{"paper_id":"2d69fa01-e6c1-4c1d-bf3a-d82e4e046cc5","body_text":"Intensity-dependent corticospinal facilitation by repetitive peripheral magnetic \nstimulation: Evidence for a major contribution of group I afferents \n \nAuthors: \nKaito Yoshidaa, Mitsuhiro Nitob*, Dai Miyazakia, Ayu Omiyaa, Kanau Shitaraa, Tadaki Kosekic, \nDaisuke Kudod, Nariyuki Murad, Hiromi Fujiib, Tomofumi Yamaguchie \n \nAffiliations: \na Graduate School of Health Sciences, Yamagata Prefectural University of Health Sciences, \nYamagata, Japan \nb Department of Occupational Therapy, Yamagata Prefectural University of Health Sciences, \nYamagata, Japan \nc Department of Rehabilitation Medicine, Yamagata Saisei Hospital, Yamagata, Japan \nd Department of Physical Therapy, Yamagata Prefectural University of Health Sciences, \nYamagata, Japan \ne Department of Physical Therapy, Human Health Sciences, Graduate School of Medicine, \nKyoto University, Kyoto, Japan \n \n*Correspondence: \nMitsuhiro Nito, OT, PhD \nDepartment of Occupational Therapy, Yamagata Prefectural University of Health Sciences, 260 \nKamiyanagi, Yamagata, 990-2212, Japan \nPhone: +81-23-686-6656 \nEmail: mnito@yachts.ac.jp \n \nORCID ID: \nMitsuhiro Nito, https://orcid.org/0000-0003-2330-2485\n \nDaisuke Kudo, https://orcid.org/0000-0001-7696-156X \nHiromi Fujii, https://orcid.org/0000-0003-0155-3873 \nTomofumi Yamaguchi, https://orcid.org/0000-0003-2206-8586 \n \n.CC-BY 4.0 International licenseavailable under a \n(which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made \nThe copyright holder for this preprintthis version posted December 29, 2025. ; https://doi.org/10.64898/2025.12.29.696815doi: bioRxiv preprint \n\n1 \n \n \n \nAbstract 1 \nBackground: Repetitive peripheral magnetic stimulation (PMS) is increasingly used in 2 \nneurorehabilitation, yet the optimal stimulation intensity for inducing corticospinal facilitation 3 \nand the underlying afferent mechanisms remain unclear. We investigated the 4 \nintensity-dependent effects of repetitive PMS on corticospinal excitability and single motor 5 \nunit responses, and tested group I afferent contribution. 6 \nMethods: Healthy participants received repetitive PMS (25 Hz; 2-s ON/2-s OFF) over the 7 \nextensor carpi radialis (ECR) in a crossover design at 0.9× motor threshold (MT), 1.2×MT, and 8 \nhigh intensity sufficient to induce maximal wrist dorsiflexion (mean 1.8×MT). Motor-evoked 9 \npotentials (MEPs) elicited by transcranial magnetic stimulation were recorded from the ECR 10 \nand flexor carpi radialis (FCR) before and during the intervention (total 15 min). The lasting 11 \neffects were assessed after 9 min of high-intensity PMS for 50 min. To examine group I 12 \nafferent contribution, the same high-intensity protocol was applied during upper-arm ischemia 13 \nafter reducing the ECR H-reflex to <10% of baseline. Sensory–motor input characteristics 14 \nacross stimulation intensities were compared using post-stimulus time histograms of ECR 15 \nsingle motor unit firings during weak voluntary contraction. 16 \nResults: High-intensity PMS significantly increased ECR MEPs after 9 min of intervention, 17 \nwhereas 1.2×MT of PMS required 15 min to induce a marked effect. PMS at 0.9×MT did not 18 \ninduce significant MEP changes. Across all intensities, the FCR MEPs remained unaltered. 19 \nECR MEPs remained markedly elevated for up to 30 min after 9 min of high-intensity PMS. In 20 \ncontrast, PMS delivered during ischemia produced no MEP enhancement. The motor unit 21 \nanalysis revealed that suprathreshold PMS elicited an early peak in firing 22 \nprobability—consistent with monosynaptic Ia excitation—whose amplitude increased with 23 \nstimulation intensity, whereas PMS at 0.9×MT produced no discernible peak. 24 \nConclusions: Repetitive PMS above MT facilitates corticospinal excitability in an 25 \nintensity-dependent manner. Facilitation was abolished during ischemia. Together with the 26 \npresence of a short-latency peak in motor unit firing via a monosynaptic pathway, this finding 27 \nsupports a major contribution of large-diameter muscle afferents, with a substantial Ia 28 \ncomponent, to PMS-induced corticospinal facilitation. 29 \n.CC-BY 4.0 International licenseavailable under a \n(which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made \nThe copyright holder for this preprintthis version posted December 29, 2025. ; https://doi.org/10.64898/2025.12.29.696815doi: bioRxiv preprint \n\n2 \n \n \n \n 30 \nKeywords: Group Ia afferents, Humans, Neuromodulation, Rehabilitation, Sensory inputs, 31 \nTranscranial magnetic stimulation  32 \n.CC-BY 4.0 International licenseavailable under a \n(which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made \nThe copyright holder for this preprintthis version posted December 29, 2025. ; https://doi.org/10.64898/2025.12.29.696815doi: bioRxiv preprint \n\n3 \n \n \n \nBACKGROUND 33 \nThe corticospinal tract is the primary pathway that mediates voluntary motor control in humans 34 \n(1, 2). Transcranial magnetic stimulation (TMS)–elicited motor-evoked potentials (MEPs) 35 \nprovide a noninvasive measure of corticospinal excitability (CSE) (3). CSE changes have been 36 \nclosely linked to motor impairment and recovery after central nervous system (CNS) lesions 37 \n(4), and to motor performance improvement (5, 6). These associations underscore CSE’s 38 \ncentral role in motor function and its modulation as a potential therapeutic target. 39 \nRepetitive peripheral magnetic stimulation (PMS) has emerged as a potential adjuvant 40 \ntherapy in physical rehabilitation and is increasingly used to promote motor recovery after 41 \nCNS lesions (7-11). Accumulating evidence suggests that repetitive PMS facilitates motor 42 \nrecovery by enhancing cortical excitability (7, 12, 13). However, despite these promising 43 \nfindings, the optimal stimulation intensity for inducing neuroplastic changes remains poorly 44 \nunderstood. Previous studies have applied various PMS intensities, including levels inducing 45 \npalpable muscle contractions or small joint movements (7, 9, 10, 12-16) and levels producing 46 \nmaximal joint movements (17, 18). Others have prescribed PMS as a fixed percentage of the 47 \nmaximal stimulator output without calibrating intensity to the target muscle response (19-22). 48 \nOverall, despite evidence supporting contraction-inducing PMS for enhanced motor recovery after 49 \nCNS lesions, systematic characterization of its physiological, stimulation intensity–dependent 50 \neffects on CSE remains unestablished. 