Acute Response of Motor Unit Firing Rates During Two Resistance Exercise Training Sessions That Include Isotonic and Isometric Muscle Actions

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Busick, Keifer A. Bass, Andrew J. Veith, Akunna E. Nwasoria, and 2 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-9544715/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Previous studies reported increases in strength following a single resistance exercise training (RET) session. The observed gains are believed to be driven by neural mechanisms. It remains unclear the role of motor unit (MU) activation on increases in isometric and isotonic strength following a single RET session. The purpose of the present study was to examine the effects of a single RET session on MU activation during isometric and isotonic muscle actions. In addition, potential differences in MU activity between isometric and isotonic muscle actions will be examined for the first dorsal interosseous muscle (FDI). Thirty-four recreationally active participants (15 males and 19 females, age = 21.25 ± 2.40 years) completed two RET sessions. The sessions were separated by 48 hours. Isometric maximal voluntary contraction (MVC) and isotonic 1-repetition maximum (1RM) tests were completed in each session. MU peak firing rates (FR) were assessed in relation to the corresponding action potential amplitude. Higher MU FRs were observed during isometric MVCs in comparison to isotonic 1RMs ( β = 13.20, p < 0.001). However, strength increases and neural adaptations were not observed during RET session 2. The findings suggest there are potential differences in MU activity between isotonic and isometric muscle actions. Additionally, the FDI may be a valuable model to attenuate the reported learning effect following acute RET. motor unit action potential amplitude firing rate isotonic isometric strength training Figures Figure 1 Figure 2 Figure 3 Figure 4 Introduction Resistance exercise training (RET) is shown to elicit improvements in strength acutely and in the long-term (Maestroni et al. 2020 ). Increases in strength during (RET) benefit performance and rehabilitation efforts (Maestroni et al. 2020 ). Chronic adaptations in strength are commonly accompanied by an increase in muscle cross-sectional area (Moritani and deVries 1979 ). However, short-term improvements in strength during (RET) typically occur without a corresponding increase in muscular strength. Subsequently, it is believed that acute increases in strength are neural in origin when hypertrophy is not occurring (Gabriel et al. 2006 ). Potentially, the reported improvements in both isometric and isotonic strength are dependent on a learning effect occurring in the nervous system (Gabriel et al. 2006 ; Ritti-Dias et al. 2011 ). Previous works have suggested maximal force output independent of contraction type may be a learned skill rather than a definitive measure of muscular strength (Rutherford and Jones 1986 ; Sale 1988 ). It is often proposed that the improvements in strength are neural in origin. This hypothesis was further validated by reports of rapid increases in strength following repeated isotonic (Benton et al. 2009 ; Ritti-Dias et al. 2011 ; Cronin and Henderson) and isometric strength testing in the absence of hypertrophy (Kroll 1962 ; Kamen and Knight 2004 ; Knight and Marmon 2008 ). This phenomenon was first reported during isometric training (Hettinger and Müller 1953 ; Kraemer et al. 2017 ). Thus, isometric familiarization testing is performed on large and small muscle groups (e.g., knee extensors and wrist flexors) (Kroll 1962 ; Kamen and Knight 2004 ; Gabriel et al. 2006 ; Knight and Marmon 2008 ). In contrast, research including isotonic muscle actions typically incorporates compound movements, such as squat and bench press variations (Cronin and Henderson 2004 ; Benton et al. 2009 ; Ritti-Dias et al. 2011 ). The large muscle groups, particularly the quadriceps, gluteus maximus, triceps brachii, and pectoralis major (Cronin and Henderson 2004 ; Benton et al. 2009 ; Ritti-Dias et al. 2011 ), make it challenging to test if increases in strength are neural in origin. A motor unit (MU) is composed of an alpha motoneuron and the muscle fibers it innervates. The excitation to MUs dictates force production (Heckman and Enoka 2012 ). The size and firing rate (FR) of active MUs are reflective of the force generated (Knight and Kamen 2004 ; Heckman and Enoka 2012 ). Increases in MU activity can enhance force production without muscle fiber hypertrophy (Kamen and Knight 2004 ). Neural adaptations of the MU, such as increases in FRs, may explain the rapid increases in strength following the initial training session (Gabriel et al. 2006 ). Large limb muscles are often the focus of resistance training studies. However, there are limitations when examining neural adaptations following resistance training. Confounding factors, such as multiple agonist, stabilizing, and antagonist muscles make it difficult to identify underlying mechanisms. In addition, large limb muscles are plastic and differ significantly across individuals based on physical activity history. The use of the first dorsal interosseus muscle (FDI) has led to important discoveries in MU research (Milner-Brown et al. 1973 ; Bigland-Ritchie et al. 1983 ; Deluca and Erim 1994 ; Kornatz et al. 2005 ). The FDI serves as an attractive muscle because it is the only muscle involved in abduction of the index finger, there is no opposing muscle, and it is not influenced by physical activity to the same extent as large muscle groups (Milner‐Brown et al. 1973; Sterczala et al. 2018 ). The purpose of the present study was to examine potential changes in neural activation from the 1st to 2nd resistance training session. Furthermore, we aim to identify potential differences in MU activation between isometric and isotonic muscle actions of the FDI. METHODS Participants Thirty-four recreationally active, college-aged individuals volunteered to participate in this study ( Mean ± SD , 15 males and 19 females, age = 21.25 ± 2.40 years, height = 172.65 ± 21.48 cm, and mass = 72.48 ± 18.35 kg). Individuals with conditions affecting upper limb motor control or cardiovascular, metabolic, and musculoskeletal disorders were excluded from participation. All participants completed a physical activity and health questionnaire and provided written informed consent prior to participation. This study was approved by the Human Subjects Committee at the University of Kansas. Research Design Participants completed two testing sessions separated by 48 hours to assess finger abduction strength of the FDI muscle. There was no familiarization visit, however, investigators demonstrated the test to the participants. Participants completed three isometric maximal voluntary contractions (MVCs) followed by an isotonic one-repetition maximum (1RM). 1RM testing continued until participants failed to achieve a full repetition as monitored by an experienced investigator. The participants matched a 45 beats-per-minute (bpm) metronome during the isotonic 1RM. Training/testing of the FDI was completed on a custom-built apparatus that included a load cell for the MVC and a pulley system for the 1RM. Strength Training/Testing Isometric testing was conducted on the participant’s right hand in a pronated position with the forearm and wrist resting flat on a table. The adjacent fingers were immobilized using a velcro strap. The thumb was secured at a 90-degree angle in a similar manner. To measure maximal force produced by the FDI muscle, participants were prompted to abduct their index finger against a force transducer (MB-100; Interface, Inc., Scottsdale, AZ). The investigator provided strong verbal encouragement during the participant’s three MVC attempts. The highest recorded 0.25 seconds of force was assigned as the maximal isometric strength of the FDI muscle. Isotonic strength was assessed with a 1RM test. The position of the wrist and forearm was consistent between the MVC and the 1RM test. In contrast, the thumb and adjacent fingers were not restrained during the 1RM. Participants were instructed to abduct their index finger at a 45-bpm tempo supplemented by a metronome. Finger abduction angle was measured via a goniometer to establish the participant’s full range of motion (ROM). A repetition was considered successful when performed in sequence with the metronome cadence through a full ROM without assistance from the adjacent fingers or wrist. The starting weight was set at 880 g and was increased until the participant failed to meet the repetition criteria. 