Enhancing corticospinal facilitation from sEMG-triggered paired intermittent theta burst stimulation and neuromuscular electrical stimulation in young and older adults

Enhancing corticospinal facilitation from sEMG-triggered paired intermittent theta burst stimulation and neuromuscular electrical stimulation in young and older adults | 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 Research Article Enhancing corticospinal facilitation from sEMG-triggered paired intermittent theta burst stimulation and neuromuscular electrical stimulation in young and older adults Millie Taylor, Matija Milosevic, Ned Jenkinson, Shin-Yi Chiou This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8077806/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 Background Pairing intermittent theta burst stimulation (iTBS) with neuromuscular electrical stimulation (NMES) can increase corticospinal excitability (CSE) of a relaxed muscle. Whether initiating stimulation with surface electromyographic activity (sEMG-triggered) during an intentional holding task enhances this effect, and whether such facilitation is preserved with ageing, remains unknown. Objective To compare the effects of different paired and unpaired stimulation protocols on corticospinal facilitation in young and older adults, and to examine acute effects on hand function in older adults. Methods Young and older adults received iTBS alone, sEMG-triggered iTBS + NMES, and sEMG-triggered sham iTBS + NMES targeting the abductor pollicis brevis (APB) of the dominant hand on separate occasions. A subset of young adults also received non-triggered iTBS + NMES. Stimulation was delivered while participants held a small cylinder between the thumb and index finger. Motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation were recorded at baseline and up to 30 min post-stimulation. In older adults, hand function was assessed using maximum voluntary contraction (MVC), Nine Hole Peg Test (NHPT), and Box and Block Test (BBT). Results sEMG-triggered iTBS + NMES produced stronger MEP facilitation than iTBS alone or sham iTBS + NMES in both age groups. sEMG-triggered and non-triggered iTBS + NMES produced comparable facilitation in young adults. No group-level improvements were observed in MVC, NHPT, or BBT. Conclusions sEMG-triggered iTBS + NMES reliably enhanced short-term corticospinal facilitation in young and older adults. The findings suggest that intentional motor involvement during stimulation, rather than brief muscle activation per se, contributes to the observed facilitation. electromyography paired associative stimulation motor cortex hand grip Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 1. Introduction The human hand is essential for most activities of daily living, and impaired hand function severely affects independence and quality of life of people. Hand rehabilitation is challenging in part due to the complexity of neural control of the hand, resulting in limited functional recovery (Pollock et al. 2014 ; Harvey et al. 2017 ). Non-invasive neuromodulation techniques, such as paired associative stimulation (PAS), have been shown to enhance corticospinal plasticity and improve hand function by strengthening corticospinal connections (Bunday et al. 2018 ; James et al. 2018 ; Rosso et al. 2022 ; Holopainen et al. 2025 ). Intermittent theta burst stimulation (iTBS) is an efficient rTMS protocol that induces long-term potentiation (LTP)-like effects using subthreshold intensities and short stimulation durations (~ 192 s or 3.2 mins), with facilitation lasting ~ 30 minutes (Huang et al. 2005 ). Cao and colleagues modified iTBS by lowering the stimulus intensity and pairing it with neuromuscular electrical stimulation (NMES). This synchronous protocol extended facilitation beyond iTBS alone, and longer application (~ 10 min) produced more robust after-effects lasting at least 30 min (Cao et al. 2022a ; Cao et al. 2022b ; Cao et al. 2025 ). These findings indicate that peripheral afferent input can enhance and prolong iTBS effects by modulating the sensorimotor cortex state (Chipchase et al. 2011 ). Voluntary contractions can also interact with plasticity induction, sometimes enhancing paired associative stimulation (PAS) Voluntary muscle contractions have been shown to enhance the effects of PAS (Yin et al. 2015 ; Bunday et al. 2018 ) but could abolish the facilitatory effect of iTBS alone (Huang et al. 2008 ; Iezzi et al. 2008 ), possibly due to ongoing neural activity in the sensorimotor cortex interacting with the induced synaptic plasticity-like processes. Clinically, surface EMG (sEMG)-triggered NMES is more effective than untriggered NMES for improving upper-limb impairment in neurological populations (de Kroon et al. 2005 ; Hara 2008 ; Schuhfried et al. 2012 ), likely because proprioceptive and cutaneous afferent feedback occur alongside attempted movements and attentional engagement, driving functional cortical reorganisation (Francisco et al. 1998 ; Cauraugh et al. 2000 ; Hong et al. 2012 ). However, this strategy has not been applied in combining with iTBS or paired iTBS and NMES (iTBS + NMES). Thus, we hypothesised that incorporating intentional motor involvement via sEMG-triggered iTBS + NMES would enhance the efficacy of iTBS + NMES in corticospinal excitability in humans. Ageing has been associated with attenuated responses to PAS and other neuromodulatory protocols (Fathi et al. 2010 ; Bhandari et al. 2016 ; Cirillo et al. 2025 ), likely due to age-related changes in intracortical excitability and sensorimotor integration. Hence, we examined corticospinal excitability of the abductor pollicis brevis (APB) in both young and older adults following sEMG-triggered paired iTBS + NMES, and compare this with sEMG-triggered iTBS sham +NMES. Additionally, we investigated whether sEMG-triggered iTBS + NMES produced greater facilitation of corticospinal excitability compared to non-triggered iTBS + NMES in healthy young adults. 2. Materials and Methods 2.1. Participants This study protocol was approved by the University of Birmingham Research Ethical Committee (ERN_22-1143) and performed in accordance with the ethical principles of the Declaration of Helsinki. Participants were recruited from the local community. Participants were excluded if they had any contraindications to the transcranial magnetic stimulation (TMS), including any neurological or psychiatric condition or disorder (Rossi et al. 2021 ). A total of twenty-two young adults and nineteen older adults took part in the study. All participants provided written informed consent and completed the Edinburgh Handedness Inventory (Oldfield 1971 ) on arrival to provide their handedness (three left-handed in total, others right-handed). 2.2. Electromyographic (EMG) Recordings Muscle activity was recorded using surface EMG (Delsys® Bagnoli-2, Delsys Inc., Boston, MA). Adhesive electrodes (19.8 mm × 35 mm) were placed over the muscle belly and in line with the approximate orientation of muscle fibres of the abductor pollicis brevis (APB; Fig. 1 A lower panel). Placement of surface EMG was following the SENIAM project guidelines (SENIAM, 1999). A ground electrode was placed on the spinus process of the 7th cervical vertebra. 2.3. Transcranial magnetic stimulation (TMS) TMS pulses were delivered via a Magstim Rapid 2 biphasic stimulator (Magstim Co. Ltd., Whitland, UK) through a figure of eight coil (loop diameter: 70 mm), handle pointing backwards and ~ 45° away from the midline (Tremblay et al. 2017 ; Cao et al. 2022a ). The coil was place over the M1 contralateral to the dominant hand to elicit motor evoked potentials (MEPs) in the APB. The optimal position (hotspot) of the coil was determined as the point where the greatest MEP amplitude was elicited in the APB with a given stimulus intensity (Rothwell et al. 1999 ; Rossini et al. 2015 ). Once the hotspot was identified, the coil position for each participant was saved in a neuro-navigation system (Brainsight, Rogue Research, Montreal, Canada); this recorded coil position was used throughout the experiment and at each assessment time-point. Active motor threshold (AMT) of the APB was determined as the lowest stimulus intensity to elicit visible MEPs in 5 out of 10 consecutive trials during an isometric contraction of 10% maximal voluntary contraction (MVC) of the APB (Rossini et al. 2015 ). EMG activity was displayed on a screen at eye level during the AMT assessment, providing visual feedback to help participants produce consistent contractions. For all measurements, a “holding posture” was defined as a posture where the participant holds a small cylindrical peg (7 mm diameter, 32 mm length) lightly between the thumb and index finger in a neutral hand position without producing visible EMG activity in the APB (Fig. 1 A; lower panel). This position was used during MEP assessments, and stimulation periods unless otherwise stated. This holding posture yielded more consistent MEP responses compared to when the hand was fully relaxed during the pilot of the study and was subsequently implemented. TMS intensity was set to evoke MEPs in the APB at peak-to-peak amplitudes of ~ 1 mV. 2.4. Intermittent Theta Burst Stimulation (iTBS) iTBS was applied using a figure-of-eight- coil (70 mm standard coil, Magstim Rapid 2 , Magstim Co. Ltd., Whitland, UK) over the motor representation of the APB in the M1 while participants maintained the same static holding posture of the hand. The iTBS paradigm consisted of three pulses which were delivered at a frequency of 50 Hz (per burst). Bursts were applied every 200 ms at 5 Hz for 2 s (10 bursts) and repeated every 10 s for a total duration of 192 s, resulting in a delivery of 600 pulses (Huang et al. 2005 ; Cao et al. 2022a ). For the sEMG-triggered iTBS + NMES condition, stimulation intensity was applied at 80% of AMT (Huang et al. 2005 ). For the sEMG-triggered sham-iTBS (iTBS sham ) + NMES condition, the coil was flipped 180° along the frontal plane (Chou et al. 2015 ), and stimulation intensity was turned down to 30% of maximal stimulator output (MSO). 2.5. Neuromuscular Electrical Stimulation NMES was administrated using the ODFS Pace stimulator (Odstock Drooped Foot Stimulator Pace, Odstock Medical Ltd., Salisbury, UK). Pulse width and frequency were 300 µs, and 50 Hz, respectively (Cao et al. 2022a ). To stimulate the APB, two disposable self-adhesive surface electrodes (3.5 cm diameter, round) were placed on the inside of the wrist over the median nerve (Fig. 1 A; upper panel) (Mangold et al. 2005 ). Electrode placement was checked to ensure it elicited visible thumb abduction. The NMES intensity (mA) was set at 120% of the threshold that elicited a visible thumb abduction, determined individually for each participant at each session (Cao et al. 2022a ). 