51 \n PMS physiological effects are thought to result primarily from the activation of 52 \nlow-threshold afferents, inducing Ia afferents from muscle spindles, Ib afferents from Golgi 53 \ntendon organs, and cutaneous afferents (23-25). Studies using peripheral electrical stimulation 54 \nshow that suprathreshold, contraction-inducing stimulation increases CSE, whereas 55 \nsubthreshold stimulation can reduce it, likely through preferential activation of inhibitory 56 \nafferent pathways (26-28). Despite expectations of similar mechanisms with PMS, afferent and 57 \nefferent fiber recruitment patterns differ from those observed with peripheral electrical 58 \nstimulation. In general, electrical stimulation with increasing strength elicits action potentials 59 \nin the largest fibers first, as they have the lowest electrical resistance, followed by 60 \nprogressively smaller fibers (23). In contrast, peripheral nerve magnetic stimulation requires a 61 \n.CC-BY 4.0 International licenseavailable under a \n(which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made \nThe copyright holder for this preprintthis version posted December 29, 2025. ; https://doi.org/10.64898/2025.12.29.696815doi: bioRxiv preprint \n\n4 \n \n \n \nhigher excitation threshold for sensory fibers than for α -motor fibers, and H-reflexes are not 62 \nelicited at intensities below the motor threshold (MT) (29). Moreover, PMS generates eddy 63 \ncurrents that directly stimulate deeper tissues without passing through the skin, suggesting 64 \nreduced recruitment of cutaneous afferents (8). Collectively, these properties imply that 65 \nsubthreshold PMS may fail to activate low-threshold muscle afferents and may therefore be 66 \ninsufficient to enhance CSE. Clarifying subthreshold PMS effects will provide insight into the 67 \nstimulation intensity–dependent mechanisms of PMS-induced neuroplasticity and help 68 \noptimize stimulation parameters for diverse clinical applications. In addition, PMS-induced 69 \nCSE modulation lacks direct afferent pathway characterization. In particular, the extent of 70 \ngroup I afferent—especially muscle spindle group Ia afferent—contributions to repetitive 71 \nPMS–induced CSE facilitation remains unclear. 72 \nTherefore, this study investigated the effects of different repetitive PMS intensities 73 \napplied to the wrist extensor muscles on CSE. Group I afferent contributions were assessed 74 \nusing ischemic nerve block and motor unit (MU) firing–based afferent input estimates. We 75 \nhypothesized that 1) suprathreshold PMS would trigger intensity-dependent CSE enhancement 76 \nand within a shorter intervention period than subthreshold PMS, and 2) these effects would be 77 \nmediated primarily by large-diameter group I afferents, with findings consistent with a major 78 \ncontribution from Ia afferents. Four experiments were conducted: Experiment 1 tested 79 \nintensity-dependent time courses of MEPs; Experiment 2 examined persistence after a short 80 \nhigh-intensity protocol; Experiment 3 probed the necessity of large-diameter afferent input 81 \nusing ischemia after marked H-reflex suppression; and Experiment 4 quantified afferent-driven 82 \nmotoneuron facilitation using single MU recordings. 83 \n 84 \nMETHODS 85 \nParticipants 86 \nThe study included 34 healthy volunteers, whose participation was distributed across four 87 \nexperiments (20 females, aged 18–34 years, 23 ± 4 years, mean ± standard deviation [SD]). 88 \nThe number of participants for Experiments 1, 2, 3, and 4 was 19 (10 females, 24.0 ± 4.2 89 \nyears), 17 (9 females, 22.9 ± 4.7 years), 5 (all males, 27.4 ± 4.2 years), and 8 (3 females, 25.6 90 \n.CC-BY 4.0 International licenseavailable under a \n(which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made \nThe copyright holder for this preprintthis version posted December 29, 2025. ; https://doi.org/10.64898/2025.12.29.696815doi: bioRxiv preprint \n\n5 \n \n \n \n± 4.1 years), respectively. Notably, six individuals participated in multiple experiments. 91 \nSpecifically, three individuals completed all four experiments. One individual completed 92 \nExperiments 1, 2, and 4, while two individuals completed Experiments 2, 3, and 4. None of the 93 \nparticipants had a history of neurological disease or received any medication affecting the CNS. 94 \nAll participants except two were considered right-handed based on Chapman’s dominant hand 95 \ntest (30). The experimental procedures were approved by the Ethics Committee of Y amagata 96 \nPrefectural University of Health Science (approval number: 2308-15 ) and followed the 97 \nDeclaration of Helsinki. Before participating in this study, all participants signed written 98 \ninformed consent forms for the experimental procedures. 99 \n 100 \nExperimental setup 101 \nElectromyographic recordings. Surface electromyographic (EMG) signals were recorded with 102 \npaired 1.0-cm diameter Ag/AgCl disk electrodes. The electrodes (1.5 cm interelectrode 103 \ndistance) were secured to the skin overlying the right extensor carpi radialis (ECR) and flexor 104 \ncarpi radialis (FCR) muscles. The EMG signals were amplified, bandpass filtered (15–1,000 105 \nHz), and sampled at 2,000 Hz for offline analysis (Micro 1401 with Signal software, 106 \nCambridge Electronic Design, Cambridge, UK). 107 \nMU discharges were recorded with a pair of needle electrodes (Seirin acupuncture needle, 108 \n0.16/i2 mm diameter, Seirin Kasei, Shizuoka, Japan) inserted into the ECR muscle belly. EMG 109 \nsignals were amplified and processed with a low-cut filter (50/i2 Hz). Audiovisual EMG 110 \npotential feedback was provided to help participants maintain stable single MU activity. Single 111 \nMU discrimination was carefully defined using the upper and lower amplitude discrimination 112 \nthresholds of the recorded potential. The EMG potential shape was displayed on an 113 \noscilloscope (TDS210, Tektronix, Tokyo, Japan) throughout the experiment. The MU 114 \ndischarges were converted into standard pulses and used for offline analysis (see Experiment 115 \n4). 116 \n 117 \nPeripheral magnetic stimulation. PMS was applied over the right ECR muscle to stimulate the 118 \nradial nerve innervating the ECR, as described previously (13). The forearm was fixed in a 119 \n.CC-BY 4.0 International licenseavailable under a \n(which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made \nThe copyright holder for this preprintthis version posted December 29, 2025. ; https://doi.org/10.64898/2025.12.29.696815doi: bioRxiv preprint \n\n6 \n \n \n \npronation position with the fingers free. A biphasic pulse of PMS was delivered using a 120 \nfigure-eight coil (Cool-B65; outer diameter 75 mm) connected to a MagPro R30 (MagVenture 121 \nA/S, Denmark). The coil was placed on the skin overlaying the right ECR muscle and 122 \npositioned perpendicular to the forearm. The stimulus intensity was expressed in multiples of 123 \nthe MT of the direct motor response (M-wave). MT was defined as the minimal stimulus 124 \nintensity required to induce an M-wave ≥  50 μ V (peak-to-peak amplitude) by a single-pulse 125 \nstimulus in at least three of five consecutive trials. The ECR muscle contraction was confirmed 126 \nby palpation, with the stimulus intensity well above the MT. 127 \n 128 \nTranscranial magnetic stimulation. TMS was applied over the left primary motor cortex using 129 \na figure-eight coil (loop diameter 70 mm) connected to Magstim 200 (Magstim Company, 130 \nWhitland, Dyfed, United Kingdom). We determined the optimal positioning to elicit MEPs in 131 \nthe ECR muscle at rest (hot spot) by moving the coil with the handle pointing backward and 132 \n45° away from the midline. The hot spot was defined as the region where the largest MEP in 133 \nthe ECR muscle could be evoked with minimal stimulus intensity (Lotze et al., 2003). Resting 134 \nMT was defined as the minimal stimulus intensity required to induce MEPs of ≥  50 μ V 135 \n(peak-to-peak amplitude) in at least three of five consecutive trials in the relaxed muscle (5). 