1RM attempts were separated by two minutes to allow for sufficient recovery. The heaviest weight performed by the participant was recorded as their 1RM. Electromyography Two different surface electromyography (sEMG) decomposition techniques were used to record MUs during isometric (Baglioni System, Delsys, Inc.) and isotonic tests (Trigno Wireless System, Delsy, Inc). Regardless of the system, the area of electrode placement was shaved, superficial dead skin was removed with adhesive tape, and the area was sterilized with an alcohol swab. Differences in electrode configuration between systems did not allow for the comparison of motor unit action potential (MUAP) amplitudes. The distance between electrodes affects the signal frequency received by the sensors (De Luca et al. 2012). However, FRs depend on time rather than the amplitude of the signal, allowing for a viable comparison between the two systems. Isometric MVC EMG Decomposition For isometric MVCs, sEMG was recorded via a surface array sensor (Delsys, Inc., Natick, MA). The sensor had five pins, 0.5 mm in diameter, housed in a 5x5 square with the fifth pin located in the center. The sEMG sensor was secured to the FDI with adhesive tape, with the reference electrode secured to the right elbow. The EMG signals were sampled at 20kHz and stored on a computer for offline analysis. The sEMG MVC signals recorded were decomposed, via the PD III algorithm (De Luca et al. 2006 ; Nawab et al. 2010 ), into independent MU action potential (AP) trains (De Luca et al. 2006 ; Nawab et al. 2010 ). Isotonic EMG Decomposition For the isotonic 1RM test, sEMG was recorded via a surface array sensor (Delsys, Inc., Natick MA) with 4 pins. The 4 electrode pins were arranged in a diamond formation, 5 mm apart. The sEMG sensor was secured to the FDI using a hypoallergenic double-sided adhesive. Signals were sampled at 2,222 Hz with 20–45 Hz sensor filtering. The signal was recorded wirelessly via the Trigno base station. The sEMG 1RM independent MUAP trains were identified using the PD III artificial intelligence algorithm designed for isotonic movements (Neuromap v.1.2; Delsys, Inc.) (Nawab et al. 2010 ; De Luca et al. 2015 ). Validation of MUAP Trains The reliability of MUAP trains from both methods was validated through a decompose-synthesize-decompose-compare (DSDC) test. The DSDC test uses a synthetic signal created by the AP waveforms and firing times taken from the MUAP train. The root mean squared value of the remaining unstructured signal is then applied as Gaussian noise, similar to the noise found in a raw sEMG signal. Once complete, the synthetic signal is decomposed for comparison with the decomposition values of the real signal. If the decomposition is accurate, the values of the two signals will be similar. To increase validity for the interpretation of MU events, only repetitions with ≥ 5 MUs and ≥ 90% accuracy between signals were included in the statistical analysis. The 90% accuracy level associates well with other user driven methods to validate accuracy of firing times, such as spike trigger averaging (Herda et al. 2020 ; Beausejour et al. 2023 ). A detailed overview can be found in previous work De Luca et al. ( 2015 ) and Herda et al. ( 2020 ). Statistical Analyses The MVC and 1RM data were imported into R (R Core Team 2024) to investigate the relationship between peak FRs and MUAP during isometric and isotonic contractions across two RET sessions (See Figs. 1 and 2 ). Peak FRs were defined as the highest recorded pulses per second (pps). MUAP amplitude was person-centered (MUAPamp_person_c) to show within-subject deviation. Person-means were reintroduced as a grand-mean centered between-subject variable (MUAPamp_person_mean_gmc). A score of 0 for MUAPamp_person_c indicates the average MUAPamp value for the individual subject. Additionally, a score of 0 for the MUAPamp_person_mean_gmc variable indicates the subject had an average MUAPamp value within the subject pool. This approach was selected to account for the between- and within-subject variability of MUAPamp. A likelihood ratio test was conducted to find the most suitable mixed-effect model. The final three mixed effect models were as follows: 1RM vs. MVC (Group x Session), MVC (Session x MUAP_person_c, MUAP_person_c x MUAP_person_mean_gmc), and 1RM (Session + MUAP_person_c x MUAP_person_mean_gmc). The linear mixed-effects model was selected to account for individual- and between-subject MU FR and AP amplitude variability that may occur within the variables of interest (i.e., session, within- and between-MUAP amplitudes/FRs). Paired sample t-tests were used to examine the potential differences in isotonic 1RM vs isometric MVC strength between RET sessions 1 and 2. Figure 1 ( a) A force tracing of an isometric maximal voluntary contraction overlaying decomposed motor unit (MU) action potential (AP) trains. The AP amplitude (mV) of larger (b) and smaller (c) recruited MUs plotted over time (ms). Figure 2 (a) The change in joint angle with its corresponding decomposed motor unit (MU) action potential (AP) trains during a 1-repetition maximum. The AP amplitude plotted over time for a larger (b) and smaller (c) recruited MU. RESULTS Thirty-two participants were included in the isometric MVC strength and MU statistical analyses, whereas 10 participants were included in the 1RM strength and MU analyses. There were several participants in the 1RM (22) and MVC (2) test with < 5 MUs recorded. Strength A paired-samples t-test indicated no differences in isometric MVC ( p = 0.8943, d = 0.01) and isotonic 1RM strength between sessions 1 to 2 ( p = 0.3843, d =-0.29). (See Fig. 3 ) Figure 3 ( a ) The mean (SD) isometric maximal voluntary contraction (MVC) torque (Nm) and ( b ) isotonic 1- repetition maximum (1RM) force (g) for resistance exercise training session 1 (S1) and 2 (S2). Isometric MVC MUAP amplitudes and FRs Initially, the final mixed-effect model found three significant 2-way interactions (FR- Session x MUAP_person_c and MUAP_person_c x MUAP_person_mean_gmc). Upon post hoc analysis, the interactions were found to be nonsignificant. In summary, a single MVC session may not be sufficient to stimulate strength or neural adaptations in the FDI. Isotonic 1RM MUAP amplitudes and FRs The final mixed effect model found no significant interactions between sessions (FR - Session + MUAP_person_c x MUAP_person_mean_gmc). The absence of interaction between MU discharge characteristics and session suggests that a single strength testing session does not result in neural adaptations. Isometric vs. Isotonic FRs Only participants who contributed to both data sets (MVC and 1RM) were included in the between-group analysis ( n = 8). The final mixed effect model revealed three significant 2-way interactions (FR- Group x Session, Group x MUAP_person_c, and MUAP_person_c x MUAP_person_mean_c). Higher FRs occurred during the MVC versus the 1RM ( β = 13.20, p < 0.0001) (See Fig. 4 ). No significant interactions were found between Group x Session. This suggests FRs are influenced by muscle contraction type. Figure 4 Firing rates (pps) regressed over motor unit action potential (MUAP) amplitudes (mV) for the ( a and b ) isotonic and isometric ( c