2.6. Experimental Procedures Figure 2 illustrates the study design. During each session, participants were seated comfortably on a chair, with both hands resting on a pillow with palms facing up (Fig. 1 A; upper panel). All the procedures were conducted on the dominant APB (three participants were left-handed and the rest were right-handed). Three brief MVCs (~ 3 seconds) of the APB was recorded at the start of each session for all participants. Participants were instructed to perform thumb abduction against the resistance of an experimenter while their forearm was affixed to minimise the movement from the upper arm with verbal encouragement throughout. There was a least 45 seconds break between trials. To compare the effects of the stimulation protocols, MEPs of the APB were assessed before (pre) and immediately (post0), 10 min (post10), 20 min (post20) and 30 min (post30) after the stimulation (Fig. 2 B) in young participants. For the older participants, MEPs were assessed before (pre) and 10 min (post10), 20 min (post20) and 30 min (post30) after the stimulation. MEP was not assessed immediately after stimulation in the older group to allow time for functional assessments (see below for details). A total of 10 MEPs were recorded at each time point. To initiate stimulation, participants briefly contracted the APB by squeezing the peg until EMG amplitude exceeded 10% of MVC (no visual feedback provided; Fig. 1 B), after which they returned to the holding posture (Fig. 1 A; lower panel) for the remainder of the stimulation period; this was defined as sEMG-triggered protocol. Screening Session All recruited participants underwent a screening session where they received iTBS only to control for variability within individuals’ responses to iTBS (Hamada et al. 2013 ). Participants who showed an increase in MEP amplitude at a post0 time point post-stim after the iTBS was categorised as responders to iTBS and enrolled in the main experiment. Responders were then invited back to take part in the main experiment which was scheduled at least 6 days following the screening session. Those who had no change or a decreased MEP amplitude after stimulation were defined as non-responders to iTBS and did not continue to the main or additional experiment. Main experiment All returning participants took part in the following sessions: 1) sEMG-triggered iTBS + NMES and 2) sEMG-triggered iTBS sham +NMES in a random order (at least 6 days apart). iTBS and NMES were delivered synchronously (Cao et al. 2022a ; Cao et al. 2025 ). Stimulation ran for 2 s, and then was off for 8 s, and this 10 s cycle was repeated 10 times. For the paired condition, participants received concurrent iTBS and NMES. For the sham condition, participants received NMES with the sham iTBS (see the description of the sham condition above). To minimise potential effects of anticipation, participants were explained that they were undergoing two very similar experimental procedures. To determine any acute effects of sEMG-triggered iTBS + NMES on hand dexterity and upper-limb function, the following functional tests were carried out: MVC, Nine Hole Peg Test (NHPT) and Box and Block Test (BBT). These tests were only conducted in older adults as we did not expect any stimulation-induced improvement in hand function in healthy young adults (Desai et al. 2025 ). These tests were conducted prior to, immediately after, and 30 minutes after the stimulation sessions. For the NHPT, participants sat in the same position as during stimulation and were instructed to use their dominant hand to place nine pegs into the holes of the pegboard as quickly as possible. The total time to insert all pegs was recorded (Wang et al. 2015 ). For the BBT, participants, in the same seated position, moved as many blocks as possible from one compartment of the box to the other within 30 seconds, ensuring each transfer crossed the partition without throwing the blocks. The total number of blocks transferred was recorded (Desrosiers et al. 1994 ). Additional experiment An additional experiment was performed in 12 healthy young participants who underwent the following sessions in random: 1) sEMG-triggered iTBS + NMES and 2) non-triggered iTBS + NMES (Fig. 2 A). The sEMG-triggered iTBS + NMES was the same as described above. For non-triggered iTBS + NMES, the stimulation protocol was initiated by the experimenter. 2.7. Data Analysis Neurophysiological data was analysed using Signal software v. 6 (Cambridge Electronic Design, UK). All data were visually inspected and frames were excluded from analysis if there was no response (no clear visible MEP from background EMG activity) or electrical interference, or abnormally high EMG activity (from sudden voluntary activation of movement) was present. Peak-to-peak amplitudes of average MEP recorded from each time point were measured and expressed as a percentage of the baseline average MEP amplitude. Pre-stimulus EMG was rectified and calculated as a mean amplitude in a 100 ms window prior to the stimulus. A total of 5.6 ± 3.2% trials in which mean pre-stimulus rectified EMG activity exceeded 2 standard deviations (SD) of the mean average rectified EMG were excluded from further analysis. MVC data was rectified, and mean amplitudes were calculated for APB muscles using a 500 ms window around the peak amplitude. NHPT was measured by time to complete the test, and BBT was measured by the number of blocks moved. 2.8. Statistical Analysis All statistical analysis was performed using SPSS (v. 29, IBM Corp., Armonk, NY, USA). Normal distributions were tested by the Shapiro–Wilk Test and sphericity tested using Mauchly’s Test. When data were not normally distributed (p < 0.05), non-parametric tests were used. Repeated measures (RM) ANOVAs were applied to evaluate the main effect of time on MEP amplitudes and pre-stimulus mean EMG amplitude obtained from the screening sessions to confirm the effects of iTBS in our participants. A mixed-model, two-way RM ANOVA was employed to test the within-subject effect of condition (sEMG-triggered iTBS + NMES vs. sEMG-triggered iTBS sham +NMES) and time (pre, post10, post20, and post30 minutes after the stimulation), and the between-subject effect of group (younger vs. older) on MEP amplitudes. To determine any effect of the stimulation on hand function, a two-way RM ANOVA was used to test any main effect of condition (sEMG-triggered iTBS + NMES vs. sEMG-triggered iTBS sham +NMES) and time (pre, post0, and post30 min after the stimulation) on MVCs, NHPT, and BBT in older participants. An independent t-test was performed to compare APB MVCs between young and older participants. Furthermore, two-way RM ANOVAs were performed in the additional experiment to evaluate the effect of condition (sEMG-triggered iTBS + NMES vs. non-triggered iTBS + NMES) and time (pre, post0, post10, post20, and post30 minutes after the stimulation) on MEP amplitudes and pre-stimulus EMG. Bonferroni post hoc tests were used to test for significant comparisons. Paired-t tests were employed to compare stimulus parameters used in different conditions. Significance was set at p < 0.05. Effect sizes are presented as partial eta-squared (η²ₚ) when appropriate, group data are presented as the mean ± SD in the text. 3. Results 3.1 Participant characteristics Of the 22 young and 19 older participants recruited, 20 young participants (8 males; 22 ± 2 years; 1 left-handed) and 16 older participants (7 males; 75 ± 4 years; 2 left-handed) were classified as responders, who showed increased MEP amplitudes after the iTBS alone, as summarized in Fig. 2 . Stimulus parameters are summarised in Table 1 . AMTs of the APB were similar between young (57 ± 6% MSO) and older (57 ± 6% MSO) participants (F(1,34) = 0.08, p = 0.78). TMS intensities used to elicit APB MEPs were the same across conditions (F(2,68) = 0.60, p = 0.55) and between groups (F(2,68) = 0.36, p = 0.70). NMES intensities did not differ between the sEMG-triggered iTBS + NMES and sEMG-triggered iTBS sham +NMES conditions (F(1,34) = 3.01; p = 0.09) or groups (F(1,34) = 0.24, p = 0.63). MVCs were the same across sessions (F(2,68) = 0.33, p = 0.72); the average APB MVC was greater in young participants (0.97 ± 0.22 mV) than in older participants (0.75 ± 0.19 mV; t(34) = − 3.22, p = 0.003). Table 1 Stimulus parameters and maximal voluntary contractions (MVCs) Screening iTBS + NMES iTBSsham + NMES Young Older Young Older Young Older AMT (%MSO) 57 ± 6 57 ± 5 57 ± 6 57 ± 6 n/a n/a NMES intensity (mA) n/a n/a 16.4 ± 6.5 22.4 ± 8.3 17.3 ± 7.2 24.5 ± 9.5 TMS intensity (%) 75 ± 10 74 ± 9 75 ± 9 76 ± 12 75 ± 10 75 ± 14 MVC (mV) 0.96 ± 0.23 0.78 ± 0.23 0.97 ± 0.25 0.70 ± 0.21 0.98 ± 0.33 0.72 ± 0.26 MSO: maximal stimulator output; NMES: neuromuscular electrical stimulation; TMS: transcranial magnetic stimulation; iTBS: intermittent theta burst stimulation. AMT was not assessed in the sham condition as all participants received iTBS at 30%MSO. NMES was not applied in screening. 3.2 Effects of the iTBS on corticospinal excitability A main effect of time was observed on MEP amplitudes (F(4,124) = 8.94, p < 0.001, η² = 0.22), with no group × time interaction (F(2,24) = 0.48, p = 0.63), indicating that young and older participants responded similarly to iTBS alone. Post-hoc tests showed significant MEP facilitation at all post-stimulation time points compared with baseline in both groups (all p 0.05). Figure 3 . Effects of iTBS on motor evoked potential (MEP). Group mean data showing MEP amplitudes of the abductor pollicis brevis (APB) before (pre) and after (post-0, post-10, post-20, and post-30) stimulation in young (n = 20) and older adults (n-16). Individual data are presented. The horizontal line within the box represents the median, and outer edges of the box represent the 25th and 75th percentiles. The upper and lower whiskers extend to 1.5 times the interquartile range. *p < 0.05 compared to the pre-stimulation. 3.3 Effects of EMG-triggered paired iTBS/NMES on corticospinal excitability Representative MEP traces from a young participant and an older participant illustrate MEP amplitudes increased to a greater extent after sEMG-triggered iTBS + NMES, compared with sEMG-triggered iTBS sham +NMES (Fig. 4 A). Group analysis revealed significant effects of condition (F(1,32) = 17.97, p < 0.001, η² = 0.36), time (F(3,96) = 25.16, p < 0.001, η² = 0.44), and their interaction (F(3,96) = 8.01, p < 0.001, η² = 0.20) on MEP amplitudes. There were no significant group × condition (F(1,32) = 3.44, p = 0.07) or group × time (F(3,96) = 0.66, p = 0.58) interactions. Post-hoc comparisons demonstrated that sEMG-triggered iTBS + NMES induced significantly greater MEP facilitation than sEMG-triggered iTBS sham +NMES at 10 min (F(1,33) = 15.93, p < 0.001, η² = 0.33), 20 min (F(1,33) = 16.28, p 0.05). 3.4 Comparisons between sEMG-triggered iTBS and NMES and iTBS alone on corticospinal excitability We found that APB MEP at 10-min post-stimulation was greater in the sEMG-triggered iTBS + NMES (188.46 ± 82.73% baseline MEP) condition than in the iTBS alone, with a moderate effect size (155.96 ± 71.09%; F(1,32) = 3.98; p = 0.06; η² = 0.11; Fig. 5 A). Pre-stimulus EMG was consistent between the conditions and across time points (all p > 0.05). 3.5 Comparisons between sEMG-triggered iTBS + NMES, non- triggered iTBS + NMES, and iTBS on corticospinal excitability There was an effect of time on MEP amplitudes (F(2,88) = 16.65, p < 0.001; η² = 0.60); MEP amplitudes were greater at post-stimulation than at pre-stimulation (all p < 0.05). Additionally, MEP amplitudes were greater after sEMG-triggered iTBS + NMES than after iTBS with a large effect size (F(1,11) = 4.53, p = 0.06; η² = 0.29; Fig. 5 B). There was no interaction between conditions and time (F(1,11) = 0.89, p = 0.53; η² = 0.08) No difference was in pre-stimulus EMG across conditions or time points (all p > 0.05). 