136 \nThe TMS intensity was set at 120% of the resting MT to measure MEPs as a CSE indicator. A 137 \ntotal of 11 MEPs was recorded in the resting condition. Each peak-to-peak amplitude was 138 \nmeasured and averaged, and the mean value among the participants was calculated for further 139 \nanalysis. 140 \n 141 \nElectrical peripheral nerve stimulation. Rectangular electrical pulses (1.0 ms) were 142 \npercutaneously delivered to the radial nerve trunk using bipolar surface electrodes (1.0 cm 143 \ndiameter, 1.5 cm interelectrode distance) positioned along the nerve trajectory at the arm’s 144 \nlateral intermuscular septum. The stimulus electrode was connected to a constant-current 145 \nstimulator (DS8R, Digitimer, Welwyn Garden City, UK). The maximum direct motor response 146 \n(M-max) was measured by supramaximal electrical stimulation (at an intensity of 120% to 147 \ninduce M-max). The inter-stimulus interval was 4–5 s. 148 \n.CC-BY 4.0 International licenseavailable under a \n(which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made \nThe copyright holder for this preprintthis version posted December 29, 2025. ; https://doi.org/10.64898/2025.12.29.696815doi: bioRxiv preprint \n\n7 \n \n \n \n 149 \nExperimental procedures 150 \nParticipants were comfortably seated, with the examined right arm supported on an armrest; 151 \nthe shoulder was slightly flexed (≈ 10°), followed by elbow flexion (≈ 90°), and forearm 152 \npronation. 153 \n 154 \nExperiment 1: Intensity-dependent effects of repetitive PMS on MEPs, M-waves 155 \nRepetitive PMS was delivered in a 2 s ON and 2 s OFF cycle, according to a previous report 156 \n(13). The stimulation frequency was set at 25 Hz for the following reasons: 1) the stimulation 157 \nfrequency of repetitive PMS was determined by afferent input from muscle spindles (Ia 158 \nafferents) of the wrist extensor muscles during voluntary movement (31); 2) our previous study 159 \ndemonstrated that 25 Hz and 50 Hz repetitive PMS produced comparable increases in MEPs 160 \n(13); and 3) in our preliminary experiment, application of 50 Hz repetitive PMS at higher 161 \nintensities caused stimulation-coil heating, which prevented some participants from completing 162 \na 15-min intervention. 163 \nThe effects of repetitive PMS intensity were assessed using conditions applied in random 164 \norder across three sessions on separate days. A crossover design was employed, and repetitive 165 \nPMS was delivered at three stimulus intensities: 0.9×MT (below the MT), 1.2×MT [(same as 166 \nNito et al. (2021) (13)], and a high intensity can induce maximal dorsiflexion movement of the 167 \nwrist (high-intensity). The stimulus intensity required to induce maximal dorsiflexion ranged 168 \nfrom 1.4 to 2.4×MT (mean ± SD: 1.8 ± 0.3×MT). The MT for the 0.9×MT, 1.2×MT, and 169 \nhigh-intensity were 18.8 ± 2.9, 19.4 ± 2.1, and 20.1 ± 3.2, respectively (F2,36 = 2.513, p = 0.10, 170 \n/g2015 /g3043\n/g2870 = 0.123). A minimum one-day interval separated sessions to minimize carry-over effects. 171 \nThe intervention comprised five sessions, and a single session of the repetitive PMS was 172 \ndelivered for 2 s ON and 2 s OFF cycle for 3 min. MEPs were measured 3 min before 173 \nintervention (baseline), just before repetitive PMS (T0), and after one session (T3), two 174 \nsessions (T6), three sessions (T9), four sessions (T12), and five sessions of repetitive PMS 175 \n(T15). The averaged MEP amplitude at each time point was normalized to the baseline and 176 \nexpressed as a percentage of the baseline value. M-max was also measured at T0 and T15. 177 \n.CC-BY 4.0 International licenseavailable under a \n(which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made \nThe copyright holder for this preprintthis version posted December 29, 2025. ; https://doi.org/10.64898/2025.12.29.696815doi: bioRxiv preprint \n\n8 \n \n \n \n 178 \nExperiment 2: Persistence of facilitation after short, high-intensity repetitive PMS 179 \nTo investigate the lasting effect of repetitive PMS, the stimulation was administered in three 180 \nconsecutive sessions, totaling 9 min, using the same stimulation cycle and frequency as in 181 \nExperiment 1. The stimulus intensity was set to induce maximal dorsiflexion of the wrist 182 \n(high-intensity). MEPs were measured 5 min before repetitive PMS (baseline), just before the 183 \nrepetitive PMS (Pre), and every 10 min for 50 min after the repetitive PMS (P0, P10, P20, P30, 184 \nP40, and P50). The averaged MEP amplitude at each time point was normalized to the baseline 185 \nand expressed as a percentage of the baseline value. 186 \n 187 \nExperiment 3: Ischemia to probe the contribution of large-diameter afferents 188 \nTo investigate the contribution of Ia afferent input to MEP enhancement, we applied repetitive 189 \nPMS during ischemia and examined its effects on MEP amplitude. A blood pressure cuff 190 \nplaced around the participant’s upper arm was inflated to 220 mmHg pressure (32). After the 191 \ninflation onset, the H-reflexes were elicited every 4 s by electrical stimulation to the radial 192 \nnerve trunk, and the stimulus intensity was set to elicit the maximal peak-to-peak amplitude of 193 \nthe H-reflex. The H-reflex, which was absent at rest, was elicited during weak isometric wrist 194 \ndorsiflexion. After the peak-to-peak amplitude of the H-reflex decreased to less than 10% of its 195 \npreischemic size (6.4% ± 2.6% M-max; 12–20 min from the onset of ischemia), repetitive 196 \nPMS was performed during ischemia in three consecutive sessions, totaling 9 min. The 197 \nstimulus intensity was set to induce maximal dorsiflexion of the wrist (high-intensity). MEPs 198 \nwere measured 5 min before repetitive PMS (baseline), just before the repetitive PMS (Pre), 199 \nand every 10 min for 50 min after the repetitive PMS (P0, P10, P20, P30, P40, and P50). The 200 \nMEP measurement at P0 was performed after confirming that M-max remained unchanged 201 \ncompared to its size before ischemia (before ischemia, 5.9 ± 2.6 mV; after ischemia, 6.2 ± 2.0 202 \nmV; p = 0.79, d = 0.126). 