and d ) muscle actions during sessions one (left) and two (right). (e) The mean (SD) y-intercept of 1RM and MVC collapsed sessions 1 and 2. * Indicates higher predicted firing rates. DISCUSSION The primary findings of this study contradict previous investigations. A single session of resistance exercise training did not increase isometric (Kroll 1962 ; Kamen and Knight 2004 ; Gabriel et al. 2006 ; Knight and Marmon 2008 ) or isotonic strength (Cronin and Henderson 2004 ; Ritti-Dias et al. 2011 ), which coincided with no changes in MU FRs. Interestingly, FRs were greater during the isometric MVC in comparison to the 1RM. The initial familiarization session can be considered a de facto RET session. Previous research reported significant strength increases in isotonic and isometric strength from visit 1 (familiarization) to visit 2 (Cronin and Henderson 2004 ; Kamen and Knight 2004 ; Knight and Marmon 2008 ; Ritti-Dias et al. 2011 ). In contrast, there were no increases in isometric and isotonic strength from session 1 to session 2. A learning effect is often cited when increases in strength occur following the initial visit during the RET program (Rutherford and Jones 1986 ; Sale 1988 ; Gabriel et al. 2006 ). The potential learning might be minimized in the FDI as it is a simple movement with no opposing or synergist muscle groups participating in the contractions. Subsequently, the FDI should be the preferred muscle if researchers want to minimize the learning effect in the research design. An inverse relationship ( R 2 = 0.56) was observed between MU FRs and AP amplitudes during the isometric MVCs. Thus, MUs with greater AP amplitudes, an indirect measure of size, exhibit lower firing rates in comparison to smaller AP amplitudes. These findings are consistent with the proposed onion control scheme (De Luca and Contessa 2012 ). Previous papers report strong relationships for large limb and fine motor muscles during isometric MVCs (De Luca and Hostage 2010 ; Miller et al. 2019 ). There are only a few studies that report relationships between MU FRs and AP amplitudes during isotonic contractions. In the present study, there were strong inverse relationships between MU FRs and AP amplitudes ( R 2 = 0.78) during the maximal isotonic 1RM contractions of the hand muscle. There are a few studies that report MU FRs and AP amplitudes during isotonic contractions for various velocities and intensities (De Luca et al. 2015 ; Rivas et al. 2025 ). For the biceps brachii (BB), De Luca et al. ( 2015 ) reported that MU activity was consistent with the onion skin control scheme during cyclic dynamic contractions, while Rivas et al. ( 2025 ) observed a strong inverse relationship during near-maximal intent isotonic contractions. In the present study, the mean firing rates for males (21.92 ± 6.87 pps) and females (23.37 ± 4.47 pps) during the isotonic 1RMs were similar to the BB (Males: 21.70 ± 4.01 pps; Females: 17.75 ± 3.17 pps) from Rivas et al. ( 2025 ). In addition, the y-intercepts from the MU FRs vs. AP amplitude relationships of the FDI and BB are similar as well. However, the slopes from the MU FR vs. AP amplitude relationships were far less negative for the FDI in comparison to the BB. Considering the much shorter recruitment range of the FDI, the less negative slopes from the MU FR vs. AP amplitude relationships are likely the result of increased rate coding as the primary mechanism to increase force output beyond moderate submaximal intensities. Future research should investigate FR behavior among muscles during 1RMs. The muscle action-related differences in FRs may be attributed to the duration of excitation to the MU. The consistent force output required in isometric actions can induce long-duration excitation of the active MUs. However, isotonic movements are biphasic, demanding variable excitation to MUs (Linnamo et al. 2003 ; Heckman et al. 2005 ; Heckman et al. 2008 ). Persistent inward currents (PICs) can increase firing rates but require 50 ms to 200 ms of excitatory input to be activated (Heckman et al. 2005 ; Heckman et al. 2008 ). The variability in excitation during isotonic muscle actions may prevent the activation of PIC channels. Conclusion Our findings suggest the FDI is a compelling model for mitigating a potential learning effect during maximal strength assessments. 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Busick","email":"","orcid":"","institution":"University of Kansas","correspondingAuthor":false,"prefix":"","firstName":"Zane","middleName":"H.","lastName":"Busick","suffix":""},{"id":634655694,"identity":"021bbd78-3d5e-431c-beb5-003807bff8ad","order_by":1,"name":"Keifer A. Bass","email":"","orcid":"","institution":"University of Kansas","correspondingAuthor":false,"prefix":"","firstName":"Keifer","middleName":"A.","lastName":"Bass","suffix":""},{"id":634655706,"identity":"dc3c2c99-2a83-49f2-bfc0-ebc115bc43f8","order_by":2,"name":"Andrew J. Veith","email":"","orcid":"","institution":"University of Kansas","correspondingAuthor":false,"prefix":"","firstName":"Andrew","middleName":"J.","lastName":"Veith","suffix":""},{"id":634655711,"identity":"050df1e6-31e6-48a3-b72e-3f48fb5a56dc","order_by":3,"name":"Akunna E. Nwasoria","email":"","orcid":"","institution":"University of Kansas","correspondingAuthor":false,"prefix":"","firstName":"Akunna","middleName":"E.","lastName":"Nwasoria","suffix":""},{"id":634655719,"identity":"3085519d-c7a5-42d5-9161-4a03021d0a7e","order_by":4,"name":"Grace E. Hicks","email":"","orcid":"","institution":"University of Kansas","correspondingAuthor":false,"prefix":"","firstName":"Grace","middleName":"E.","lastName":"Hicks","suffix":""},{"id":634655722,"identity":"db81f462-45e3-462b-bbf3-a06f51ee2a7a","order_by":5,"name":"Trent J. Herda","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAqElEQVRIiWNgGAWjYFACHiBmY5CTgLGJ0ADRYky6lsQZRGuxZ+A9+LiizC595owExgdv24iyhS/Z8My55NzZEgnMhnOJ08JjJtnYxpw7TyKBTZqXSC3mPxvb6tPlJBLYfxOrxYyxse1wgjTQFmbitBzmMZZsOHfccGbPw2bJOeeI0MLe3mP4saGsWl7iePLBD2/KiNDCwAxnMTYQo34UjIJRMApGATEAAPOgK6VyIIGFAAAAAElFTkSuQmCC","orcid":"","institution":"University of Kansas","correspondingAuthor":true,"prefix":"","firstName":"Trent","middleName":"J.","lastName":"Herda","suffix":""}],"badges":[],"createdAt":"2026-04-27 17:09:08","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-9544715/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-9544715/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":108975683,"identity":"13d145ba-6b33-4a83-8663-76e184d0ccf6","added_by":"auto","created_at":"2026-05-11 10:57:14","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":131527,"visible":true,"origin":"","legend":"\u003cp\u003e(\u003cem\u003ea) \u003c/em\u003eA force tracing of an isometric maximal voluntary contraction overlaying decomposed motor unit (MU) action potential (AP) trains. The AP amplitude (mV) of larger (b) and smaller (c) recruited MUs plotted over time (ms).\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-9544715/v1/6356611afee3c38851437473.png"},{"id":108975681,"identity":"8f00d2f2-3648-4344-91dc-6e9bf612390e","added_by":"auto","created_at":"2026-05-11 10:57:14","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":141513,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003e(a) \u003c/em\u003eThe change in joint angle with its corresponding decomposed motor unit (MU) action potential (AP) trains during a 1-repetition maximum. The AP amplitude plotted over time for a larger (b) and smaller (c) recruited MU.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-9544715/v1/56b2758095e037571c87cde1.png"},{"id":108976004,"identity":"72449ab4-6272-46fa-8d40-3255821a8ea8","added_by":"auto","created_at":"2026-05-11 10:58:36","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":59458,"visible":true,"origin":"","legend":"\u003cp\u003e(\u003cem\u003ea\u003c/em\u003e) The mean (SD) isometric maximal voluntary contraction (MVC) torque (Nm) and (\u003cem\u003eb\u003c/em\u003e) isotonic 1- repetition maximum (1RM) force (g) for resistance exercise training session 1 (S1) and 2 (S2).