3.6 Effects of the iTBS and NMES on hand motor function in older adults One MVC value from a participant was excluded due to technical error. Among 15 older participants, there was an effect of time on APB MVCs (F(2,28) = 6.96, p = 0.004) but no effect of condition (F(1,14) = 0.00, p = 0.98) or time × condition interaction (F(2,28) = 1.25, p = 0.30). MVC of the APB were lower at 30 min after iTBS sham +NMES (Z = − 2.50, p = 0.024; Table 2 ). Table 2 Functional test in older participants. sEMG-triggered iTBS + NMES sEMG-triggered iTBS sham +NMES Test Baseline Post0 Post30 Baseline Post0 Post30 NHPT (seconds) 16.9 ± 5.2 16.1 ± 3.5 15.0 ± 3.9 17.7 ± 7.4 16.6 ± 4.5 16.5 ± 6.7 BBT (number) 22 ± 7 23 ± 7 23 ± 7 24 ± 7 24 ± 6 25 ± 7 MVC (mV) 0.71 ± 0.21 0.68 ± 0.25 0.68 ± 0.26 0.72 ± 0.24 0.67 ± 0.21 0.63 ± 0.21 NHPT: nine-hole peg test; BBT: box and block test; MVC: maximal voluntary contraction. Data are presented as mean ± SD. All 16 older participants completed the NHPT; 10 (63%) improved after iTBS + NMES and 7 (44%) after iTBS sham +NMES. Fourteen older participants completed the BBT due to the availability of the equipment. Twelve (67%) improved after iTBS + NMES and seven (47%) after iTBS sham +NMES. However, the amount of improvement did not reach to the statistically significant level (all p > 0.05; Table 2 ). 4. Discussion We show that sEMG-triggered iTBS + NMES, delivered during a holding posture, facilitated corticospinal excitability in the APB muscle of both young and older adults. Compared with sham, it induced greater MEP facilitation lasting up to 30 min, suggesting enhanced corticospinal synaptic efficacy. The magnitude and duration of facilitation were similar across age groups, indicating preserved short-term associative plasticity with aging. The facilitatory effect was stronger with sEMG-triggered iTBS + NMES than with iTBS alone (moderate effect size). In young adults, sEMG-triggered and non-triggered iTBS + NMES produced comparable facilitation, both exceeding iTBS alone (moderate-to-strong effect size). Despite these neurophysiological effects, no measurable improvements in hand dexterity were observed after a single session, suggesting that acute facilitation does not immediately translate into functional gains. 4.1. Corticospinal facilitation after synchronous iTBS and NMES in both young and older adults Motor cortical plasticity depends on the precise timing of converging sensory and motor activity, a principle described by spike-timing-dependent plasticity (Markram et al. 1997 ; Müller-Dahlhaus et al. 2010 ). In humans, this has been modelled using PAS, which typically involves time-locked pairing of TMS with peripheral nerve stimulation (PNS). Some studies have suggested that the effectiveness of PAS depends on carefully individualized interstimulus intervals (ISIs), calculated from central and peripheral conduction times (Bunday and Perez 2012 ; Urbin et al. 2017 ). Others, however, have reported a physiologically relevant window (~ 40–60 ms) in which synaptic efficacy can still be strengthened without precious individualised ISIs (Baranyi and Feher 1981 ; Stefan et al. 2000 ; Taylor and Martin 2009 ). Although our protocol did not individually calibrate conduction times, we still observed facilitation of the corticospinal excitability following synchronous iTBS + NMES. This is consistent with the evidence suggesting that strict millisecond precision may not be essential when paired stimuli converge on overlapping neural populations (Baranyi and Feher 1981 ; Stefan et al. 2000 ) and in keeping with prior work using non-invasive iTBS (Cao et al., 2022a , b , 2025 ) and focused ultrasound stimulation (Desai et al. 2025 ) combined with functional electrical on corticospinal excitability in healthy adults. Taken together, participants constantly exhibited greater MEP facilitation after iTBS + NMES than after iTBS alone, suggesting that afferent volleys provided an additional effect on corticospinal facilitation. Previous studies reporting that voluntary muscle contractions abolished the facilitatory effects of iTBS typically employed sustained contractions (Huang et al. 2008 ; Iezzi et al. 2008 ). Conversely, our protocol used a brief, transient contraction only at the beginning of the stimulation period to trigger the paired iTBS + NMES. This difference likely explains why the brief contraction in our study did not interfere with the facilitatory effects. Additionally, our facilitatory effects exceeded those reported by prior work which paired iTBS with NMES in relaxed forearm muscles (Cao et al. 2022a ). The enhanced effect in our study is unlikely to be due to the brief muscle contraction used to trigger the paired stimulation, as sEMG-triggered iTBS + NMES did not yield greater facilitation than non-triggered iTBS + NMES in our study. Research has shown that even minimal tonic contractions can engage the corticospinal tract and enhance LTP-like plasticity (Turton and Lemon 1999 ). Therefore, we propose that the intentional motor involvement, i.e., maintaining a holding posture by lightly holding a small peg between the thumb and index finger, may explain the longer-lasting facilitation observed in our study compared to previous work. High-frequency and frequency-matched pairing may also contribute to the magnitude and persistence of facilitation. We paired 50 Hz NMES with 50 Hz iTBS bursts, a frequency known to enhance motor cortical excitability (Huang et al. 2005 ; Chou et al. 2015 ), mimic natural motor unit firing in small hand muscles (Farina et al. 2008 ), and recruit central pathways (Chipchase et al. 2011 ). Research reported that 10 Hz rTMS paired with 10 Hz sensory stimulation facilitated corticospinal excitability, whereas mismatched frequencies (10 Hz rTMS with 3 Hz stimulation) did not (Zhong et al. 2021 ). These features likely maximised the probability of pre- and postsynaptic coactivation, producing the observed LTP-like effects. Our findings extend prior reports that iTBS produces comparable facilitation in young and older adults (Dickins et al. 2015 ; Cristancho et al. 2020 ; Slan et al. 2024 ), in contrast to studies using single-pulse PAS, which reported less effective to elicit facilitation in older participants (Fathi et al. 2010 ; Cirillo et al. 2025 ). The preserved facilitation in older adults observed in our study may reflect the additional engagement of the corticospinal pathways, even if minimal, by maintaining the peg between the thumb and index finger during the stimulation. Functionally, no significant group-level improvements were seen in hand dexterity after a single session. Most older participants in our study performed within or above the normative range at baseline, likely creating a ceiling effect. Nevertheless, a higher proportion of participants improved their NHPT and BBT scores after iTBS + NMES compared with iTBS sham +NMES, suggesting that repeated sessions or application in individuals with lower baseline performance could produce measurable behavioural benefits. 4.2. Functional implications and limitations Our results highlight sEMG-triggered iTBS + NMES as a practical, robust PAS protocol with translational potential for neurorehabilitation. Notably, our approach did not require individualised ISI calibration, was quick (~ 3 mins of paired stimulation), and was delivered with minimal voluntary effort, yet still produced consistent facilitation across age groups. These features make the protocol particularly suitable for clinical application in individuals with impaired hand function, such as those recovering from stroke or spinal cord injury, where voluntary movement may be limited. Future investigations are required to establish the effects of multi-session interventions combined with task-specific training for functional motor recovery in clinical populations. This study has several limitations. First, the older adult cohort had near-normal hand dexterity, which may have limited our ability to detect functional improvements. Including larger and more diverse samples and individuals with impaired hand function would enhance the robustness and generalisability of the findings. Second, we did not assess corticospinal excitability during complete hand relaxation, nor did we quantify the effort required to maintain the static hand posture. As a result, we cannot definitively attribute the prolonged facilitation to the inclusion of the static motor task. Finally, the relative contributions of cortical versus spinal mechanisms remain unclear. Future studies incorporating neurophysiological measures could help elucidate the underlying mechanisms and identify the conditions under which this protocol is most effective. 5. Conclusions A single session of sEMG-triggered iTBS + NMES, delivered during a simple holding task, produced robust corticospinal facilitation in both young and older adults, lasting ~ 30 min longer than NMES or iTBS alone after only 3 min of stimulation. This effect likely reflects the convergence of cortical and peripheral inputs with matched high-frequency stimulation, engaging descending motor pathways. These findings highlight sEMG-triggered iTBS + NMES as a rapid and promising method to induce plasticity in corticospinal pathways of the hand. Declarations Acknowledgement: We are grateful to all the study participants for their time and generosity. We also thank the Central for Human Brain Health for kindly providing lab space and equipment free of charge. Funding: This project is funded by the Royal Society IES\R3\223125 - International Exchanges 2022 Round 3 (MM and SC). The views expressed are those of the author(s) and not necessarily those of the funder. Data availability: The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request. Conflict of interest: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement: Millie Taylor: Data curation, Formal analysis, Investigation, Writing – original draft. Matija Milosevic: Conceptualization, Funding acquisition, Methodology, Writing – review and editing. Ned Jenkinson: Conceptualization, Methodology, Supervision, Writing – review and editing. 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07:14:12","extension":"html","order_by":21,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":141094,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-8077806/v1/17dc5cccefdfb53c74102ed3.html"},{"id":96693754,"identity":"f001f77a-2a93-40ac-a1b3-bf6b0f4cf971","added_by":"auto","created_at":"2025-11-25 07:14:11","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":319179,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003e(A) \u003c/strong\u003eExperimental setup and electrode placement (upper panel) with the holding posture (lower panel). (B) Paradigm of surface EMG-triggered (sEMG-triggered) paired intermittent theta burst stimulation (iTBS) and neuromuscular electrical stimulation (NMES). Stimulation is initiated when abductor pollicis brevis (APB) activity exceeds 10% of maximal voluntary contraction (MVC), after which participants return to the holding posture.\u003c/p\u003e","description":"","filename":"Figure1.png","url":"https://assets-eu.researchsquare.com/files/rs-8077806/v1/8bedd0b1dfe6493f0d3cc3bc.png"},{"id":96711395,"identity":"800d1d04-e375-477d-bdec-54273b5ec50e","added_by":"auto","created_at":"2025-11-25 10:11:57","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":181779,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eStudy diagram. (A) \u003c/strong\u003eExperimental setup.