203 \n 204 \nExperiment 4: Estimation of afferent-driven motoneuron facilitation using MU recordings 205 \nTo compare sensory input magnitude across PMS intensities, conditioning stimulation effects 206 \n.CC-BY 4.0 International licenseavailable under a \n(which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made \nThe copyright holder for this preprintthis version posted December 29, 2025. ; https://doi.org/10.64898/2025.12.29.696815doi: bioRxiv preprint \n\n9 \n \n \n \non a single MU were investigated (33). This approach is based on the premise that group Ia 207 \nafferents have a monosynaptic connection to almost all homonymous motoneurons (34), 208 \nenabling us to estimate the level of Ia afferent input by analyzing MU firings. Poststimulus 209 \ntime histograms (PSTHs) (bin width 0.2 ms or 0.5 ms) of the discharge of a voluntarily 210 \nactivated MU (approximately 100-ms firing interval) were constructed for the period ranging 211 \nfrom 15 to 50/i2 ms after conditioning stimulation. ECR MU discharges were recorded during 212 \nisometric wrist extension at <5% maximal voluntary contraction. MU discharges were detected 213 \nby the computer at intervals of approximately every 0.7 s, triggering the stimulator. The 214 \nconditioning stimulation was triggered with a delay of about 70/i2 ms after voluntary MU 215 \nactivation. The delay was set at a time that easily affected the next MU firing. In other words, 216 \nthe delay was set so that the afferent volleys by the conditioning stimulation would arrive at 217 \nthe motoneuron around the latest period of hyperpolarization due to the previous MU firing 218 \nand just before voluntarily driven discharge of the motoneuron. Additionally, a 219 \nfiring-probability histogram was constructed under a no-stimulation control condition. The 220 \ncontrol and stimulated situations were alternated randomly (the same number of triggers) 221 \nwithin a sequence. Each sequence generally comprised 200–800 stimulated and control 222 \nsituations. Conditioning stimulation effects were obtained by subtracting control-condition 223 \ntrigger counts from poststimulation values in each bin. Histogram subtraction was used to 224 \ngenerate a cumulative sum curve confirming facilitation (35). 225 \nConditioning PMS was applied to the radial nerve innervating the ECR. PSTHs were 226 \nconstructed using stimulation intensities of 0.9, 1.2, 1.5, and 1.8× MT to investigate PMS 227 \nintensity effects on the firing probability of ECR MUs. For each single MU, PSTHs were 228 \nconstructed for each stimulation intensity, with the order of intensities randomized across 229 \nmeasurements. 230 \n 231 \nStatistical analysis 232 \nThe Shapiro–Wilk test was used to confirm that the normalized MEPs and M-max followed a 233 \nnormal distribution. In Experiment 1, a two-way repeated-measures mixed model analysis of 234 \nvariance (ANOV A) was used with INTENSITY (0.9×MT, 1.2×MT, and high-intensity) and 235 \n.CC-BY 4.0 International licenseavailable under a \n(which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made \nThe copyright holder for this preprintthis version posted December 29, 2025. ; https://doi.org/10.64898/2025.12.29.696815doi: bioRxiv preprint \n\n10 \n \n \n \nTIME (T0, T3, T6, T9, T12, T15) as factors to compare normalized MEPs. When a significant 236 \ninteraction or main effects were determined, a two-way repeated-measures mixed model 237 \nANOV A was similarly used with INTENSITY (0.9×MT, 1.2×MT, and high-intensity) and 238 \nTIME (T0 and T15) as factors to compare M-max. 239 \nExperiment 2 assessed lasting repetitive PMS effects on MEPs using a repeated-measures 240 \nmixed model ANOVA with TIME as the main factor (Pre, P0, P10, P20, P30, P40, and P50). 241 \nOwing to the small sample size (n = 5), Experiment 3 reports individual data without 242 \nstatistical analysis. 243 \nIn Experiment 4, effect latency and duration were defined as consecutive bins showing 244 \nincreased firing probability and were determined from fluctuations in cumulative sum curves 245 \nby visual inspection from at least two investigators. Differences between poststimulation and 246 \ncontrol firing probabilities across consecutive-bin time windows were assessed using χ 2-test. A 247 \nrepeated-measures mixed model ANOVA with main factor INTENSITY (0.9×MT, 1.2×MT, 248 \n1.5×MT, and high-intensity) was used to compare peak amplitudes derived from cumulative 249 \ndifference histograms. An unpaired t-test was performed to compare the peak duration induced 250 \nby PMS and electrical stimulation. 251 \nStatistical significance was set at p < 0.05 for all comparisons. When significant main 252 \neffects or interactions were found, post-hoc comparisons were performed using a paired t-test 253 \nwith Bonferroni adjustments. Effect sizes were reported as partial η ² (/g2015 /g3043\n/g2870) for ANOVA and 254 \nCohen’s d for the t-test. All statistical analyses, except the χ 2-test, were performed using SPSS 255 \n30 (IBM, Armonk, NY , USA). The χ 2-test was conducted using a PSTH analysis program 256 \n(MTS0014, Gigatex, Japan). Group data are presented as mean ± SD in the text. 257 \n 258 \n 259 \nRESULTS 260 \nExperiment 1: Intensity-dependent effects of repetitive PMS on MEPs, M-waves 261 \nFigure 1 shows the time courses of the normalized MEPs. Notably, ECR MEP amplitudes were 262 \ndifferentially modulated by stimulation intensity (Figure 1A). Two-way repeated-measures 263 \nmixed model ANOVA revealed a significant main effect of INTENSITY (F2,36 = 11.524, p < 264 \n.CC-BY 4.0 International licenseavailable under a \n(which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made \nThe copyright holder for this preprintthis version posted December 29, 2025. ; https://doi.org/10.64898/2025.12.29.696815doi: bioRxiv preprint \n\n11 \n \n \n \n0.001, /g2015 /g3043\n/g2870 = 0.192) and TIME (F5,90 = 13.312, p < 0.001, /g2015 /g3043\n/g2870 = 0.470), and their interaction 265 \n(F10,180 = 2.688, p = 0.004, /g2015 /g3043\n/g2870 = 0.167) on the MEP amplitudes of the ECR. Time-factor 266 \ncomparisons revealed no changes following repetitive PMS at 0.9×MT, even after a total 267 \n15-min intervention (F5,90 = 0.885, p = 0.494, /g2015 /g3043\n/g2870 = 0.047). In contrast, repetitive PMS at 268 \n1.2×MT increased MEPs at T15 compared to T0 (p < 0.001, d = 1.047), whereas repetitive 269 \nPMS at high-intensity induced an earlier facilitation, with MEP amplitudes increasing at T9 (p 270 \n= 0.002, d = 0.997), T12 (p < 0.001, d = 1.585), and T15 compared to T0 (p < 0.001, d = 271 \n1.240). Comparison of the MEPs of ECR among different conditions showed a significant 272 \ndifference between 0.9×MT and high intensity repetitive PMS at T12 (p = 0.013, d = 0.859) 273 \nand T15 (p = 0.002, d = 0.800), but no significant difference was observed in other 274 \ncombinations (p > 0.05). 275 \nFCR MEP amplitudes remained unaltered under any of these conditions (Figure 1B). A 276 \ntwo-way repeated-measures mixed model ANOV A revealed no significant main effects of 277 \nINTENSITY (F2,36 = 0.594, p = 0.104, /g2015 /g3043\n/g2870 = 0.033) or TIME (F 5,90 = 0.820, p = 0.536, /g2015 /g3043\n/g2870 = 278 \n0.062), and their interaction (F10,180 = 0.968, p = 0.473, /g2015 /g3043\n/g2870 = 0.051) on the FCR MEP 279 \namplitudes. 