\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-9544715/v1/3627ec0808094dc1208de1f5.png"},{"id":108975710,"identity":"851b53d8-7cee-41b7-9454-7415533d25ef","added_by":"auto","created_at":"2026-05-11 10:57:18","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":163999,"visible":true,"origin":"","legend":"\u003cp\u003eFiring rates (pps) regressed over motor unit action potential (MUAP) amplitudes (mV) for the (\u003cem\u003ea and b\u003c/em\u003e) isotonic and isometric (\u003cem\u003ec and d\u003c/em\u003e) muscle actions during sessions one (left) and two (right). (e) The mean (SD) y-intercept of 1RM and MVC collapsed sessions 1 and 2. * Indicates higher predicted firing rates.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-9544715/v1/c1dc4c44404d95309908b2d9.png"},{"id":108978806,"identity":"375ded5a-651e-430c-bad7-86d2b3e3e55e","added_by":"auto","created_at":"2026-05-11 11:48:46","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":609979,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-9544715/v1/0b0cc517-c6f8-49a6-914e-e123e98f78e0.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Acute Response of Motor Unit Firing Rates During Two Resistance Exercise Training Sessions That Include Isotonic and Isometric Muscle Actions","fulltext":[{"header":"Introduction","content":"\u003cp\u003eResistance exercise training (RET) is shown to elicit improvements in strength acutely and in the long-term (Maestroni et al. \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Increases in strength during (RET) benefit performance and rehabilitation efforts (Maestroni et al. \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Chronic adaptations in strength are commonly accompanied by an increase in muscle cross-sectional area (Moritani and deVries \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e1979\u003c/span\u003e). However, short-term improvements in strength during (RET) typically occur without a corresponding increase in muscular strength. Subsequently, it is believed that acute increases in strength are neural in origin when hypertrophy is not occurring (Gabriel et al. \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2006\u003c/span\u003e). Potentially, the reported improvements in both isometric and isotonic strength are dependent on a learning effect occurring in the nervous system (Gabriel et al. \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; Ritti-Dias et al. \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2011\u003c/span\u003e).\u003c/p\u003e \u003cp\u003ePrevious works have suggested maximal force output independent of contraction type may be a learned skill rather than a definitive measure of muscular strength (Rutherford and Jones \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e1986\u003c/span\u003e; Sale \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e1988\u003c/span\u003e). It is often proposed that the improvements in strength are neural in origin. This hypothesis was further validated by reports of rapid increases in strength following repeated isotonic (Benton et al. \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2009\u003c/span\u003e; Ritti-Dias et al. \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Cronin and Henderson) and isometric strength testing in the absence of hypertrophy (Kroll \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e1962\u003c/span\u003e; Kamen and Knight \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2004\u003c/span\u003e; Knight and Marmon \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2008\u003c/span\u003e). This phenomenon was first reported during isometric training (Hettinger and M\u0026uuml;ller \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e1953\u003c/span\u003e; Kraemer et al. \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). Thus, isometric familiarization testing is performed on large and small muscle groups (e.g., knee extensors and wrist flexors) (Kroll \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e1962\u003c/span\u003e; Kamen and Knight \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2004\u003c/span\u003e; Gabriel et al. \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; Knight and Marmon \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2008\u003c/span\u003e). In contrast, research including isotonic muscle actions typically incorporates compound movements, such as squat and bench press variations (Cronin and Henderson \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2004\u003c/span\u003e; Benton et al. \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2009\u003c/span\u003e; Ritti-Dias et al. \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). The large muscle groups, particularly the quadriceps, gluteus maximus, triceps brachii, and pectoralis major (Cronin and Henderson \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2004\u003c/span\u003e; Benton et al. \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2009\u003c/span\u003e; Ritti-Dias et al. \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2011\u003c/span\u003e), make it challenging to test if increases in strength are neural in origin.\u003c/p\u003e \u003cp\u003eA motor unit (MU) is composed of an alpha motoneuron and the muscle fibers it innervates. The excitation to MUs dictates force production (Heckman and Enoka \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). The size and firing rate (FR) of active MUs are reflective of the force generated (Knight and Kamen \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2004\u003c/span\u003e; Heckman and Enoka \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). Increases in MU activity can enhance force production without muscle fiber hypertrophy (Kamen and Knight \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2004\u003c/span\u003e). Neural adaptations of the MU, such as increases in FRs, may explain the rapid increases in strength following the initial training session (Gabriel et al. \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2006\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eLarge limb muscles are often the focus of resistance training studies. However, there are limitations when examining neural adaptations following resistance training. Confounding factors, such as multiple agonist, stabilizing, and antagonist muscles make it difficult to identify underlying mechanisms. In addition, large limb muscles are plastic and differ significantly across individuals based on physical activity history. The use of the first dorsal interosseus muscle (FDI) has led to important discoveries in MU research (Milner-Brown et al. \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e1973\u003c/span\u003e; Bigland-Ritchie et al. \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e1983\u003c/span\u003e; Deluca and Erim \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e1994\u003c/span\u003e; Kornatz et al. \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2005\u003c/span\u003e). The FDI serves as an attractive muscle because it is the only muscle involved in abduction of the index finger, there is no opposing muscle, and it is not influenced by physical activity to the same extent as large muscle groups (Milner‐Brown et al. 1973; Sterczala et al. \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). The purpose of the present study was to examine potential changes in neural activation from the 1st to 2nd resistance training session. Furthermore, we aim to identify potential differences in MU activation between isometric and isotonic muscle actions of the FDI.