\u003cstrong\u003e (B) \u003c/strong\u003eStimulus paradigm. *Older participants undergo functional tests but no TMS assessment immediately after the completion of the stimulation. \u003csup\u003e$\u003c/sup\u003eOlder participants undergo both functional tests and TMS assessment at 30 min post-stimulation. Young participants undergo TMS assessments only at all time points.\u003c/p\u003e","description":"","filename":"Figure2.png","url":"https://assets-eu.researchsquare.com/files/rs-8077806/v1/86a3ee55dcaca9fd87f49bf3.png"},{"id":96711156,"identity":"2e450332-2c47-4d6c-99ed-4c059e2c86cb","added_by":"auto","created_at":"2025-11-25 10:11:43","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":215059,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eEffects of iTBS on motor evoked potential (MEP). \u003c/strong\u003eGroup mean data showing MEP amplitudes of the abductor pollicis brevis (APB) before (pre) and after (post-0, post-10, post-20, and post-30) stimulation in young (n=20) and older adults (n-16). Individual data are presented. The horizontal line within the box represents the median, and outer edges of the box represent the 25th and 75th percentiles. The upper and lower whiskers extend to 1.5 times the interquartile range. *p \u0026lt; 0.05 compared to the pre-stimulation.\u003c/p\u003e","description":"","filename":"Figure3.png","url":"https://assets-eu.researchsquare.com/files/rs-8077806/v1/7cda5e2edf621a623555893d.png"},{"id":96693762,"identity":"3742edca-7445-4d13-a573-91557ee6bce0","added_by":"auto","created_at":"2025-11-25 07:14:11","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":582189,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eMotor evoked potential (MEP) amplitudes in iTBS+NMES and iTBS\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003esham\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e+NMES.\u003c/strong\u003e\u0026nbsp; \u003cstrong\u003e(A)\u003c/strong\u003e MEP traces recorded from abductor pollicis brevis (APB) of a representative young participant and an older participant. Note that MEP amplitudes are greater after s-EMG triggered iTBS+NMES than after sEMG-triggered iTBS\u003csub\u003esham\u003c/sub\u003e+NMES in both participants. \u003cstrong\u003e(B)\u003c/strong\u003e Group data from young participants. \u003cstrong\u003e(C)\u003c/strong\u003e Group data from older participants. Individual data are presented. The horizontal line within the box represents the median, and outer edges of the box represent the 25th and 75th percentiles. The upper and lower whiskers extend to 1.5 times the interquartile range. *p \u0026lt; 0.05 compared to the pre-stimulation. \u003csup\u003e§\u003c/sup\u003ep \u0026lt; 0.05 compared between the conditions.\u003c/p\u003e","description":"","filename":"Figure4.png","url":"https://assets-eu.researchsquare.com/files/rs-8077806/v1/43e9a01cb0723060710fd4d8.png"},{"id":96710942,"identity":"69d58b7c-4640-455a-ad44-cfa1124681d1","added_by":"auto","created_at":"2025-11-25 10:11:26","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":139801,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eMotor evoked potentials (MEPs) across conditions. (A)\u003c/strong\u003eGroup data of young and older participants from iTBS alone and sEMG-triggered iTBS+NMES. \u003cstrong\u003e(B)\u003c/strong\u003e Group data of young participants (n=12) from iTBS, non-triggered iTBS+NMES and sEMG-triggered iTBS+NMES in young adults (n=12).\u003cstrong\u003e \u003c/strong\u003eError bars indicating standard errors of mean are presented. η²: partial eta-square.\u003c/p\u003e","description":"","filename":"Figure5.png","url":"https://assets-eu.researchsquare.com/files/rs-8077806/v1/6f4887ec36f9630bed425f82.png"},{"id":98436054,"identity":"9dee3a1d-2b21-4d83-af8a-3375e1e83244","added_by":"auto","created_at":"2025-12-17 16:54:49","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2312561,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8077806/v1/9c413df0-689f-434a-af5d-831fe62a0b1a.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Enhancing corticospinal facilitation from sEMG-triggered paired intermittent theta burst stimulation and neuromuscular electrical stimulation in young and older adults","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eThe human hand is essential for most activities of daily living, and impaired hand function severely affects independence and quality of life of people. Hand rehabilitation is challenging in part due to the complexity of neural control of the hand, resulting in limited functional recovery (Pollock et al. \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Harvey et al. \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). Non-invasive neuromodulation techniques, such as paired associative stimulation (PAS), have been shown to enhance corticospinal plasticity and improve hand function by strengthening corticospinal connections (Bunday et al. \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; James et al. \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Rosso et al. \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Holopainen et al. \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2025\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eIntermittent theta burst stimulation (iTBS) is an efficient rTMS protocol that induces long-term potentiation (LTP)-like effects using subthreshold intensities and short stimulation durations (~\u0026thinsp;192 s or 3.2 mins), with facilitation lasting\u0026thinsp;~\u0026thinsp;30 minutes (Huang et al. \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2005\u003c/span\u003e). Cao and colleagues modified iTBS by lowering the stimulus intensity and pairing it with neuromuscular electrical stimulation (NMES). This synchronous protocol extended facilitation beyond iTBS alone, and longer application (~\u0026thinsp;10 min) produced more robust after-effects lasting at least 30 min (Cao et al. \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2022a\u003c/span\u003e; Cao et al. \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2022b\u003c/span\u003e; Cao et al. \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). These findings indicate that peripheral afferent input can enhance and prolong iTBS effects by modulating the sensorimotor cortex state (Chipchase et al. \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). Voluntary contractions can also interact with plasticity induction, sometimes enhancing paired associative stimulation (PAS)\u003c/p\u003e\u003cp\u003eVoluntary muscle contractions have been shown to enhance the effects of PAS (Yin et al. \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Bunday et al. \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2018\u003c/span\u003e) but could abolish the facilitatory effect of iTBS alone (Huang et al. \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2008\u003c/span\u003e; Iezzi et al. \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2008\u003c/span\u003e), possibly due to ongoing neural activity in the sensorimotor cortex interacting with the induced synaptic plasticity-like processes. Clinically, surface EMG (sEMG)-triggered NMES is more effective than untriggered NMES for improving upper-limb impairment in neurological populations (de Kroon et al. \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2005\u003c/span\u003e; Hara \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2008\u003c/span\u003e; Schuhfried et al. \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2012\u003c/span\u003e), likely because proprioceptive and cutaneous afferent feedback occur alongside attempted movements and attentional engagement, driving functional cortical reorganisation (Francisco et al. \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e1998\u003c/span\u003e; Cauraugh et al. \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2000\u003c/span\u003e; Hong et al. \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). However, this strategy has not been applied in combining with iTBS or paired iTBS and NMES (iTBS\u0026thinsp;+\u0026thinsp;NMES). Thus, we hypothesised that incorporating intentional motor involvement via sEMG-triggered iTBS\u0026thinsp;+\u0026thinsp;NMES would enhance the efficacy of iTBS\u0026thinsp;+\u0026thinsp;NMES in corticospinal excitability in humans.\u003c/p\u003e\u003cp\u003eAgeing has been associated with attenuated responses to PAS and other neuromodulatory protocols (Fathi et al. \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Bhandari et al. \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Cirillo et al. \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2025\u003c/span\u003e), likely due to age-related changes in intracortical excitability and sensorimotor integration. Hence, we examined corticospinal excitability of the abductor pollicis brevis (APB) in both young and older adults following sEMG-triggered paired iTBS\u0026thinsp;+\u0026thinsp;NMES, and compare this with sEMG-triggered iTBS\u003csub\u003esham\u003c/sub\u003e+NMES. Additionally, we investigated whether sEMG-triggered iTBS\u0026thinsp;+\u0026thinsp;NMES produced greater facilitation of corticospinal excitability compared to non-triggered iTBS\u0026thinsp;+\u0026thinsp;NMES in healthy young adults.\u003c/p\u003e"},{"header":"2. Materials and Methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003e2.1. Participants\u003c/h2\u003e\u003cp\u003e This study protocol was approved by the University of Birmingham Research Ethical Committee (ERN_22-1143) and performed in accordance with the ethical principles of the Declaration of Helsinki. Participants were recruited from the local community. Participants were excluded if they had any contraindications to the transcranial magnetic stimulation (TMS), including any neurological or psychiatric condition or disorder (Rossi et al. \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). A total of twenty-two young adults and nineteen older adults took part in the study. All participants provided written informed consent and completed the Edinburgh Handedness Inventory (Oldfield \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e1971\u003c/span\u003e) on arrival to provide their handedness (three left-handed in total, others right-handed).\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\u003ch2\u003e2.2. Electromyographic (EMG) Recordings\u003c/h2\u003e\u003cp\u003eMuscle activity was recorded using surface EMG (Delsys\u0026reg; Bagnoli-2, Delsys Inc., Boston, MA). Adhesive electrodes (19.8 mm \u0026times; 35 mm) were placed over the muscle belly and in line with the approximate orientation of muscle fibres of the abductor pollicis brevis (APB; Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eA lower panel). Placement of surface EMG was following the SENIAM project guidelines (SENIAM, 1999). A ground electrode was placed on the spinus process of the 7th cervical vertebra.