280 \nThe M-max amplitudes remained unchanged under any of these conditions. The mean 281 \nvalues of M-max at T0 and T15 were 7.9 ± 3.2 mV and 8.2 ± 3.2 mV for 0.9×MT repetitive 282 \nPMS, 7.5 ± 2.6 mV and 7.5 ± 2.8 mV for 1.2×MT repetitive PMS, 7.3 ± 2.4 mV and 7.7 ± 2.7 283 \nmV for high-intensity repetitive PMS, respectively. A two-way repeated-measures mixed 284 \nmodel ANOVA revealed no significant main effects of INTENSITY (F2,36 = 1.242, p = 0.301, 285 \n/g2015 /g3043\n/g2870 = 0.065) or TIME (F1,18 = 2.065, p = 0.168, /g2015 /g3043\n/g2870 = 0.103), and their interaction (F 2,36 = 0.432, 286 \np = 0.652, /g2015 /g3043\n/g2870 = 0.023). 287 \n 288 \nExperiment 2: Persistence of facilitation after short, high-intensity repetitive PMS 289 \nFigure 2 shows the changes in MEPs of ECR before and after 9 min of repetitive PMS. 290 \nRepetitive PMS markedly increased MEPs in stimulated ECR, whereas no significant changes 291 \nwere observed in FCR. A repeated-measures mixed model ANOV A for ECR MEPs revealed a 292 \nsignificant main effect of Time (F6,96 = 5.153, p < 0.001, /g2015 /g3043\n/g2870 = 0.244). Post-hoc tests revealed 293 \n.CC-BY 4.0 International licenseavailable under a \n(which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made \nThe copyright holder for this preprintthis version posted December 29, 2025. ; https://doi.org/10.64898/2025.12.29.696815doi: bioRxiv preprint \n\n12 \n \n \n \nthat ECR MEPs were significantly increased at P0 (p < 0.001, d = 1.174), P10 (p < 0.001, d = 294 \n1.484), P20 (p = 0.024, d = 0.968), and P30 (p = 0.037, d = 0.725) compared to Pre values, 295 \nwhereas no significant changes were observed at other time points (p > 0.05). In contrast, a 296 \nrepeated-measures mixed model ANOVA for FCR MEPs revealed no significant main effect of 297 \nTime (F6,96 = 0.985, p = 0.440, /g2015 /g3043\n/g2870 = 0.059). 298 \n 299 \nExperiment 3: Ischemia to probe the contribution of large-diameter afferents 300 \nFigure 3 illustrates the individual changes in ECR MEPs for five participants who participated 301 \nin Experiments 2 and 3. As shown in Figure 3A, which indicates the individual data underlying 302 \nFigure 2, a robust increase in MEPs was confirmed. Conversely, repetitive PMS applied after 303 \nischemic blockade of Ia afferent input produced no observable MEP changes (Figure 3B), 304 \nsuggesting a contribution of Ia afferent input to repetitive PMS–induced MEP augmentation. 305 \n 306 \nExperiment 4: Estimation of afferent-driven motoneuron facilitation using MU recordings 307 \nFigure 4 shows a representative facilitation induced by PMS with various intensities. A 308 \nsignificant peak (representing an increase in the firing probability) of PSTH was observed 309 \n22.5–24.0 ms after PMS using 1.8×MT (p/i2 </i2 0.001). In this MU, a peak was observed at the 310 \nsame latency across all stimulus intensities from 1.2×MT to 1.8×MT, whereas the peak size, as 311 \nindicated by the cumulative sum curve, decreased upon PMS intensity reduction. The PMS at 312 \n0.9×MT did not affect the MU firing probability (p > 0.05). A total of 27 MUs from eight 313 \nparticipants were studied. Because stable MU firing could not be maintained over prolonged 314 \nperiods, the number of analyzed MUs varied slightly across stimulation intensities. A 315 \nsignificant peak was observed in all 25 MUs when PMS was applied at 1.8×MT (100%), in 17 316 \nof 24 MUs at 1.5×MT (71%), and in 14 of 25 MUs at 1.2×MT (56%). In contrast, no 317 \nsignificant peak was detected in any of the 25 MUs using PMS at 0.9×MT (0%) (Figure 4B). 318 \nFurthermore, the peak amplitudes obtained from the cumulative sums of the difference 319 \nhistograms increased progressively with stimulation intensity (Figure 4C). A 320 \nrepeated-measures mixed model ANOVA revealed a significant main effect of INTENSITY 321 \n(F3,66.065=23.936, p<0.001, /g2015 /g3043\n/g2870=0.486). These findings suggest that higher PMS intensities are 322 \n.CC-BY 4.0 International licenseavailable under a \n(which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made \nThe copyright holder for this preprintthis version posted December 29, 2025. ; https://doi.org/10.64898/2025.12.29.696815doi: bioRxiv preprint \n\n13 \n \n \n \nassociated with greater afferent input, whereas PMS delivered below the MT does not elicit 323 \nafferent input. 324 \nFigure 5 shows an example of the effects of PMS and electrical stimulation of the radial 325 \nnerve trunk on single MU firings. The latency of the peak was 22.0 ms for PMS and 20.4 ms 326 \nfor electrical stimulation, and the duration was 2.4 ms and 2.4 ms, respectively. A total of 15 327 \nMUs from 8 participants were studied. The difference latency of the peaks was 21.4 ± 2.0 ms 328 \nfor PMS and 20.4 ± 2.0 ms for electrical stimulation, respectively, indicating that the electrical 329 \nstimulation–induced peak latency was 1.0 ± 0.7 ms shorter than that of PMS. Considering the 330 \ndistance between the two stimulation sites (8.2 ± 3.4 cm), the central latency of the 331 \nPMS-induced peak can be considered equivalent to that of monosynaptic Ia facilitation 332 \ninduced by electrical stimulation (36). The peak duration was 2.4 ± 1.0 ms for PMS and 2.2 ± 333 \n0.8 ms for electrical stimulation, respectively, indicating no significant difference between the 334 \ntwo conditions (p = 0.402 using unpaired t-test, d = 0.157). These findings suggest that PMS 335 \nactivates Ia afferents and generates excitatory postsynaptic potentials (EPSPs), with the initial 336 \ncomponent mediated by a monosynaptic pathway. 337 \n 338 \n 339 \nDISCUSSION 340 \nThe present study aimed to clarify how varying intensities of repetitive PMS applied to wrist 341 \nextensor muscles influence CSE. Our results yielded three main findings. First, repetitive PMS 342 \nfacilitated CSE in an intensity-dependent manner, with high-intensity stimulation inducing 343 \nearlier facilitation than 1.2×MT, while 0.9×MT produced no effect. Second, the facilitation was 344 \nmuscle-specific and not attributable to peripheral changes, as FCR MEPs and ECR M-max 345 \nremained unchanged. Third, the abolition of facilitation during ischemia together with a 346 \nshort-latency peak in MU firing via a monosynaptic path supports a major contribution of 347 \nlarge-diameter muscle afferents, consistent with a substantial Ia component, to PMS-induced 348 \ncorticospinal facilitation. 