\u003c/p\u003e"},{"header":"METHODS","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eParticipants\u003c/h2\u003e \u003cp\u003eThirty-four recreationally active, college-aged individuals volunteered to participate in this study (\u003cem\u003eMean\u003c/em\u003e\u0026thinsp;\u0026plusmn;\u0026thinsp;\u003cem\u003eSD\u003c/em\u003e, 15 males and 19 females, age\u0026thinsp;=\u0026thinsp;21.25\u0026thinsp;\u0026plusmn;\u0026thinsp;2.40 years, height\u0026thinsp;=\u0026thinsp;172.65\u0026thinsp;\u0026plusmn;\u0026thinsp;21.48 cm, and mass\u0026thinsp;=\u0026thinsp;72.48\u0026thinsp;\u0026plusmn;\u0026thinsp;18.35 kg). Individuals with conditions affecting upper limb motor control or cardiovascular, metabolic, and musculoskeletal disorders were excluded from participation. All participants completed a physical activity and health questionnaire and provided written informed consent prior to participation. This study was approved by the Human Subjects Committee at the University of Kansas.\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eResearch Design\u003c/h3\u003e\n\u003cp\u003eParticipants completed two testing sessions separated by 48 hours to assess finger abduction strength of the FDI muscle. There was no familiarization visit, however, investigators demonstrated the test to the participants. Participants completed three isometric maximal voluntary contractions (MVCs) followed by an isotonic one-repetition maximum (1RM). 1RM testing continued until participants failed to achieve a full repetition as monitored by an experienced investigator. The participants matched a 45 beats-per-minute (bpm) metronome during the isotonic 1RM. Training/testing of the FDI was completed on a custom-built apparatus that included a load cell for the MVC and a pulley system for the 1RM.\u003c/p\u003e\n\u003ch3\u003eStrength Training/Testing\u003c/h3\u003e\n\u003cp\u003e Isometric testing was conducted on the participant\u0026rsquo;s right hand in a pronated position with the forearm and wrist resting flat on a table. The adjacent fingers were immobilized using a velcro strap. The thumb was secured at a 90-degree angle in a similar manner. To measure maximal force produced by the FDI muscle, participants were prompted to abduct their index finger against a force transducer (MB-100; Interface, Inc., Scottsdale, AZ). The investigator provided strong verbal encouragement during the participant\u0026rsquo;s three MVC attempts. The highest recorded 0.25 seconds of force was assigned as the maximal isometric strength of the FDI muscle.\u003c/p\u003e \u003cp\u003eIsotonic strength was assessed with a 1RM test. The position of the wrist and forearm was consistent between the MVC and the 1RM test. In contrast, the thumb and adjacent fingers were not restrained during the 1RM. Participants were instructed to abduct their index finger at a 45-bpm tempo supplemented by a metronome. Finger abduction angle was measured via a goniometer to establish the participant\u0026rsquo;s full range of motion (ROM). A repetition was considered successful when performed in sequence with the metronome cadence through a full ROM without assistance from the adjacent fingers or wrist. The starting weight was set at 880 g and was increased until the participant failed to meet the repetition criteria. 1RM attempts were separated by two minutes to allow for sufficient recovery. The heaviest weight performed by the participant was recorded as their 1RM.\u003c/p\u003e\n\u003ch3\u003eElectromyography\u003c/h3\u003e\n\u003cp\u003eTwo different surface electromyography (sEMG) decomposition techniques were used to record MUs during isometric (Baglioni System, Delsys, Inc.) and isotonic tests (Trigno Wireless System, Delsy, Inc). Regardless of the system, the area of electrode placement was shaved, superficial dead skin was removed with adhesive tape, and the area was sterilized with an alcohol swab.\u003c/p\u003e \u003cp\u003eDifferences in electrode configuration between systems did not allow for the comparison of motor unit action potential (MUAP) amplitudes. The distance between electrodes affects the signal frequency received by the sensors (De Luca et al. 2012). However, FRs depend on time rather than the amplitude of the signal, allowing for a viable comparison between the two systems.\u003c/p\u003e\n\u003ch3\u003eIsometric MVC EMG Decomposition\u003c/h3\u003e\n\u003cp\u003eFor isometric MVCs, sEMG was recorded via a surface array sensor (Delsys, Inc., Natick, MA). The sensor had five pins, 0.5 mm in diameter, housed in a 5x5 square with the fifth pin located in the center. The sEMG sensor was secured to the FDI with adhesive tape, with the reference electrode secured to the right elbow. The EMG signals were sampled at 20kHz and stored on a computer for offline analysis. The sEMG MVC signals recorded were decomposed, via the PD III algorithm (De Luca et al. \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; Nawab et al. \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2010\u003c/span\u003e), into independent MU action potential (AP) trains (De Luca et al. \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; Nawab et al. \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2010\u003c/span\u003e).\u003c/p\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003eIsotonic EMG Decomposition\u003c/h2\u003e \u003cp\u003eFor the isotonic 1RM test, sEMG was recorded via a surface array sensor (Delsys, Inc., Natick MA) with 4 pins. The 4 electrode pins were arranged in a diamond formation, 5 mm apart. The sEMG sensor was secured to the FDI using a hypoallergenic double-sided adhesive. Signals were sampled at 2,222 Hz with 20\u0026ndash;45 Hz sensor filtering. The signal was recorded wirelessly via the Trigno base station. The sEMG 1RM independent MUAP trains were identified using the PD III artificial intelligence algorithm designed for isotonic movements (Neuromap v.1.2; Delsys, Inc.) (Nawab et al. \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; De Luca et al. \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2015\u003c/span\u003e).\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eValidation of MUAP Trains\u003c/h3\u003e\n\u003cp\u003eThe reliability of MUAP trains from both methods was validated through a decompose-synthesize-decompose-compare (DSDC) test. The DSDC test uses a synthetic signal created by the AP waveforms and firing times taken from the MUAP train. The root mean squared value of the remaining unstructured signal is then applied as Gaussian noise, similar to the noise found in a raw sEMG signal. Once complete, the synthetic signal is decomposed for comparison with the decomposition values of the real signal. If the decomposition is accurate, the values of the two signals will be similar. To increase validity for the interpretation of MU events, only repetitions with \u0026ge;\u0026thinsp;5 MUs and \u0026ge;\u0026thinsp;90% accuracy between signals were included in the statistical analysis. The 90% accuracy level associates well with other user driven methods to validate accuracy of firing times, such as spike trigger averaging (Herda et al. \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Beausejour et al. \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). A detailed overview can be found in previous work De Luca et al. (\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2015\u003c/span\u003e) and Herda et al. (\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2020\u003c/span\u003e).\u003c/p\u003e\n\u003ch3\u003eStatistical Analyses\u003c/h3\u003e\n\u003cp\u003eThe MVC and 1RM data were imported into R (R Core Team 2024) to investigate the relationship between peak FRs and MUAP during isometric and isotonic contractions across two RET sessions (See Figs.