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\u003ch2\u003e2.3. Transcranial magnetic stimulation (TMS)\u003c/h2\u003e\u003cp\u003eTMS pulses were delivered via a Magstim Rapid\u003csup\u003e2\u003c/sup\u003e biphasic stimulator (Magstim Co. Ltd., Whitland, UK) through a figure of eight coil (loop diameter: 70 mm), handle pointing backwards and ~\u0026thinsp;45\u0026deg; away from the midline (Tremblay et al. \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2017\u003c/span\u003e; Cao et al. \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2022a\u003c/span\u003e). The coil was place over the M1 contralateral to the dominant hand to elicit motor evoked potentials (MEPs) in the APB. The optimal position (hotspot) of the coil was determined as the point where the greatest MEP amplitude was elicited in the APB with a given stimulus intensity (Rothwell et al. \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e1999\u003c/span\u003e; Rossini et al. \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). Once the hotspot was identified, the coil position for each participant was saved in a neuro-navigation system (Brainsight, Rogue Research, Montreal, Canada); this recorded coil position was used throughout the experiment and at each assessment time-point. Active motor threshold (AMT) of the APB was determined as the lowest stimulus intensity to elicit visible MEPs in 5 out of 10 consecutive trials during an isometric contraction of 10% maximal voluntary contraction (MVC) of the APB (Rossini et al. \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). EMG activity was displayed on a screen at eye level during the AMT assessment, providing visual feedback to help participants produce consistent contractions.\u003c/p\u003e\u003cp\u003eFor all measurements, a \u0026ldquo;holding posture\u0026rdquo; was defined as a posture where the participant holds a small cylindrical peg (7 mm diameter, 32 mm length) lightly between the thumb and index finger in a neutral hand position without producing visible EMG activity in the APB (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eA; lower panel). This position was used during MEP assessments, and stimulation periods unless otherwise stated. This holding posture yielded more consistent MEP responses compared to when the hand was fully relaxed during the pilot of the study and was subsequently implemented. TMS intensity was set to evoke MEPs in the APB at peak-to-peak amplitudes of ~\u0026thinsp;1 mV.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\u003ch2\u003e2.4. Intermittent Theta Burst Stimulation (iTBS)\u003c/h2\u003e\u003cp\u003eiTBS was applied using a figure-of-eight- coil (70 mm standard coil, Magstim Rapid\u003csup\u003e2\u003c/sup\u003e, Magstim Co. Ltd., Whitland, UK) over the motor representation of the APB in the M1 while participants maintained the same static holding posture of the hand. The iTBS paradigm consisted of three pulses which were delivered at a frequency of 50 Hz (per burst). Bursts were applied every 200 ms at 5 Hz for 2 s (10 bursts) and repeated every 10 s for a total duration of 192 s, resulting in a delivery of 600 pulses (Huang et al. \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2005\u003c/span\u003e; Cao et al. \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2022a\u003c/span\u003e). For the sEMG-triggered iTBS\u0026thinsp;+\u0026thinsp;NMES condition, stimulation intensity was applied at 80% of AMT (Huang et al. \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2005\u003c/span\u003e). For the sEMG-triggered sham-iTBS (iTBS\u003csub\u003esham\u003c/sub\u003e)\u0026thinsp;+\u0026thinsp;NMES condition, the coil was flipped 180\u0026deg; along the frontal plane (Chou et al. \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2015\u003c/span\u003e), and stimulation intensity was turned down to 30% of maximal stimulator output (MSO).\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\u003ch2\u003e2.5. Neuromuscular Electrical Stimulation\u003c/h2\u003e\u003cp\u003eNMES was administrated using the ODFS Pace stimulator (Odstock Drooped Foot Stimulator Pace, Odstock Medical Ltd., Salisbury, UK). Pulse width and frequency were 300 \u0026micro;s, and 50 Hz, respectively (Cao et al. \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2022a\u003c/span\u003e). To stimulate the APB, two disposable self-adhesive surface electrodes (3.5 cm diameter, round) were placed on the inside of the wrist over the median nerve (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eA; upper panel) (Mangold et al. \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2005\u003c/span\u003e). Electrode placement was checked to ensure it elicited visible thumb abduction. The NMES intensity (mA) was set at 120% of the threshold that elicited a visible thumb abduction, determined individually for each participant at each session (Cao et al. \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2022a\u003c/span\u003e).\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\u003ch2\u003e2.6. Experimental Procedures\u003c/h2\u003e\u003cp\u003eFigure \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e illustrates the study design. During each session, participants were seated comfortably on a chair, with both hands resting on a pillow with palms facing up (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eA; upper panel). All the procedures were conducted on the dominant APB (three participants were left-handed and the rest were right-handed). Three brief MVCs (~\u0026thinsp;3 seconds) of the APB was recorded at the start of each session for all participants. Participants were instructed to perform thumb abduction against the resistance of an experimenter while their forearm was affixed to minimise the movement from the upper arm with verbal encouragement throughout. There was a least 45 seconds break between trials.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eTo compare the effects of the stimulation protocols, MEPs of the APB were assessed before (pre) and immediately (post0), 10 min (post10), 20 min (post20) and 30 min (post30) after the stimulation (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eB) in young participants. For the older participants, MEPs were assessed before (pre) and 10 min (post10), 20 min (post20) and 30 min (post30) after the stimulation. MEP was not assessed immediately after stimulation in the older group to allow time for functional assessments (see below for details). A total of 10 MEPs were recorded at each time point.\u003c/p\u003e\u003cp\u003e To initiate stimulation, participants briefly contracted the APB by squeezing the peg until EMG amplitude exceeded 10% of MVC (no visual feedback provided; Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eB), after which they returned to the holding posture (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eA; lower panel) for the remainder of the stimulation period; this was defined as sEMG-triggered protocol.\u003c/p\u003e\u003cp\u003e\u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003eScreening Session\u003c/span\u003e\u003c/p\u003e\u003cp\u003eAll recruited participants underwent a screening session where they received iTBS only to control for variability within individuals\u0026rsquo; responses to iTBS (Hamada et al. \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2013\u003c/span\u003e). Participants who showed an increase in MEP amplitude at a post0 time point post-stim after the iTBS was categorised as responders to iTBS and enrolled in the main experiment. Responders were then invited back to take part in the main experiment which was scheduled at least 6 days following the screening session. Those who had no change or a decreased MEP amplitude after stimulation were defined as non-responders to iTBS and did not continue to the main or additional experiment.\u003c/p\u003e\u003cp\u003e\u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003eMain experiment\u003c/span\u003e\u003c/p\u003e\u003cp\u003eAll returning participants took part in the following sessions: 1) sEMG-triggered iTBS\u0026thinsp;+\u0026thinsp;NMES and 2) sEMG-triggered iTBS\u003csub\u003esham\u003c/sub\u003e+NMES in a random order (at least 6 days apart). iTBS and NMES were delivered synchronously (Cao et al. \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2022a\u003c/span\u003e; Cao et al. \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). Stimulation ran for 2 s, and then was off for 8 s, and this 10 s cycle was repeated 10 times. For the paired condition, participants received concurrent iTBS and NMES. For the sham condition, participants received NMES with the sham iTBS (see the description of the sham condition above). To minimise potential effects of anticipation, participants were explained that they were undergoing two very similar experimental procedures.\u003c/p\u003e\u003cp\u003eTo determine any acute effects of sEMG-triggered iTBS\u0026thinsp;+\u0026thinsp;NMES on hand dexterity and upper-limb function, the following functional tests were carried out: MVC, Nine Hole Peg Test (NHPT) and Box and Block Test (BBT). These tests were only conducted in older adults as we did not expect any stimulation-induced improvement in hand function in healthy young adults (Desai et al. \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). These tests were conducted prior to, immediately after, and 30 minutes after the stimulation sessions.\u003c/p\u003e\u003cp\u003eFor the NHPT, participants sat in the same position as during stimulation and were instructed to use their dominant hand to place nine pegs into the holes of the pegboard as quickly as possible. The total time to insert all pegs was recorded (Wang et al. \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). For the BBT, participants, in the same seated position, moved as many blocks as possible from one compartment of the box to the other within 30 seconds, ensuring each transfer crossed the partition without throwing the blocks. The total number of blocks transferred was recorded (Desrosiers et al. \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e1994\u003c/span\u003e).\u003c/p\u003e\u003cp\u003e\u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003eAdditional experiment\u003c/span\u003e\u003c/p\u003e\u003cp\u003eAn additional experiment was performed in 12 healthy young participants who underwent the following sessions in random: 1) sEMG-triggered iTBS\u0026thinsp;+\u0026thinsp;NMES and 2) non-triggered iTBS\u0026thinsp;+\u0026thinsp;NMES (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eA). The sEMG-triggered iTBS\u0026thinsp;+\u0026thinsp;NMES was the same as described above. For non-triggered iTBS\u0026thinsp;+\u0026thinsp;NMES, the stimulation protocol was initiated by the experimenter.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e\u003ch2\u003e2.7. Data Analysis\u003c/h2\u003e\u003cp\u003eNeurophysiological data was analysed using Signal software v. 6 (Cambridge Electronic Design, UK). All data were visually inspected and frames were excluded from analysis if there was no response (no clear visible MEP from background EMG activity) or electrical interference, or abnormally high EMG activity (from sudden voluntary activation of movement) was present. Peak-to-peak amplitudes of average MEP recorded from each time point were measured and expressed as a percentage of the baseline average MEP amplitude. Pre-stimulus EMG was rectified and calculated as a mean amplitude in a 100 ms window prior to the stimulus. A total of 5.6\u0026thinsp;\u0026plusmn;\u0026thinsp;3.2% trials in which mean pre-stimulus rectified EMG activity exceeded 2 standard deviations (SD) of the mean average rectified EMG were excluded from further analysis. MVC data was rectified, and mean amplitudes were calculated for APB muscles using a 500 ms window around the peak amplitude. NHPT was measured by time to complete the test, and BBT was measured by the number of blocks moved.