349 \n 350 \nGroup I Afferents Responsible for CSE Enhancement 351 \n.CC-BY 4.0 International licenseavailable under a \n(which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made \nThe copyright holder for this preprintthis version posted December 29, 2025. ; https://doi.org/10.64898/2025.12.29.696815doi: bioRxiv preprint \n\n14 \n \n \n \nGroup I afferents from wrist extensor muscles are most likely the afferent fibers responsible for 352 \nCSE enhancement. In the present study, we demonstrated that a suprathreshold PMS increased 353 \nMEPs. Specifically, high-intensity PMS-induced increased MEPs, achieving this effect only 354 \nafter a short duration of intervention. Furthermore, MU firing analysis showed similar PSTH 355 \npeak latencies across suprathreshold PMS intensities, whereas peak amplitudes increased with 356 \nstimulation intensity. Additionally, the PSTH duration was comparable to that induced by 357 \nelectrical stimulation to the radial nerve trunk, which is known to elicit a monosynaptic 358 \ncomponent mediated by Ia afferents (37, 38), regardless of the stimulation intensity of PMS. 359 \nThe peak amplitude and duration of the PSTH are considered to reflect the size and rise time of 360 \nthe composite EPSPs, respectively (39, 40). Higher PMS intensities may have recruited a 361 \ngreater number of Ia afferents, thereby eliciting larger EPSPs. Consequently, stronger 362 \nintensities of PMS may have induced increased CSE within a shorter intervention period 363 \ncompared to weaker intensities. Ischemic nerve block is known to preferentially attenuate 364 \nlarge-diameter afferents (32). Accordingly, the absence of MEP facilitation during ischemia 365 \nsupports a major contribution of large-diameter muscle afferents (i.e., group I pathways) to 366 \nPMS-induced corticospinal facilitation. Although ischemia does not isolate Ia afferents 367 \nexclusively, the monosynaptic timing and narrow duration of MU facilitation are compatible 368 \nwith a substantial Ia component. Because repetitive PMS was delivered after the H-reflex was 369 \nreduced to <10% of baseline, these findings collectively suggest that attenuation of 370 \nlarge-diameter afferent input abolished the facilitatory effect on CSE. 371 \nPMS at subthreshold intensities did not alter CSE or MU firings in the present study. This 372 \ninterpretation is supported by a previous report by Panizza et al. (1992) (29), which 373 \ndemonstrated two key findings regarding PMS: First, sensory fibers exhibit a higher threshold 374 \nthan motor fibers. Second, the H-reflex—elicited by monosynaptic excitation from the 375 \nhomonymous group Ia afferents—is not induced at intensities lower than the MT. Additionally, 376 \nprevious studies on electrical stimulation have shown contrasting effects: suprathreshold 377 \nelectrical stimulation of the median nerve trunk at the wrist increases MEPs, whereas 378 \nsubthreshold stimulation decreases MEPs (26-28). Furthermore, cutaneous nerve stimulation 379 \nof the index finger originating from the median nerve also decreases MEPs (26). Comparable 380 \n.CC-BY 4.0 International licenseavailable under a \n(which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made \nThe copyright holder for this preprintthis version posted December 29, 2025. ; https://doi.org/10.64898/2025.12.29.696815doi: bioRxiv preprint \n\n15 \n \n \n \nconduction velocities between the fastest cutaneous and group Ia afferents (41) suggest similar 381 \nrecruitment thresholds. Therefore, the inhibitory effect of subthreshold stimulation on CSE 382 \nmay result from the preferential activation of cutaneous afferents rather than of group Ia 383 \nafferents. In contrast, PMS may be more effective in increasing CSE by preferentially 384 \nactivating Ia afferents and providing stronger ascending proprioceptive input. Although PMS 385 \nwas applied transcutaneously and may activate cu taneous afferents beneath the stimulation site, 386 \nmagnetically induced eddy currents directly stimulate deeper tissues and recruit superficial 387 \ncutaneous afferents less effectively than electrical stimulation (8). Thus, cutaneous afferent 388 \ncontributions at the stimulation site are likely smaller than those typically recruited by 389 \nperipheral electrical stimulation, although this study did not directly quantify such observation. 390 \nTherefore, these findings suggest that subthreshold PMS is insufficient to excite the sensory 391 \nafferents necessary to modulate CSE, thereby failing to induce either enhancement or 392 \nsuppression of MEPs. 393 \n 394 \nChanges in CSE Induced by Repetitive PMS 395 \nPrevious studies (7, 12, 13) have revealed that repetitive PMS at suprathreshold intensity 396 \nincreases MEPs. Moreover, reductions in short-latency intracortical inhibition and increases in 397 \nintracortical facilitation have been reported following repetitive PMS (12, 13). In contrast, 398 \nMEPs evoked by transmastoid electrical stimulation were not altered by repetitive PMS 399 \n(Omiya et al., under review), which is thought to reflect changes in the efficiency of synaptic 400 \ntransmission between the cortex and spinal motor neurons or the excitability of spinal motor 401 \nneurons themselves (42, 43). Furthermore, the excitability of the spinal reflex loop in the 402 \nmuscle receiving PMS appears to remain unchanged, as demonstrated using the H-reflex (13). 403 \nIn addition, M-max remained unchanged following repetitive PMS, indicating preserved 404 \nneuromuscular transmission efficacy. Overall, these findings suggest that augmented MEPs are 405 \npredominantly attributable to changes at supraspinal levels (e.g., cortical and/or corticospinal 406 \nsynaptic efficacy), although the present design does not fully exclude contributions from spinal 407 \nmechanisms. 408 \nPrior work demonstrated sustained MEP enhancement for up to 60 min after 15 min of 409 \n.CC-BY 4.0 International licenseavailable under a \n(which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made \nThe copyright holder for this preprintthis version posted December 29, 2025. ; https://doi.org/10.64898/2025.12.29.696815doi: bioRxiv preprint \n\n16 \n \n \n \nrepetitive PMS at 1.2×MT (13). In contrast, the present study demonstrated that higher 410 \nintensities of PMS-induced greater increases in MEPs following only 9 min of the intervention, 411 \nbut the effect persisted for only 30 min. These findings suggest a trade-off between the 412 \nintensity and duration of PMS-induced effects: higher intensities can induce greater increases 413 \nin CSE within a shorter intervention time, but the effects are less sustained compared with 414 \nthose induced by lower intensities. Therefore, high-intensity PMS, capable of inducing rapid 415 \nincreases in CSE, may provide practical advantages in clinical settings with time constraints. A 416 \nlimitation of the present study is that the duration of CSE changes following high-intensity 417 \nPMS applied over longer intervention periods (e.g., 15 min) was not investigated. It is 418 \nconceivable that extending the duration of high-intensity stimulation may prolong CSE 419 \nenhancement. Future studies are warranted to determine whether longer high-intensity PMS 420 \nprotocols can combine the advantages of rapid induction and sustained effects. 