a faster recovery from DOMS compared to older individuals or those with lower levels of physical activity. Despite this, the presence of mechanical hyperalgesia and mild muscle soreness did confirm the presence of DOMS in our sample. Moreover, while the positioning of individuals could have allowed for compensation using hip and gluteal muscles during the task, we aimed to minimise such compensatory movements in our protocol. This was achieved by restricting movement with straps over the pelvis and thighs and by instructing participants to rely mainly on their trunk extensor muscles as they performed the task. Another limitation of the study is the use of rectified sEMG to estimate neural drive to muscles. While sEMG signals can be influenced by various factors, the rationale behind choosing this method has been previously detailed \u003csup\u003e22\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eAnother important consideration is the specific load, speed, and measurement time points chosen for this study. Although changes were observed and DOMS was induced by the eccentric exercise protocol, employing an eccentric protocol with maximal effort and faster contractions, along with incorporating immediate and 2-hour post-task measurements, might have revealed additional insights, particularly in terms of HDsEMG measures related to thoracolumbar ES activity, as indicated in previous research for limb muscles \u003csup\u003e3\u003c/sup\u003e. Lastly, it would be beneficial to confirm our speculation regarding the contribution of lower limb musculature to the resultant torque and gain insights into the behaviour of synergistic muscles. The positioning of the electrodes, coupled with the seated posture of the participants, restricted our ability to place additional electrodes on these lower limb muscles.\u003c/p\u003e \u003c/div\u003e"},{"header":"5. CONCLUSION","content":"\u003cp\u003eThis study uniquely demonstrates that in the presence of DOMS, individuals exhibit improved torque steadiness during trunk concentric and eccentric extension contractions, likely due to adaptations of movement and muscle recruitment strategies, influenced by a learning effect from initial training exposure. This also shows that contrary to individuals with CLBP who may lack the motor resources to compensate for motor control impairments, resulting in reduced torque steadiness, pain-free individuals can make adjustments that lead to improvements in torque steadiness. Importantly, the study also reveals that movement patterns differ significantly between contractions in the presence of DOMS. Increased lumbar flexion was observed in the more challenging eccentric contractions, whereas in concentric contractions, there was a noticeable reduction in thoracolumbar (sagittal) movement. This variation in behaviour could suggest a strategy of protecting the involved tissues and learned efficiency to optimise torque steadiness performance. Collectively, the findings of this work underscore the significance of adaptive strategies in response to DOMS and their influence on muscular performance.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eACKNOWLEDGEMENTS\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWe extend our gratitude to our colleague Ignacio Contreras-Hernandez for his assistance in data collection for this research. Our appreciation also goes to every participant involved in the study. This research was undertaken without the support of any external funding.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAUTHOR CONTRIBUTIONS\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eMA, DF, and E.M.-V. conceived and designed research; M.A. and N.H.-J. performed experiments; M.A., D.J.-G., and E.M.-V. analysed data; MA, DF, and E.M.-V. interpreted results of experiments; M.A. prepared figures: M.A. drafted manuscript; M.A., D.J.-G., N.H.-J., D.F., and E.M.-V. edited and revised manuscript; M.A., D.J.-G., N.H.-J., D.F., and E.M.-V. approved final version of manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFUNDING INFORMATION\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis study was conducted with no external funding sources.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCONFLICT OF INTEREST STATEMENT\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors confirm that there are no conflicts of interest regarding the publication of this paper, financial or otherwise.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eDATA AVAILABILITY STATEMENT\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eResearch data can be shared upon reasonable request.