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e\u003ch2\u003e2.8. Statistical Analysis\u003c/h2\u003e\u003cp\u003eAll statistical analysis was performed using SPSS (v. 29, IBM Corp., Armonk, NY, USA). Normal distributions were tested by the Shapiro\u0026ndash;Wilk Test and sphericity tested using Mauchly\u0026rsquo;s Test. When data were not normally distributed (p\u0026thinsp;\u0026lt;\u0026thinsp;0.05), non-parametric tests were used. Repeated measures (RM) ANOVAs were applied to evaluate the main effect of time on MEP amplitudes and pre-stimulus mean EMG amplitude obtained from the screening sessions to confirm the effects of iTBS in our participants. A mixed-model, two-way RM ANOVA was employed to test the within-subject effect of condition (sEMG-triggered iTBS\u0026thinsp;+\u0026thinsp;NMES vs. sEMG-triggered iTBS\u003csub\u003esham\u003c/sub\u003e+NMES) and time (pre, post10, post20, and post30 minutes after the stimulation), and the between-subject effect of group (younger vs. older) on MEP amplitudes. To determine any effect of the stimulation on hand function, a two-way RM ANOVA was used to test any main effect of condition (sEMG-triggered iTBS\u0026thinsp;+\u0026thinsp;NMES vs. sEMG-triggered iTBS\u003csub\u003esham\u003c/sub\u003e+NMES) and time (pre, post0, and post30 min after the stimulation) on MVCs, NHPT, and BBT in older participants. An independent t-test was performed to compare APB MVCs between young and older participants. Furthermore, two-way RM ANOVAs were performed in the additional experiment to evaluate the effect of condition (sEMG-triggered iTBS\u0026thinsp;+\u0026thinsp;NMES vs. non-triggered iTBS\u0026thinsp;+\u0026thinsp;NMES) and time (pre, post0, post10, post20, and post30 minutes after the stimulation) on MEP amplitudes and pre-stimulus EMG. Bonferroni post hoc tests were used to test for significant comparisons. Paired-t tests were employed to compare stimulus parameters used in different conditions. Significance was set at p\u0026thinsp;\u0026lt;\u0026thinsp;0.05. Effect sizes are presented as partial eta-squared (η\u0026sup2;ₚ) when appropriate, group data are presented as the mean\u0026thinsp;\u0026plusmn;\u0026thinsp;SD in the text.\u003c/p\u003e\u003c/div\u003e"},{"header":"3. Results","content":"\u003cdiv id=\"Sec12\" class=\"Section2\"\u003e\u003ch2\u003e3.1 Participant characteristics\u003c/h2\u003e\u003cp\u003eOf the 22 young and 19 older participants recruited, 20 young participants (8 males; 22\u0026thinsp;\u0026plusmn;\u0026thinsp;2 years; 1 left-handed) and 16 older participants (7 males; 75\u0026thinsp;\u0026plusmn;\u0026thinsp;4 years; 2 left-handed) were classified as responders, who showed increased MEP amplitudes after the iTBS alone, as summarized in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. Stimulus parameters are summarised in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. AMTs of the APB were similar between young (57\u0026thinsp;\u0026plusmn;\u0026thinsp;6% MSO) and older (57\u0026thinsp;\u0026plusmn;\u0026thinsp;6% MSO) participants (F(1,34)\u0026thinsp;=\u0026thinsp;0.08, p\u0026thinsp;=\u0026thinsp;0.78). TMS intensities used to elicit APB MEPs were the same across conditions (F(2,68)\u0026thinsp;=\u0026thinsp;0.60, p\u0026thinsp;=\u0026thinsp;0.55) and between groups (F(2,68)\u0026thinsp;=\u0026thinsp;0.36, p\u0026thinsp;=\u0026thinsp;0.70). NMES intensities did not differ between the sEMG-triggered iTBS\u0026thinsp;+\u0026thinsp;NMES and sEMG-triggered iTBS\u003csub\u003esham\u003c/sub\u003e+NMES conditions (F(1,34)\u0026thinsp;=\u0026thinsp;3.01; p\u0026thinsp;=\u0026thinsp;0.09) or groups (F(1,34)\u0026thinsp;=\u0026thinsp;0.24, p\u0026thinsp;=\u0026thinsp;0.63). MVCs were the same across sessions (F(2,68)\u0026thinsp;=\u0026thinsp;0.33, p\u0026thinsp;=\u0026thinsp;0.72); the average APB MVC was greater in young participants (0.97\u0026thinsp;\u0026plusmn;\u0026thinsp;0.22 mV) than in older participants (0.75\u0026thinsp;\u0026plusmn;\u0026thinsp;0.19 mV; t(34)\u0026thinsp;=\u0026thinsp;\u0026minus;\u0026thinsp;3.22, p\u0026thinsp;=\u0026thinsp;0.003).\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eStimulus parameters and maximal voluntary contractions (MVCs)\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"7\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e\u0026nbsp;\u003c/th\u003e\u003cth align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e\u003cp\u003eScreening\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colspan=\"2\" nameend=\"c5\" namest=\"c4\"\u003e\u003cp\u003eiTBS\u0026thinsp;+\u0026thinsp;NMES\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colspan=\"2\" nameend=\"c7\" namest=\"c6\"\u003e\u003cp\u003eiTBSsham\u0026thinsp;+\u0026thinsp;NMES\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003e\u003cb\u003eYoung\u003c/b\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003e\u003cb\u003eOlder\u003c/b\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003e\u003cb\u003eYoung\u003c/b\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003e\u003cb\u003eOlder\u003c/b\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c6\"\u003e\u003cp\u003e\u003cb\u003eYoung\u003c/b\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c7\"\u003e\u003cp\u003e\u003cb\u003eOlder\u003c/b\u003e\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eAMT (%MSO)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e57\u0026thinsp;\u0026plusmn;\u0026thinsp;6\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e57\u0026thinsp;\u0026plusmn;\u0026thinsp;5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e\u003cp\u003e57\u0026thinsp;\u0026plusmn;\u0026thinsp;6\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e\u003cp\u003e57\u0026thinsp;\u0026plusmn;\u0026thinsp;6\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003en/a\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003en/a\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eNMES intensity (mA)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003en/a\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003en/a\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e\u003cp\u003e16.4\u0026thinsp;\u0026plusmn;\u0026thinsp;6.5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e\u003cp\u003e22.4\u0026thinsp;\u0026plusmn;\u0026thinsp;8.3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e17.3\u0026thinsp;\u0026plusmn;\u0026thinsp;7.2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e24.5\u0026thinsp;\u0026plusmn;\u0026thinsp;9.5\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eTMS intensity (%)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e75\u0026thinsp;\u0026plusmn;\u0026thinsp;10\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e74\u0026thinsp;\u0026plusmn;\u0026thinsp;9\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e\u003cp\u003e75\u0026thinsp;\u0026plusmn;\u0026thinsp;9\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e\u003cp\u003e76\u0026thinsp;\u0026plusmn;\u0026thinsp;12\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e75\u0026thinsp;\u0026plusmn;\u0026thinsp;10\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e75\u0026thinsp;\u0026plusmn;\u0026thinsp;14\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eMVC (mV)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e0.96\u0026thinsp;\u0026plusmn;\u0026thinsp;0.23\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0.78\u0026thinsp;\u0026plusmn;\u0026thinsp;0.23\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e\u003cp\u003e0.97\u0026thinsp;\u0026plusmn;\u0026thinsp;0.25\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e\u003cp\u003e0.70\u0026thinsp;\u0026plusmn;\u0026thinsp;0.21\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e0.98\u0026thinsp;\u0026plusmn;\u0026thinsp;0.33\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e0.72\u0026thinsp;\u0026plusmn;\u0026thinsp;0.26\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003ctfoot\u003e\u003ctr\u003e\u003ctd colspan=\"7\"\u003eMSO: maximal stimulator output; NMES: neuromuscular electrical stimulation; TMS: transcranial magnetic stimulation; iTBS: intermittent theta burst stimulation. AMT was not assessed in the sham condition as all participants received iTBS at 30%MSO. NMES was not applied in screening.\u003c/td\u003e\u003c/tr\u003e\u003c/tfoot\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec13\" class=\"Section2\"\u003e\u003ch2\u003e3.2 Effects of the iTBS on corticospinal excitability\u003c/h2\u003e\u003cp\u003eA main effect of time was observed on MEP amplitudes (F(4,124)\u0026thinsp;=\u0026thinsp;8.94, p\u0026thinsp;\u0026lt;\u0026thinsp;0.001, η\u0026sup2; = 0.22), with no group \u0026times; time interaction (F(2,24)\u0026thinsp;=\u0026thinsp;0.48, p\u0026thinsp;=\u0026thinsp;0.63), indicating that young and older participants responded similarly to iTBS alone. Post-hoc tests showed significant MEP facilitation at all post-stimulation time points compared with baseline in both groups (all p\u0026thinsp;\u0026lt;\u0026thinsp;0.05; Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). Pre-stimulus EMG amplitudes remained the same across all time points (p\u0026thinsp;\u0026gt;\u0026thinsp;0.05).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eFigure \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. \u003cb\u003eEffects of iTBS on motor evoked potential (MEP).\u003c/b\u003e Group mean data showing MEP amplitudes of the abductor pollicis brevis (APB) before (pre) and after (post-0, post-10, post-20, and post-30) stimulation in young (n\u0026thinsp;=\u0026thinsp;20) and older adults (n-16). Individual data are presented. The horizontal line within the box represents the median, and outer edges of the box represent the 25th and 75th percentiles. The upper and lower whiskers extend to 1.5 times the interquartile range. *p\u0026thinsp;\u0026lt;\u0026thinsp;0.05 compared to the pre-stimulation.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec14\" class=\"Section2\"\u003e\u003ch2\u003e3.3 Effects of EMG-triggered paired iTBS/NMES on corticospinal excitability\u003c/h2\u003e\u003cp\u003eRepresentative MEP traces from a young participant and an older participant illustrate MEP amplitudes increased to a greater extent after sEMG-triggered iTBS\u0026thinsp;+\u0026thinsp;NMES, compared with sEMG-triggered iTBS\u003csub\u003esham\u003c/sub\u003e+NMES (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eA). Group analysis revealed significant effects of condition (F(1,32)\u0026thinsp;=\u0026thinsp;17.97, p\u0026thinsp;\u0026lt;\u0026thinsp;0.001, η\u0026sup2; = 0.36), time (F(3,96)\u0026thinsp;=\u0026thinsp;25.16, p\u0026thinsp;\u0026lt;\u0026thinsp;0.001, η\u0026sup2; = 0.44), and their interaction (F(3,96)\u0026thinsp;=\u0026thinsp;8.01, p\u0026thinsp;\u0026lt;\u0026thinsp;0.001, η\u0026sup2; = 0.20) on MEP amplitudes. There were no significant group \u0026times; condition (F(1,32)\u0026thinsp;=\u0026thinsp;3.44, p\u0026thinsp;=\u0026thinsp;0.07) or group \u0026times; time (F(3,96)\u0026thinsp;=\u0026thinsp;0.66, p\u0026thinsp;=\u0026thinsp;0.58) interactions. Post-hoc comparisons demonstrated that sEMG-triggered iTBS\u0026thinsp;+\u0026thinsp;NMES induced significantly greater MEP facilitation than sEMG-triggered iTBS\u003csub\u003esham\u003c/sub\u003e+NMES at 10 min (F(1,33)\u0026thinsp;=\u0026thinsp;15.93, p\u0026thinsp;\u0026lt;\u0026thinsp;0.001, η\u0026sup2; = 0.33), 20 min (F(1,33)\u0026thinsp;=\u0026thinsp;16.28, p\u0026thinsp;\u0026lt;\u0026thinsp;0.001, η\u0026sup2; = 0.33), and 30 min (F(1,33)\u0026thinsp;=\u0026thinsp;6.88, p\u0026thinsp;=\u0026thinsp;0.01, η\u0026sup2; = 0.17) post-stimulation relative to baseline (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eB). Pre-stimulus EMG was the same between conditions and across time points (all p\u0026thinsp;\u0026gt;\u0026thinsp;0.05).