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e and \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). Peak FRs were defined as the highest recorded pulses per second (pps). MUAP amplitude was person-centered (MUAPamp_person_c) to show within-subject deviation. Person-means were reintroduced as a grand-mean centered between-subject variable (MUAPamp_person_mean_gmc). A score of 0 for MUAPamp_person_c indicates the average MUAPamp value for the individual subject. Additionally, a score of 0 for the MUAPamp_person_mean_gmc variable indicates the subject had an average MUAPamp value within the subject pool. This approach was selected to account for the between- and within-subject variability of MUAPamp. A likelihood ratio test was conducted to find the most suitable mixed-effect model. The final three mixed effect models were as follows: 1RM vs. MVC (Group x Session), MVC (Session x MUAP_person_c, MUAP_person_c x MUAP_person_mean_gmc), and 1RM (Session\u0026thinsp;+\u0026thinsp;MUAP_person_c x MUAP_person_mean_gmc). The linear mixed-effects model was selected to account for individual- and between-subject MU FR and AP amplitude variability that may occur within the variables of interest (i.e., session, within- and between-MUAP amplitudes/FRs). Paired sample t-tests were used to examine the potential differences in isotonic 1RM vs isometric MVC strength between RET sessions 1 and 2.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e (\u003cem\u003ea)\u003c/em\u003e A force tracing of an isometric maximal voluntary contraction overlaying decomposed motor unit (MU) action potential (AP) trains. The AP amplitude (mV) of larger (b) and smaller (c) recruited MUs plotted over time (ms).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e \u003cem\u003e(a)\u003c/em\u003e The change in joint angle with its corresponding decomposed motor unit (MU) action potential (AP) trains during a 1-repetition maximum. The AP amplitude plotted over time for a larger (b) and smaller (c) recruited MU.\u003c/p\u003e"},{"header":"RESULTS","content":"\u003cp\u003eThirty-two participants were included in the isometric MVC strength and MU statistical analyses, whereas 10 participants were included in the 1RM strength and MU analyses. There were several participants in the 1RM (22) and MVC (2) test with \u0026lt;\u0026thinsp;5 MUs recorded.\u003c/p\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003eStrength\u003c/h2\u003e \u003cp\u003eA paired-samples t-test indicated no differences in isometric MVC (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.8943, \u003cem\u003ed\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.01) and isotonic 1RM strength between sessions 1 to 2 (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.3843, \u003cem\u003ed\u003c/em\u003e =-0.29). (See Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e)\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e (\u003cem\u003ea\u003c/em\u003e) The mean (SD) isometric maximal voluntary contraction (MVC) torque (Nm) and (\u003cem\u003eb\u003c/em\u003e) isotonic 1- repetition maximum (1RM) force (g) for resistance exercise training session 1 (S1) and 2 (S2).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section2\"\u003e \u003ch2\u003eIsometric MVC MUAP amplitudes and FRs\u003c/h2\u003e \u003cp\u003eInitially, the final mixed-effect model found three significant 2-way interactions (FR- Session x MUAP_person_c and MUAP_person_c x MUAP_person_mean_gmc). Upon post hoc analysis, the interactions were found to be nonsignificant. In summary, a single MVC session may not be sufficient to stimulate strength or neural adaptations in the FDI.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec14\" class=\"Section2\"\u003e \u003ch2\u003eIsotonic 1RM MUAP amplitudes and FRs\u003c/h2\u003e \u003cp\u003eThe final mixed effect model found no significant interactions between sessions (FR - Session\u0026thinsp;+\u0026thinsp;MUAP_person_c x MUAP_person_mean_gmc). The absence of interaction between MU discharge characteristics and session suggests that a single strength testing session does not result in neural adaptations.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec15\" class=\"Section2\"\u003e \u003ch2\u003eIsometric vs. Isotonic FRs\u003c/h2\u003e \u003cp\u003eOnly participants who contributed to both data sets (MVC and 1RM) were included in the between-group analysis (\u003cem\u003en\u003c/em\u003e\u0026thinsp;=\u0026thinsp;8). The final mixed effect model revealed three significant 2-way interactions (FR- Group x Session, Group x MUAP_person_c, and MUAP_person_c x MUAP_person_mean_c). Higher FRs occurred during the MVC versus the 1RM (\u003cem\u003eβ\u003c/em\u003e\u0026thinsp;=\u0026thinsp;13.20, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.0001) (See Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). No significant interactions were found between Group x Session. This suggests FRs are influenced by muscle contraction type.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e Firing rates (pps) regressed over motor unit action potential (MUAP) amplitudes (mV) for the (\u003cem\u003ea and b\u003c/em\u003e) isotonic and isometric (\u003cem\u003ec and d\u003c/em\u003e) muscle actions during sessions one (left) and two (right). (e) The mean (SD) y-intercept of 1RM and MVC collapsed sessions 1 and 2. * Indicates higher predicted firing rates.\u003c/p\u003e \u003c/div\u003e"},{"header":"DISCUSSION","content":"\u003cp\u003eThe primary findings of this study contradict previous investigations. A single session of resistance exercise training did not increase isometric (Kroll \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e1962\u003c/span\u003e; Kamen and Knight \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2004\u003c/span\u003e; Gabriel et al. \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; Knight and Marmon \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2008\u003c/span\u003e) or isotonic strength (Cronin and Henderson \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2004\u003c/span\u003e; Ritti-Dias et al. \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2011\u003c/span\u003e), which coincided with no changes in MU FRs. Interestingly, FRs were greater during the isometric MVC in comparison to the 1RM.\u003c/p\u003e \u003cp\u003eThe initial familiarization session can be considered a de facto RET session. Previous research reported significant strength increases in isotonic and isometric strength from visit 1 (familiarization) to visit 2 (Cronin and Henderson \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2004\u003c/span\u003e; Kamen and Knight \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2004\u003c/span\u003e; Knight and Marmon \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2008\u003c/span\u003e; Ritti-Dias et al. \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). In contrast, there were no increases in isometric and isotonic strength from session 1 to session 2. A learning effect is often cited when increases in strength occur following the initial visit during the RET program (Rutherford and Jones \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e1986\u003c/span\u003e; Sale \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e1988\u003c/span\u003e; Gabriel et al. \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2006\u003c/span\u003e). The potential learning might be minimized in the FDI as it is a simple movement with no opposing or synergist muscle groups participating in the contractions. Subsequently, the FDI should be the preferred muscle if researchers want to minimize the learning effect in the research design.