421 \n 422 \nMethodological Considerations 423 \nDespite these strengths, several methodological considerations warrant acknowledgment. First, 424 \nthe ischemic nerve block procedure in Experiment 3 was performed in a small sample (n = 5). 425 \nThis small sample size limits the generalizability of our findings that large-diameter afferent 426 \ninput—with a substantial Ia component—mainly contributes to PMS-induced facilitation. 427 \nSecond, despite marked ischemia-induced H-reflex suppression indicating reduced group Ia 428 \nafferent input, potential contributions from other large-diameter afferents, including group Ib 429 \nafferents, remain possible. These afferents may also have been partially affected, potentially 430 \ncontributing to the observed increase in MEPs (23-25). Third, repetitive PMS–induced coil 431 \nheating constitutes a methodological limitation. At frequencies higher than 25 Hz or at higher 432 \nstimulation intensities, continuous intervention may become challenging due to the rapid rise 433 \nin coil temperature. Therefore, the parameters used in this study were selected to balance the 434 \ninduction of corticospinal facilitation with the physical capacities of the stimulator. 435 \n 436 \nClinical Implications 437 \nNumerous studies have reported that repetitive PMS contributes to improved motor 438 \n.CC-BY 4.0 International licenseavailable under a \n(which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made \nThe copyright holder for this preprintthis version posted December 29, 2025. ; https://doi.org/10.64898/2025.12.29.696815doi: bioRxiv preprint \n\n17 \n \n \n \ndysfunction following CNS lesions (7, 9-11, 16, 18-20, 22), suggesting promising therapeutic 439 \npotential. Repetitive PMS–induced increases in CSE likely contribute, at least in part, to these 440 \nfunctional improvements (7). 441 \nBeyond these potential benefits for functional recovery, PMS offers practical advantages 442 \nfor clinical application: PMS can directly stimulate deep tissues without penetrating the skin to 443 \ninduce eddy currents (8), and is therefore considered less likely to cause discomfort during 444 \nstimulation. Indeed, none of the participants reported any discomfort during PMS, even at an 445 \nintensity sufficient to elicit maximal wrist dorsiflexion, and no participant found it difficult to 446 \ncontinue the experiment. 447 \nWhile the present findings provide evidence for the underlying mechanism of PMS, its 448 \nclinical applicability requires further investigation. The present study examined only the 449 \nimmediate effects of repetitive PMS on wrist extensors in healthy individuals; whether similar 450 \nfacilitatory effects occur in individuals with CNS lesion–related motor dysfunction or are 451 \nsustained across repeated, longer-term interventions remains to be determined. Furthermore, 452 \nalthough we demonstrated that repetitive PMS increases CSE, the functional implications of 453 \nthis enhancement remain to be fully elucidated. Future studies incorporating behavioral or 454 \nfunctional outcome measures should clarify whether PMS-induced increases in excitability 455 \ntranslate into meaningful improvements in motor performance or clinical function. 456 \n 457 \nCONCLUSIONS 458 \nRepetitive PMS at intensities above the MT enhances CSE, likely mediated predominantly by 459 \nlarge-diameter muscle afferents (group I afferents), consistent with a substantial Ia component. 460 \nShort-duration interventions at higher stimulation intensities produce greater CSE 461 \nenhancement than lower-intensity stimulation. In addition, stronger stimulation intensities are 462 \nassociated with greater afferent input. These findings suggest that group Ia afferents from 463 \nmuscle spindles markedly contribute to CSE modulation. These results provide insight into the 464 \nmechanisms underlying PMS-induced neuroplasticity and may help optimize stimulation 465 \nparameters for clinical applications in neurorehabilitation. 466 \n 467 \n.CC-BY 4.0 International licenseavailable under a \n(which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made \nThe copyright holder for this preprintthis version posted December 29, 2025. ; https://doi.org/10.64898/2025.12.29.696815doi: bioRxiv preprint \n\n18 \n \n \n \nLIST OF ABBREVIATIONS 468 \nCSE: corticospinal excitability 469 \nCNS: central nervous system 470 \nECR: extensor carpi radialis 471 \nEMG: electromyography 472 \nEPSP: excitatory postsynaptic potential 473 \nFCR: flexor carpi radialis 474 \nMEP: motor-evoked potential 475 \nM-max: maximum direct motor response 476 \nMT: motor threshold 477 \nMU: motor unit 478 \nPMS: peripheral magnetic stimulation 479 \nPSTH: poststimulus time histogram 480 \nTMS: transcranial magnetic stimulation 481 \n 482 \nData availability 483 \nThe datasets supporting the conclusions of this article are available from the corresponding 484 \nauthor on reasonable request. 485 \n 486 \nAcknowledgments 487 \nThe authors thank all the participants who engaged in this study. 488 \n 489 \nGrants 490 \nM.N. was supported by JSPS KAKENHI (Grant Number: 22K17628) and by a grant from the 491 \nWatanabe Foundation. 492 \n 493 \nDisclosures 494 \nThe authors declare that they have no competing interests. 495 \n 496 \n.CC-BY 4.0 International licenseavailable under a \n(which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. 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Terminals of single Ia fibers: location, density, and 606 \ndistribution within a pool of 300 homonymous motoneurons. J Neurophysiol 34: 171–187, 607 \n1971. 608 \n35. Ellaway PH. Cumulative sum technique and its application to the analysis of peristimulus 609 \ntime histograms. Electroencephalogr Clin Neurophysiol 45: 302–304, 1978. 610 \n36. Shinozaki K, Nito M, Kobayashi S, Hayashi M, Miyasaka T, Hashizume W, Shindo 611 \nM, and Naito A. Monosynaptic facilitation of group I afferents between brachioradialis 612 \nand extensor carpi radialis in humans. Neurosci Res 114: 30–34, 2017. 613 \n37. Birnbaum A, and Ashby P. Postsynaptic potentials in individual soleus motoneurons in 614 \nman produced by achilles tendon taps and electrical stimulation of tibial nerve. 615 \n.CC-BY 4.0 International licenseavailable under a \n(which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made \nThe copyright holder for this preprintthis version posted December 29, 2025. ; https://doi.org/10.64898/2025.12.29.696815doi: bioRxiv preprint \n\n23 \n \n \n \nElectroencephalogr Clin Neurophysiol 54: 469–471, 1982. 616 \n38. Burke D, Gandevia SC, and McKeon B. The afferent volleys responsible for spinal 617 \nproprioceptive reflexes in man. J Physiol 339: 535–552, 1983. 618 \n39. Ashby P, and Zilm D. Characteristics of postsynaptic potentials produced in single 619 \nhuman motoneurons by homonymous group 1 volleys. Exp Brain Res 47: 41–48, 1982. 620 \n40. Fetz EE, and Gustafsson B. Relation between shapes of post-synaptic potentials and 621 \nchanges in firing probability of cat motoneurones. J Physiol 341: 387–410, 1983. 