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec15\" class=\"Section2\"\u003e\u003ch2\u003e3.4 Comparisons between sEMG-triggered iTBS and NMES and iTBS alone on corticospinal excitability\u003c/h2\u003e\u003cp\u003eWe found that APB MEP at 10-min post-stimulation was greater in the sEMG-triggered iTBS\u0026thinsp;+\u0026thinsp;NMES (188.46\u0026thinsp;\u0026plusmn;\u0026thinsp;82.73% baseline MEP) condition than in the iTBS alone, with a moderate effect size (155.96\u0026thinsp;\u0026plusmn;\u0026thinsp;71.09%; F(1,32)\u0026thinsp;=\u0026thinsp;3.98; p\u0026thinsp;=\u0026thinsp;0.06; η\u0026sup2; = 0.11; Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eA). Pre-stimulus EMG was consistent between the conditions and across time points (all p\u0026thinsp;\u0026gt;\u0026thinsp;0.05).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec16\" class=\"Section2\"\u003e\u003ch2\u003e3.5 Comparisons between sEMG-triggered iTBS\u0026thinsp;+\u0026thinsp;NMES, non- triggered iTBS\u0026thinsp;+\u0026thinsp;NMES, and iTBS on corticospinal excitability\u003c/h2\u003e\u003cp\u003eThere was an effect of time on MEP amplitudes (F(2,88)\u0026thinsp;=\u0026thinsp;16.65, p\u0026thinsp;\u0026lt;\u0026thinsp;0.001; η\u0026sup2; = 0.60); MEP amplitudes were greater at post-stimulation than at pre-stimulation (all p\u0026thinsp;\u0026lt;\u0026thinsp;0.05). Additionally, MEP amplitudes were greater after sEMG-triggered iTBS\u0026thinsp;+\u0026thinsp;NMES than after iTBS with a large effect size (F(1,11)\u0026thinsp;=\u0026thinsp;4.53, p\u0026thinsp;=\u0026thinsp;0.06; η\u0026sup2; = 0.29; Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eB). There was no interaction between conditions and time (F(1,11)\u0026thinsp;=\u0026thinsp;0.89, p\u0026thinsp;=\u0026thinsp;0.53; η\u0026sup2; = 0.08) No difference was in pre-stimulus EMG across conditions or time points (all p\u0026thinsp;\u0026gt;\u0026thinsp;0.05).\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec17\" class=\"Section2\"\u003e\u003ch2\u003e3.6 Effects of the iTBS and NMES on hand motor function in older adults\u003c/h2\u003e\u003cp\u003eOne MVC value from a participant was excluded due to technical error. Among 15 older participants, there was an effect of time on APB MVCs (F(2,28)\u0026thinsp;=\u0026thinsp;6.96, p\u0026thinsp;=\u0026thinsp;0.004) but no effect of condition (F(1,14)\u0026thinsp;=\u0026thinsp;0.00, p\u0026thinsp;=\u0026thinsp;0.98) or time \u0026times; condition interaction (F(2,28)\u0026thinsp;=\u0026thinsp;1.25, p\u0026thinsp;=\u0026thinsp;0.30). MVC of the APB were lower at 30 min after iTBS\u003csub\u003esham\u003c/sub\u003e+NMES (Z\u0026thinsp;=\u0026thinsp;\u0026minus;\u0026thinsp;2.50, p\u0026thinsp;=\u0026thinsp;0.024; Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e).\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eFunctional test in older participants.\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"7\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e\u003cth align=\"left\" colspan=\"3\" nameend=\"c4\" namest=\"c2\"\u003e\u003cp\u003esEMG-triggered iTBS\u0026thinsp;+\u0026thinsp;NMES\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colspan=\"3\" nameend=\"c7\" namest=\"c5\"\u003e\u003cp\u003esEMG-triggered iTBS\u003csub\u003esham\u003c/sub\u003e+NMES\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eTest\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e\u003cb\u003eBaseline\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e\u003cb\u003ePost0\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e\u003cb\u003ePost30\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e\u003cb\u003eBaseline\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e\u003cb\u003ePost0\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e\u003cb\u003ePost30\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eNHPT (seconds)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e16.9\u0026thinsp;\u0026plusmn;\u0026thinsp;5.2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e16.1\u0026thinsp;\u0026plusmn;\u0026thinsp;3.5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e15.0\u0026thinsp;\u0026plusmn;\u0026thinsp;3.9\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e17.7\u0026thinsp;\u0026plusmn;\u0026thinsp;7.4\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e16.6\u0026thinsp;\u0026plusmn;\u0026thinsp;4.5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e16.5\u0026thinsp;\u0026plusmn;\u0026thinsp;6.7\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eBBT (number)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e22\u0026thinsp;\u0026plusmn;\u0026thinsp;7\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e23\u0026thinsp;\u0026plusmn;\u0026thinsp;7\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e23\u0026thinsp;\u0026plusmn;\u0026thinsp;7\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e24\u0026thinsp;\u0026plusmn;\u0026thinsp;7\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e24\u0026thinsp;\u0026plusmn;\u0026thinsp;6\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e25\u0026thinsp;\u0026plusmn;\u0026thinsp;7\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eMVC (mV)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e0.71\u0026thinsp;\u0026plusmn;\u0026thinsp;0.21\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0.68\u0026thinsp;\u0026plusmn;\u0026thinsp;0.25\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.68\u0026thinsp;\u0026plusmn;\u0026thinsp;0.26\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e0.72\u0026thinsp;\u0026plusmn;\u0026thinsp;0.24\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e0.67\u0026thinsp;\u0026plusmn;\u0026thinsp;0.21\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e0.63\u0026thinsp;\u0026plusmn;\u0026thinsp;0.21\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003ctfoot\u003e\u003ctr\u003e\u003ctd colspan=\"7\"\u003eNHPT: nine-hole peg test; BBT: box and block test; MVC: maximal voluntary contraction. Data are presented as mean\u0026thinsp;\u0026plusmn;\u0026thinsp;SD.\u003c/td\u003e\u003c/tr\u003e\u003c/tfoot\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eAll 16 older participants completed the NHPT; 10 (63%) improved after iTBS\u0026thinsp;+\u0026thinsp;NMES and 7 (44%) after iTBS\u003csub\u003esham\u003c/sub\u003e+NMES. Fourteen older participants completed the BBT due to the availability of the equipment. Twelve (67%) improved after iTBS\u0026thinsp;+\u0026thinsp;NMES and seven (47%) after iTBS\u003csub\u003esham\u003c/sub\u003e+NMES. However, the amount of improvement did not reach to the statistically significant level (all p\u0026thinsp;\u0026gt;\u0026thinsp;0.05; Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e).\u003c/p\u003e\u003c/div\u003e"},{"header":"4. Discussion","content":"\u003cp\u003eWe show that sEMG-triggered iTBS\u0026thinsp;+\u0026thinsp;NMES, delivered during a holding posture, facilitated corticospinal excitability in the APB muscle of both young and older adults. Compared with sham, it induced greater MEP facilitation lasting up to 30 min, suggesting enhanced corticospinal synaptic efficacy. The magnitude and duration of facilitation were similar across age groups, indicating preserved short-term associative plasticity with aging. The facilitatory effect was stronger with sEMG-triggered iTBS\u0026thinsp;+\u0026thinsp;NMES than with iTBS alone (moderate effect size). In young adults, sEMG-triggered and non-triggered iTBS\u0026thinsp;+\u0026thinsp;NMES produced comparable facilitation, both exceeding iTBS alone (moderate-to-strong effect size). Despite these neurophysiological effects, no measurable improvements in hand dexterity were observed after a single session, suggesting that acute facilitation does not immediately translate into functional gains.\u003c/p\u003e\u003cdiv id=\"Sec19\" class=\"Section2\"\u003e\u003ch2\u003e4.1. Corticospinal facilitation after synchronous iTBS and NMES in both young and older adults\u003c/h2\u003e\u003cp\u003eMotor cortical plasticity depends on the precise timing of converging sensory and motor activity, a principle described by spike-timing-dependent plasticity (Markram et al. \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e1997\u003c/span\u003e; M\u0026uuml;ller-Dahlhaus et al. \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2010\u003c/span\u003e). In humans, this has been modelled using PAS, which typically involves time-locked pairing of TMS with peripheral nerve stimulation (PNS). Some studies have suggested that the effectiveness of PAS depends on carefully individualized interstimulus intervals (ISIs), calculated from central and peripheral conduction times (Bunday and Perez \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2012\u003c/span\u003e; Urbin et al. \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). Others, however, have reported a physiologically relevant window (~\u0026thinsp;40\u0026ndash;60 ms) in which synaptic efficacy can still be strengthened without precious individualised ISIs (Baranyi and Feher \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1981\u003c/span\u003e; Stefan et al. \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e2000\u003c/span\u003e; Taylor and Martin \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2009\u003c/span\u003e). Although our protocol did not individually calibrate conduction times, we still observed facilitation of the corticospinal excitability following synchronous iTBS\u0026thinsp;+\u0026thinsp;NMES. This is consistent with the evidence suggesting that strict millisecond precision may not be essential when paired stimuli converge on overlapping neural populations (Baranyi and Feher \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1981\u003c/span\u003e; Stefan et al. \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e2000\u003c/span\u003e) and in keeping with prior work using non-invasive iTBS (Cao et al., \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2022a\u003c/span\u003e, \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003eb\u003c/span\u003e, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2025\u003c/span\u003e) and focused ultrasound stimulation (Desai et al. \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2025\u003c/span\u003e) combined with functional electrical on corticospinal excitability in healthy adults. Taken together, participants constantly exhibited greater MEP facilitation after iTBS\u0026thinsp;+\u0026thinsp;NMES than after iTBS alone, suggesting that afferent volleys provided an additional effect on corticospinal facilitation.