\u003c/p\u003e \u003cp\u003eAn inverse relationship (\u003cem\u003eR\u003c/em\u003e\u003csup\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;0.56) was observed between MU FRs and AP amplitudes during the isometric MVCs. Thus, MUs with greater AP amplitudes, an indirect measure of size, exhibit lower firing rates in comparison to smaller AP amplitudes. These findings are consistent with the proposed onion control scheme (De Luca and Contessa \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). Previous papers report strong relationships for large limb and fine motor muscles during isometric MVCs (De Luca and Hostage \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Miller et al. \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2019\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThere are only a few studies that report relationships between MU FRs and AP amplitudes during isotonic contractions. In the present study, there were strong inverse relationships between MU FRs and AP amplitudes (\u003cem\u003eR\u003c/em\u003e\u003csup\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sup\u003e= 0.78) during the maximal isotonic 1RM contractions of the hand muscle. There are a few studies that report MU FRs and AP amplitudes during isotonic contractions for various velocities and intensities (De Luca et al. \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Rivas et al. \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). For the biceps brachii (BB), De Luca et al. (\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2015\u003c/span\u003e) reported that MU activity was consistent with the onion skin control scheme during cyclic dynamic contractions, while Rivas et al. (\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2025\u003c/span\u003e) observed a strong inverse relationship during near-maximal intent isotonic contractions.\u003c/p\u003e \u003cp\u003eIn the present study, the mean firing rates for males (21.92\u0026thinsp;\u0026plusmn;\u0026thinsp;6.87 pps) and females (23.37\u0026thinsp;\u0026plusmn;\u0026thinsp;4.47 pps) during the isotonic 1RMs were similar to the BB (Males: 21.70\u0026thinsp;\u0026plusmn;\u0026thinsp;4.01 pps; Females: 17.75\u0026thinsp;\u0026plusmn;\u0026thinsp;3.17 pps) from Rivas et al. (\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). In addition, the y-intercepts from the MU FRs vs. AP amplitude relationships of the FDI and BB are similar as well. However, the slopes from the MU FR vs. AP amplitude relationships were far less negative for the FDI in comparison to the BB. Considering the much shorter recruitment range of the FDI, the less negative slopes from the MU FR vs. AP amplitude relationships are likely the result of increased rate coding as the primary mechanism to increase force output beyond moderate submaximal intensities. Future research should investigate FR behavior among muscles during 1RMs.\u003c/p\u003e \u003cp\u003eThe muscle action-related differences in FRs may be attributed to the duration of excitation to the MU. The consistent force output required in isometric actions can induce long-duration excitation of the active MUs. However, isotonic movements are biphasic, demanding variable excitation to MUs (Linnamo et al. \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2003\u003c/span\u003e; Heckman et al. \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2005\u003c/span\u003e; Heckman et al. \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2008\u003c/span\u003e). Persistent inward currents (PICs) can increase firing rates but require 50 ms to 200 ms of excitatory input to be activated (Heckman et al. \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2005\u003c/span\u003e; Heckman et al. \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2008\u003c/span\u003e). The variability in excitation during isotonic muscle actions may prevent the activation of PIC channels.\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eOur findings suggest the FDI is a compelling model for mitigating a potential learning effect during maximal strength assessments. Additionally, the strong inverse relationship between MU FR and AP amplitude in the isotonic 1RM is consistent with recruitment schemes found during isometric actions. Interestingly, there were lower FRs in the isotonic contractions. This may be due to rate coding of the FDI during isotonic actions. Future investigations should examine the FR characteristics of muscles during a 1RM.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cem\u003eCompeting Interests\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no conflict of interest.\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eFunding\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eNo funding was received for conducting this study.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eBeausejour JP, Bohlen P, Harmon KK, Girts RM, Pagan JI, Hahs-Vaughn DL, Herda TJ, Stock MS (2023) A comparison of techniques for verifying the accuracy of precision decomposition-derived relationships between motor unit firing rates and recruitment thresholds from surface EMG signals. Exp Brain Res 241(10):2547\u0026ndash;2560. https://doi.org/10.1007/s00221-023-06694-7\u003c/li\u003e\n \u003cli\u003eBenton MJ, Swan PD, Peterson MD (2009) Evaluation of Multiple One Repetition Maximum Strength Trials in Untrained Women. J Strength Cond Res 23(5):1503\u0026ndash;1507. https://doi.org/10.1519/JSC.0b013e3181b338b3\u003c/li\u003e\n \u003cli\u003eBigland-Ritchie B, Johansson R, Lippold OC, Woods JJ (1983) Contractile speed and EMG changes during fatigue of sustained maximal voluntary contractions. J Neurophysiol 50(1):313\u0026ndash;324. https://doi.org/10.1152/jn.1983.50.1.313\u003c/li\u003e\n \u003cli\u003eCronin JB, Henderson Maximal strength and power assessment in novice weight trainers\u003c/li\u003e\n \u003cli\u003eCronin JB, Henderson ME (2004) Maximal strength and power assessment in novice weight trainers. J Strength Cond Res 18(1):48\u0026ndash;52\u003c/li\u003e\n \u003cli\u003eDe Luca CJ, Adam A, Wotiz R, Gilmore LD, Nawab SH (2006) Decomposition of Surface EMG Signals. J Neurophysiol 96(3):1646\u0026ndash;1657. https://doi.org/10.1152/jn.00009.2006\u003c/li\u003e\n \u003cli\u003eDe Luca CJ, Chang S-S, Roy SH, Kline JC, Nawab SH (2015) Decomposition of surface EMG signals from cyclic dynamic contractions. J Neurophysiol 113(6):1941\u0026ndash;1951. https://doi.org/10.1152/jn.00555.2014\u003c/li\u003e\n \u003cli\u003eDe Luca CJ, Contessa P (2012) Hierarchical control of motor units in voluntary contractions. J Neurophysiol 107(1):178\u0026ndash;195. https://doi.org/10.1152/jn.00961.2010\u003c/li\u003e\n \u003cli\u003eDe Luca CJ, Hostage EC (2010) Relationship Between Firing Rate and Recruitment Threshold of Motoneurons in Voluntary Isometric Contractions. J Neurophysiol 104(2):1034\u0026ndash;1046. https://doi.org/10.1152/jn.01018.2009\u003c/li\u003e\n \u003cli\u003eDeluca C, Erim Z (1994) Common drive of motor units in regulation of muscle force. Trends Neurosci 17(7):299\u0026ndash;305. https://doi.org/10.1016/0166-2236(94)90064-7\u003c/li\u003e\n \u003cli\u003eGabriel DA, Kamen G, Frost G (2006) Neural Adaptations to Resistive Exercise: Mechanisms and Recommendations for Training Practices. Sports Med 36(2):133\u0026ndash;149. https://doi.org/10.2165/00007256-200636020-00004\u003c/li\u003e\n \u003cli\u003eHeckman CJ, Enoka RM (2012) Motor Unit. Compr Physiol 2(4):2629\u0026ndash;2682. https://doi.org/10.1002/j.2040-4603.2012.tb00465.x\u003c/li\u003e\n \u003cli\u003eHeckman CJ, Gorassini MA, Bennett DJ (2005) Persistent inward currents in motoneuron dendrites: Implications for motor output. Muscle Nerve 31(2):135\u0026ndash;156. https://doi.org/10.1002/mus.20261\u003c/li\u003e\n \u003cli\u003eHeckman CJ, Johnson M, Mottram C, Schuster J (2008) Persistent Inward Currents in Spinal Motoneurons and Their Influence on Human Motoneuron Firing Patterns. The Neuroscientist 14(3):264\u0026ndash;275. https://doi.org/10.1177/1073858408314986\u003c/li\u003e\n \u003cli\u003eHerda TJ, Parra ME, Miller JD, Sterczala AJ, Kelly MR (2020) Measuring the accuracies of motor unit firing times and action potential waveforms derived from surface electromyographic decomposition. J Electromyogr Kinesiol 52:102421. https://doi.org/10.1016/j.jelekin.2020.102421\u003c/li\u003e\n \u003cli\u003eHettinger T, M\u0026uuml;ller EA (1953) Muskelleistung und muskeltraining. Arbeitsphysiologie 