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eORCID\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eMichail Arvanitidis: https://orcid.org/0000-0002-3339-6668\u003c/p\u003e\n\u003cp\u003eDavid Jim\u0026eacute;nez-Grande: https://orcid.org/0000-0001-5454-7667\u003c/p\u003e\n\u003cp\u003eNad\u0026egrave;ge Haouidji-Javaux : https://orcid.org/0000-0002-3167-5292\u003c/p\u003e\n\u003cp\u003eDeborah Falla: https://orcid.org/0000-0003-1689-6190\u003c/p\u003e\n\u003cp\u003eEduardo Martinez-Valdes: https://orcid.org/0000-0002-5790-7514\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eHotfiel, T. \u003cem\u003eet al.\u003c/em\u003e Advances in Delayed-Onset Muscle Soreness (DOMS): Part I: Pathogenesis and Diagnostics. 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Physiology 31, 300\u0026ndash;312, doi:\u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1152/physiol.00049.2014\u003c/span\u003e\u003cspan address=\"10.1152/physiol.00049.2014\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2016).\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"delayed onset of muscle soreness, exercise-induced muscle damage, eccentric, concentric, high-density surface EMG, torque steadiness","lastPublishedDoi":"10.21203/rs.3.rs-4426332/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4426332/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eWe aimed to assess high-density surface electromyography (HDsEMG)-torque relationships in the presence of delayed onset trunk muscle soreness (DOMS) and the effect of these relationships on torque steadiness (TS) and lumbar movement during concentric/eccentric submaximal trunk extension contractions. Twenty healthy individuals attended three laboratory sessions (24 hours apart). HDsEMG signals were recorded unilaterally from the thoracolumbar erector spinae with two 64-electrode grids. HDsEMG-torque signal relationships were explored via coherence (0-5Hz) and cross-correlation analyses. Principal component analysis was used for HDsEMG-data dimensionality reduction and improvement of HDsEMG-torque-based estimations. DOMS did not reduce either concentric or eccentric trunk extensor muscle strength. However, in the presence of DOMS, improved TS, alongside an altered HDsEMG-torque relationship and kinematic changes were observed, in a contraction-dependent manner. For eccentric trunk extension, improved TS was observed, with greater lumbar flexion movement and a reduction in δ-band HDsEMG-torque coherence and cross-correlation. For concentric trunk extensions, TS improvements were observed alongside reduced thoracolumbar sagittal movement. DOMS does not seem to impair the ability to control trunk muscle force, however, perceived soreness induced changes in lumbar movement and muscle recruitment strategies, which could alter motor performance if the exposure to pain is maintained in the long term.\u003c/p\u003e","manuscriptTitle":"Eccentric exercise-induced delayed onset trunk muscle soreness alters high-density surface EMG- torque relationships and lumbar kinematics","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-06-03 14:29:47","doi":"10.21203/rs.3.rs-4426332/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2024-06-17T06:14:50+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-06-14T17:29:43+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-06-12T19:45:43+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"7426276865420299996650305170921615755","date":"2024-06-05T13:34:46+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"14689367409429771702905540724359926142","date":"2024-06-05T12:07:12+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2024-06-05T02:07:28+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2024-06-05T02:03:11+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"","date":"2024-05-17T18:52:10+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2024-05-17T07:00:01+00:00","index":"","fulltext":""},{"type":"submitted","content":"Scientific Reports","date":"2024-05-15T15:54:41+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"32c1358e-d0f3-443d-a6eb-2e9cab5bdbdb","owner":[],"postedDate":"June 3rd, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":32464926,"name":"Biological sciences/Neuroscience/Motor control"},{"id":32464927,"name":"Biological sciences/Physiology/Neurophysiology"}],"tags":[],"updatedAt":"2024-08-12T16:06:19+00:00","versionOfRecord":{"articleIdentity":"rs-4426332","link":"https://doi.org/10.1038/s41598-024-69050-x","journal":{"identity":"scientific-reports","isVorOnly":false,"title":"Scientific Reports"},"publishedOn":"2024-08-10 15:58:09","publishedOnDateReadable":"August 10th, 2024"},"versionCreatedAt":"2024-06-03 14:29:47","video":"","vorDoi":"10.1038/s41598-024-69050-x","vorDoiUrl":"https://doi.org/10.1038/s41598-024-69050-x","workflowStages":[]},"version":"v1","identity":"rs-4426332","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4426332","identity":"rs-4426332","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}