\u003c/p\u003e\u003cp\u003ePrevious studies reporting that voluntary muscle contractions abolished the facilitatory effects of iTBS typically employed sustained contractions (Huang et al. \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2008\u003c/span\u003e; Iezzi et al. \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2008\u003c/span\u003e). Conversely, our protocol used a brief, transient contraction only at the beginning of the stimulation period to trigger the paired iTBS\u0026thinsp;+\u0026thinsp;NMES. This difference likely explains why the brief contraction in our study did not interfere with the facilitatory effects. Additionally, our facilitatory effects exceeded those reported by prior work which paired iTBS with NMES in relaxed forearm muscles (Cao et al. \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2022a\u003c/span\u003e). The enhanced effect in our study is unlikely to be due to the brief muscle contraction used to trigger the paired stimulation, as sEMG-triggered iTBS\u0026thinsp;+\u0026thinsp;NMES did not yield greater facilitation than non-triggered iTBS\u0026thinsp;+\u0026thinsp;NMES in our study. Research has shown that even minimal tonic contractions can engage the corticospinal tract and enhance LTP-like plasticity (Turton and Lemon \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e1999\u003c/span\u003e). Therefore, we propose that the intentional motor involvement, i.e., maintaining a holding posture by lightly holding a small peg between the thumb and index finger, may explain the longer-lasting facilitation observed in our study compared to previous work.\u003c/p\u003e\u003cp\u003eHigh-frequency and frequency-matched pairing may also contribute to the magnitude and persistence of facilitation. We paired 50 Hz NMES with 50 Hz iTBS bursts, a frequency known to enhance motor cortical excitability (Huang et al. \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2005\u003c/span\u003e; Chou et al. \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2015\u003c/span\u003e), mimic natural motor unit firing in small hand muscles (Farina et al. \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2008\u003c/span\u003e), and recruit central pathways (Chipchase et al. \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). Research reported that 10 Hz rTMS paired with 10 Hz sensory stimulation facilitated corticospinal excitability, whereas mismatched frequencies (10 Hz rTMS with 3 Hz stimulation) did not (Zhong et al. \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). These features likely maximised the probability of pre- and postsynaptic coactivation, producing the observed LTP-like effects.\u003c/p\u003e\u003cp\u003eOur findings extend prior reports that iTBS produces comparable facilitation in young and older adults (Dickins et al. \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Cristancho et al. \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Slan et al. \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2024\u003c/span\u003e), in contrast to studies using single-pulse PAS, which reported less effective to elicit facilitation in older participants (Fathi et al. \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Cirillo et al. \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). The preserved facilitation in older adults observed in our study may reflect the additional engagement of the corticospinal pathways, even if minimal, by maintaining the peg between the thumb and index finger during the stimulation.\u003c/p\u003e\u003cp\u003eFunctionally, no significant group-level improvements were seen in hand dexterity after a single session. Most older participants in our study performed within or above the normative range at baseline, likely creating a ceiling effect. Nevertheless, a higher proportion of participants improved their NHPT and BBT scores after iTBS\u0026thinsp;+\u0026thinsp;NMES compared with iTBS\u003csub\u003esham\u003c/sub\u003e+NMES, suggesting that repeated sessions or application in individuals with lower baseline performance could produce measurable behavioural benefits.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec20\" class=\"Section2\"\u003e\u003ch2\u003e4.2. Functional implications and limitations\u003c/h2\u003e\u003cp\u003eOur results highlight sEMG-triggered iTBS\u0026thinsp;+\u0026thinsp;NMES as a practical, robust PAS protocol with translational potential for neurorehabilitation. Notably, our approach did not require individualised ISI calibration, was quick (~\u0026thinsp;3 mins of paired stimulation), and was delivered with minimal voluntary effort, yet still produced consistent facilitation across age groups. These features make the protocol particularly suitable for clinical application in individuals with impaired hand function, such as those recovering from stroke or spinal cord injury, where voluntary movement may be limited. Future investigations are required to establish the effects of multi-session interventions combined with task-specific training for functional motor recovery in clinical populations.\u003c/p\u003e\u003cp\u003eThis study has several limitations. First, the older adult cohort had near-normal hand dexterity, which may have limited our ability to detect functional improvements. Including larger and more diverse samples and individuals with impaired hand function would enhance the robustness and generalisability of the findings. Second, we did not assess corticospinal excitability during complete hand relaxation, nor did we quantify the effort required to maintain the static hand posture. As a result, we cannot definitively attribute the prolonged facilitation to the inclusion of the static motor task. Finally, the relative contributions of cortical versus spinal mechanisms remain unclear. Future studies incorporating neurophysiological measures could help elucidate the underlying mechanisms and identify the conditions under which this protocol is most effective.\u003c/p\u003e\u003c/div\u003e"},{"header":"5. Conclusions","content":"\u003cp\u003eA single session of sEMG-triggered iTBS\u0026thinsp;+\u0026thinsp;NMES, delivered during a simple holding task, produced robust corticospinal facilitation in both young and older adults, lasting\u0026thinsp;~\u0026thinsp;30 min longer than NMES or iTBS alone after only 3 min of stimulation. This effect likely reflects the convergence of cortical and peripheral inputs with matched high-frequency stimulation, engaging descending motor pathways. These findings highlight sEMG-triggered iTBS\u0026thinsp;+\u0026thinsp;NMES as a rapid and promising method to induce plasticity in corticospinal pathways of the hand.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgement:\u0026nbsp;\u003c/strong\u003eWe are grateful to all the study participants for their time and generosity. We also thank the Central for Human Brain Health for kindly providing lab space and equipment free of charge.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding:\u0026nbsp;\u003c/strong\u003eThis project is funded by the Royal Society IES\\R3\\223125 - International Exchanges 2022 Round 3 (MM and SC). The views expressed are those of the author(s) and not necessarily those of the funder.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData availability:\u0026nbsp;\u003c/strong\u003eThe datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConflict of interest:\u0026nbsp;\u003c/strong\u003eThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCRediT authorship contribution statement:\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eMillie Taylor: Data curation, Formal analysis, Investigation, Writing \u0026ndash; original draft.\u003c/p\u003e\n\u003cp\u003eMatija Milosevic: Conceptualization, Funding acquisition, Methodology, Writing \u0026ndash; review and editing.\u003c/p\u003e\n\u003cp\u003eNed Jenkinson: Conceptualization, Methodology, Supervision, Writing \u0026ndash; review and editing.\u003c/p\u003e\n\u003cp\u003eShin-Yi Chiou: Project administration, Resources, Conceptualization, Funding acquisition, Methodology, Data curation, Formal analysis, Investigation, Writing \u0026ndash; original draft.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eBaranyi A, Feher O (1981) Long-term facilitation of excitatory synaptic transmission in single motor cortical neurones of the cat produced by repetitive pairing of synaptic potentials and action potentials following intracellular stimulation. 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Brain Stimul 14:884\u0026ndash;894. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1016/j.brs.2021.05.005\u003c/span\u003e\u003cspan address=\"10.1016/j.brs.2021.05.005\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\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":"electromyography, paired associative stimulation, motor cortex, hand grip","lastPublishedDoi":"10.21203/rs.3.rs-8077806/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8077806/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003ch2\u003eBackground\u003c/h2\u003e\u003cp\u003ePairing intermittent theta burst stimulation (iTBS) with neuromuscular electrical stimulation (NMES) can increase corticospinal excitability (CSE) of a relaxed muscle. Whether initiating stimulation with surface electromyographic activity (sEMG-triggered) during an intentional holding task enhances this effect, and whether such facilitation is preserved with ageing, remains unknown.\u003c/p\u003e\u003ch2\u003eObjective\u003c/h2\u003e\u003cp\u003eTo compare the effects of different paired and unpaired stimulation protocols on corticospinal facilitation in young and older adults, and to examine acute effects on hand function in older adults.\u003c/p\u003e\u003ch2\u003eMethods\u003c/h2\u003e\u003cp\u003eYoung and older adults received iTBS alone, sEMG-triggered iTBS\u0026thinsp;+\u0026thinsp;NMES, and sEMG-triggered sham iTBS\u0026thinsp;+\u0026thinsp;NMES targeting the abductor pollicis brevis (APB) of the dominant hand on separate occasions. A subset of young adults also received non-triggered iTBS\u0026thinsp;+\u0026thinsp;NMES. Stimulation was delivered while participants held a small cylinder between the thumb and index finger. Motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation were recorded at baseline and up to 30 min post-stimulation. In older adults, hand function was assessed using maximum voluntary contraction (MVC), Nine Hole Peg Test (NHPT), and Box and Block Test (BBT).\u003c/p\u003e\u003ch2\u003eResults\u003c/h2\u003e\u003cp\u003esEMG-triggered iTBS\u0026thinsp;+\u0026thinsp;NMES produced stronger MEP facilitation than iTBS alone or sham iTBS\u0026thinsp;+\u0026thinsp;NMES in both age groups. sEMG-triggered and non-triggered iTBS\u0026thinsp;+\u0026thinsp;NMES produced comparable facilitation in young adults. No group-level improvements were observed in MVC, NHPT, or BBT.\u003c/p\u003e\u003ch2\u003eConclusions\u003c/h2\u003e\u003cp\u003esEMG-triggered iTBS\u0026thinsp;+\u0026thinsp;NMES reliably enhanced short-term corticospinal facilitation in young and older adults. The findings suggest that intentional motor involvement during stimulation, rather than brief muscle activation per se, contributes to the observed facilitation.\u003c/p\u003e","manuscriptTitle":"Enhancing corticospinal facilitation from sEMG-triggered paired intermittent theta burst stimulation and neuromuscular electrical stimulation in young and older adults","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-11-25 07:14:07","doi":"10.21203/rs.3.rs-8077806/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":"f82483e6-916b-42d2-bd7d-854c7467be36","owner":[],"postedDate":"November 25th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2025-12-16T03:54:08+00:00","versionOfRecord":[],"versionCreatedAt":"2025-11-25 07:14:07","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8077806","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8077806","identity":"rs-8077806","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}