622 \n41. Macefield G, Gandevia SC, and Burke D. Conduction velocities of muscle and 623 \ncutaneous afferents in the upper and lower limbs of human subjects. Brain 112 ( Pt 6): 624 \n1519–1532, 1989. 625 \n42. Taylor JL, and Gandevia SC. Noninvasive stimulation of the human corticospinal tract. 626 \nJ Appl Physiol (1985) 96: 1496–1503, 2004. 627 \n43. Ugawa Y, Rothwell JC, Day BL, Thompson PD, and Marsden CD. Percutaneous 628 \nelectrical stimulation of corticospinal pathways at the level of the pyramidal decussation 629 \nin humans. Ann Neurol 29: 418–427, 1991. 630 \n 631 \nFIGURE LEGENDS 632 \nFigure 1. Effects of different intensities of repetitive peripheral magnetic stimulation 633 \n(PMS) on motor-evoked potentials (MEPs). 634 \nChanges in MEP amplitudes of the extensor carpi radialis (ECR; A) and flexor carpi radialis 635 \n(FCR; B) muscles were recorded from 19 participants. The repetitive PMS was delivered at 636 \nthree intensities: below motor threshold (0.9×MT; white circle), above motor threshold 637 \n(1.2×MT; light gray circle), and the intensity inducing maximum wrist dorsiflexion 638 \n(high-intensity, dark gray circle). The ordinate shows MEP amplitude normalized to the 639 \nbaseline value for each participant, and the abscissa shows the time point of measurements: 640 \nbefore repetitive PMS (T0), after one session (T3), two sessions (T6), three sessions (T9), four 641 \nsessions (T12), and five sessions of repetitive PMS (T15). Each circle and error bar represent 642 \nthe mean value and SD, respectively. Asterisks indicate significant differences compared with 643 \n“T0,” and the daggers indicate significant differences compared with the other intensities (p < 644 \n.CC-BY 4.0 International licenseavailable under a \n(which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made \nThe copyright holder for this preprintthis version posted December 29, 2025. ; https://doi.org/10.64898/2025.12.29.696815doi: bioRxiv preprint \n\n24 \n \n \n \n0.05). 645 \n 646 \nFigure 2. Lasting effects of repetitive PMS on MEPs. 647 \nTime courses of MEP changes in the ECR (A) and FCR (B) muscles were recorded from 17 648 \nparticipants. The ordinates show MEP amplitude normalized to baseline amplitude for each 649 \nparticipant, and the abscissae show the time point of measurements: before (Pre) and after the 650 \nrepetitive PMS (P0–P50). Each circle and error bar represent the mean value and SD, 651 \nrespectively. Asterisks indicate significant differences compared with “Pre.” 652 \n 653 \nFigure 3. Effects of ischemia on MEP facilitation induced by repetitive PMS. 654 \nThe time courses of MEP changes in the ECR were recorded from five participants without (A) 655 \nand with ischemia (B). Individual MEP changes and their mean values are represented as gray 656 \nlines and circles, respectively. The ordinate and abscissa are the same as those in Figure 2. 657 \n 658 \nFigure 4. Changes in the firing probability of an ECR motor unit following different 659 \nintensities of PMS. 660 \nA, Left: Time histograms were obtained without PMS (white bar) or with PMS to the radial 661 \nnerve innervating the ECR (black bar) within a sequence. Each histogram was constructed 662 \nfrom 600 triggers. Middle: Each column (dark gray bar) represents the difference between the 663 \nsituations with and without PMS. The analysis time window (light gray area) was determined 664 \nfrom several consecutive bins of the time histograms obtained during 1.8×MT PMS. Right: 665 \nCumulative sums are obtained from each subtracted histogram. The ordinates represent the 666 \nnumber of counts as a percentage of the number of triggers, and the abscissae represent the 667 \nlatency after stimulation. Bin width: 0.5 ms. B, Stacked bar graph showing the proportion of 668 \nmotor units that exhibited significant peaks in the time histograms under each stimulation 669 \nintensity. The numbers in parentheses indicate the number of motor units. C, Changes in the 670 \npeak amplitude obtained by cumulative sums within the analysis time window of the difference 671 \nhistograms under different stimulation intensities. Individual changes and their mean values 672 \nare represented as gray lines and circles, respectively. Asterisks indicate significant differences 673 \n.CC-BY 4.0 International licenseavailable under a \n(which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made \nThe copyright holder for this preprintthis version posted December 29, 2025. ; https://doi.org/10.64898/2025.12.29.696815doi: bioRxiv preprint \n\n25 \n \n \n \nrevealed by post-hoc tests (p < 0.05). 674 \n 675 \nFigure 5. Changes in the firing probability of an ECR motor unit following electrical and 676 \nmagnetic stimulation. 677 \nTime histograms were obtained by subtracting the situation without stimulation from that with 678 \nstimulation. Each histogram was constructed from 400 triggers. The ordinate and abscissa are 679 \nthe same as in Figure 4A. Bin width: 0.2 ms. The difference between the latencies of peaks 680 \ninduced by electrical stimulation (20.4 ms) and magnetic stimulation (22.0 ms) was 1.6 ms. 681 \nThe distance between the two stimulation sites was 10 cm.  682 \n.CC-BY 4.0 International licenseavailable under a \n(which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made \nThe copyright holder for this preprintthis version posted December 29, 2025. ; https://doi.org/10.64898/2025.12.29.696815doi: bioRxiv preprint \n\n26 \n \n \n \n  683 \n.CC-BY 4.0 International licenseavailable under a \n(which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made \nThe copyright holder for this preprintthis version posted December 29, 2025. ; https://doi.org/10.64898/2025.12.29.696815doi: bioRxiv preprint \n\n27 \n \n \n \n  684 \n.CC-BY 4.0 International licenseavailable under a \n(which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made \nThe copyright holder for this preprintthis version posted December 29, 2025. ; https://doi.org/10.64898/2025.12.29.696815doi: bioRxiv preprint \n\n28 \n \n \n \n  685 \n.CC-BY 4.0 International licenseavailable under a \n(which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made \nThe copyright holder for this preprintthis version posted December 29, 2025. ; https://doi.org/10.64898/2025.12.29.696815doi: bioRxiv preprint \n\n29 \n \n \n \n  686 \n.CC-BY 4.0 International licenseavailable under a \n(which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made \nThe copyright holder for this preprintthis version posted December 29, 2025. ; https://doi.org/10.64898/2025.12.29.696815doi: bioRxiv preprint \n\n30 \n \n \n \n 687 \n.CC-BY 4.0 International licenseavailable under a \n(which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made \nThe copyright holder for this preprintthis version posted December 29, 2025. ; https://doi.org/10.64898/2025.12.29.696815doi: bioRxiv preprint","source_license":"CC-BY-4.0","license_restricted":false}