15(2):111\u0026ndash;126\u003c/li\u003e\n \u003cli\u003eKamen G, Knight CA (2004) Training-Related Adaptations in Motor Unit Discharge Rate in Young and Older Adults. J Gerontol A Biol Sci Med Sci 59(12):1334\u0026ndash;1338. https://doi.org/10.1093/gerona/59.12.1334\u003c/li\u003e\n \u003cli\u003eKnight CA, Kamen G (2004) Enhanced motor unit rate coding with improvements in a force-matching task. J Electromyogr Kinesiol 14(6):619\u0026ndash;629. https://doi.org/10.1016/j.jelekin.2004.04.005\u003c/li\u003e\n \u003cli\u003eKnight CA, Marmon AR (2008) Neural Training for Quick Strength Gains in the Elderly: Strength as a Learned Skill. J Strength Cond Res 22(6):1869\u0026ndash;1875. https://doi.org/10.1519/JSC.0b013e318182186c\u003c/li\u003e\n \u003cli\u003eKornatz KW, Christou EA, Enoka RM (2005) Practice reduces motor unit discharge variability in a hand muscle and improves manual dexterity in old adults. J Appl Physiol 98(6):2072\u0026ndash;2080. https://doi.org/10.1152/japplphysiol.01149.2004\u003c/li\u003e\n \u003cli\u003eKraemer WJ, Ratamess NA, Flanagan SD, Shurley JP, Todd JS, Todd TC (2017) Understanding the Science of Resistance Training: An Evolutionary Perspective. Sports Med 47(12):2415\u0026ndash;2435. https://doi.org/10.1007/s40279-017-0779-y\u003c/li\u003e\n \u003cli\u003eKroll W (1962) Reliability of a Selected Measure of Human Strength. Res Q Am Assoc Health Phys Educ Recreat 33(3):410\u0026ndash;417. https://doi.org/10.1080/10671188.1962.10616472\u003c/li\u003e\n \u003cli\u003eLinnamo V, Moritani T, Nicol C, Komi PV (2003) Motor unit activation patterns during isometric, concentric and eccentric actions at different force levels. J Electromyogr Kinesiol 13(1):93\u0026ndash;101. https://doi.org/10.1016/S1050-6411(02)00063-9\u003c/li\u003e\n \u003cli\u003eMaestroni L, Read P, Bishop C, Papadopoulos K, Suchomel TJ, Comfort P, Turner A (2020) The Benefits of Strength Training on Musculoskeletal System Health: Practical Applications for Interdisciplinary Care. Sports Med 50(8):1431\u0026ndash;1450. https://doi.org/10.1007/s40279-020-01309-5\u003c/li\u003e\n \u003cli\u003eMiller JD, Sterczala AJ, Trevino MA, Wray ME, Dimmick HL, Herda TJ (2019) Motor unit action potential amplitudes and firing rates during repetitive muscle actions of the first dorsal interosseous in children and adults. Eur J Appl Physiol 119(4):1007\u0026ndash;1018. https://doi.org/10.1007/s00421-019-04090-0\u003c/li\u003e\n \u003cli\u003eMilner‐Brown HS, Stein RB, Yemm R (1973) The contractile properties of human motor units during voluntary isometric contractions. J Physiol 228(2):285\u0026ndash;306. https://doi.org/10.1113/jphysiol.1973.sp010087\u003c/li\u003e\n \u003cli\u003eMoritani T, deVries HA (1979) Neural factors versus hypertrophy in the time course of muscle strength gain. Am J Phys Med Rehabil 58(3)\u003c/li\u003e\n \u003cli\u003eNawab SH, Chang S-S, De Luca CJ (2010) High-yield decomposition of surface EMG signals. Clin Neurophysiol 121(10):1602\u0026ndash;1615. https://doi.org/10.1016/j.clinph.2009.11.092\u003c/li\u003e\n \u003cli\u003eRitti-Dias RM, Avelar A, Salvador EP, Cyrino ES (2011) Influence of Previous Experience on Resistance Training on Reliability of One-Repetition Maximum Test. J Strength Cond Res 25(5):1418\u0026ndash;1422. https://doi.org/10.1519/JSC.0b013e3181d67c4b\u003c/li\u003e\n \u003cli\u003eRivas CA, Voskuil CC, Hahs-Vaughn DL, Stock MS, Carr JC (2025) Motor unit activity during maximal-intent dynamic muscle actions varies by intensity and sex in healthy adults. J Neurophysiol 134(3):940\u0026ndash;951. https://doi.org/10.1152/jn.00184.2025\u003c/li\u003e\n \u003cli\u003eRutherford OM, Jones DA (1986) The role of learning and coordination in strength training. Eur J Appl Physiol 55(1):100\u0026ndash;105. https://doi.org/10.1007/BF00422902\u003c/li\u003e\n \u003cli\u003eSale D (1988) Neural adaptation to resistance training. Med Sci Sports Exerc 20(5):S135\u0026ndash;S145\u003c/li\u003e\n \u003cli\u003eSterczala AJ, Miller JD, Trevino MA, Dimmick HL, Herda TJ (2018) Differences in the motor unit firing rates and amplitudes in relation to recruitment thresholds during submaximal contractions of the first dorsal interosseous between chronically resistance-trained and physically active men. Appl Physiol Nutr Metab 43(8):759\u0026ndash;768. https://doi.org/10.1139/apnm-2017-0646\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"motor unit, action potential amplitude, firing rate, isotonic, isometric, strength training","lastPublishedDoi":"10.21203/rs.3.rs-9544715/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-9544715/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003ePrevious studies reported increases in strength following a single resistance exercise training (RET) session. The observed gains are believed to be driven by neural mechanisms. It remains unclear the role of motor unit (MU) activation on increases in isometric and isotonic strength following a single RET session. The purpose of the present study was to examine the effects of a single RET session on MU activation during isometric and isotonic muscle actions. In addition, potential differences in MU activity between isometric and isotonic muscle actions will be examined for the first dorsal interosseous muscle (FDI). Thirty-four recreationally active participants (15 males and 19 females, age = 21.25 ± 2.40 years) completed two RET sessions. The sessions were separated by 48 hours. Isometric maximal voluntary contraction (MVC) and isotonic 1-repetition maximum (1RM) tests were completed in each session. MU peak firing rates (FR) were assessed in relation to the corresponding action potential amplitude. Higher MU FRs were observed during isometric MVCs in comparison to isotonic 1RMs (\u003cem\u003eβ\u003c/em\u003e = 13.20, \u003cem\u003ep\u003c/em\u003e \u0026lt; 0.001). However, strength increases and neural adaptations were not observed during RET session 2. The findings suggest there are potential differences in MU activity between isotonic and isometric muscle actions. Additionally, the FDI may be a valuable model to attenuate the reported learning effect following acute RET.\u003c/p\u003e","manuscriptTitle":"Acute Response of Motor Unit Firing Rates During Two Resistance Exercise Training Sessions That Include Isotonic and Isometric Muscle Actions","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-05-11 10:53:56","doi":"10.21203/rs.3.rs-9544715/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"b3668af4-61dd-479b-990e-77820e5cc7a2","owner":[],"postedDate":"May 11th, 2026","published":true,"recentEditorialEvents":[{"type":"editorInvitedReview","content":"","date":"2026-05-19T00:10:41+00:00","index":40,"fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-05-05T08:53:22+00:00","index":39,"fulltext":""},{"type":"reviewerAgreed","content":"289690625311463334691577259702992997861","date":"2026-05-04T02:19:18+00:00","index":38,"fulltext":""},{"type":"reviewerAgreed","content":"16378501801695431160872080060730997064","date":"2026-05-03T18:10:23+00:00","index":36,"fulltext":""},{"type":"reviewerAgreed","content":"288635214646551728779998067515501901552","date":"2026-05-01T22:08:10+00:00","index":32,"fulltext":""},{"type":"reviewerAgreed","content":"294070749762881438086803421723070863436","date":"2026-05-01T13:56:31+00:00","index":29,"fulltext":""},{"type":"reviewersInvited","content":"23","date":"2026-05-01T13:52:52+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2026-04-29T03:32:30+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2026-04-29T03:32:06+00:00","index":"","fulltext":""}],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2026-05-11T10:53:58+00:00","versionOfRecord":[],"versionCreatedAt":"2026-05-11 10:53:56","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-9544715","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-9544715","identity":"rs-9544715","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}