stroke causes motor and sensory impairments that downgrade the quality of life . impaired 
 movement of the arm is common and is often the most troublesome problem experienced by 
 stroke survivors . a recent systematic review1 reported that the most promising intervention for restoring arm 
 function was high - repetition doses of task - oriented training , such as constraint - induced 
 movement therapy , mental practice , and robotics . however , a long duration is necessary for 
 these treatments to be effective , and the adaptation range is limited . sharma et al.2 suggested that cortical excitability should 
 be modulated before motor practice to promote motor recovery or motor learning in stroke 
 rehabilitation . there are two techniques to modulate cortical excitability : ( 1 ) non - invasive 
 brain stimulation such as repetitive transcranial magnetic stimulation ( tms ) or transcranial 
 direct current stimulation , and ( 2 ) using somatosensory inputs from peripheral parts of the 
 body elicited by vibration or electrical stimulation . sensory input from a peripheral nerve changes the excitability of the cerebral cortex3,4,5 . the acquisition of accurate motor 
 performance and motor learning of new motor skills requires somatosensory input . electrical 
 stimulation can generate somatosensory signals and is easy to use in the clinical setting , 
 and some beneficial effects of electrical stimulation interventions have been reported . however , electrical stimulation parameters such as stimulation intensity , frequency , and 
 duration were not consistent across studies , making it difficult for a systematic 
 review6 to summarize the results . electrical stimulation can increase the activity of efferent motor fibers and/or increase 
 the activity of afferent sensory fibers . peripheral sensory nerve stimulation ( pns ) is an 
 electrical stimulation technique that increases the activity of afferent sensory fibers . pns 
 increases the excitability of the corticospinal pathway to the hand in healthy subjects , as 
 evaluated by tms delivered over the hand area of the motor cortex4 , 5 , 7 , 8 . pns has been proposed as a 
 supplemental therapy to facilitate the motor functions of stroke patients , such as 
 pinching9 , swallowing10 , and performing hand tasks11 , 12 . in many studies , it has been reported that long - duration stimulation is necessary to change 
 the excitability of the corticospinal tract . the shortest reported effective stimulation 
 duration was the 45  min reported by macky et al.7 . although pns is a potentially useful therapy , the pns protocols 
 that have been described are not practical for inpatient rehabilitation , as task - specific 
 training is conducted after 2  h of pns . uehara et al.13 , 14 reported that high - frequency and short - duration ( 90  s ) electrical 
 muscle stimulation ( ems ) applied to the muscles of the human hand reduced excessive motor 
 activity , i.e. , muscle hypertonia , by modulating the neural activity of the sensorimotor 
 system . they evaluated the potential benefits of high - frequency ems for hand motor skills , 
 including whether or not the stimulation reduced non - essential motor activity potentially 
 existing in the acquired motor skill , so as to upgrade the stabilized motor performance . in a previous behavioral study15 , we 
 compared the effect of ems for 120  s and pns for 1  h before performance of a motor skill and 
 found that although both improved motor performance , the improvement was greater after ems 
 than after pns . we hypothesized that short - duration high - frequency ems before motor practice 
 would change the excitability of the corticospinal tract , just as has been reported for pns . therefore , the purpose of this study was to investigate the effect of short - duration 
 high - frequency ems on corticospinal excitability through the measurement of motor evoked 
 potentials ( mep ) in young healthy subjects . eleven healthy subjects participated in this study ( two females , nine males ; age range 
 2329  years ) . all subjects received an 
 explanation of the experimental aims and procedures and their provided written informed 
 consent . the epidemiologic research ethics committee of gunma university faculty of medicine 
 approved this protocol ( no . the subjects were seated comfortably in a reclining chair and placed their right hand on a 
 table . they were instructed to completely relax their hand and arm muscles throughout the 
 experimental session . surface electromyograms were recorded from the right abductor pollicis 
 brevis ( apb ) muscle and first dorsal interosseous ( fdi ) muscle using disposable 
 silver - silver chloride surface electrodes ( 5-mm diameter ) . the electromyogram signals were 
 amplified , filtered with a bandwidth of 203,000  hz , and sampled at 10 khz using an a / d 
 converter ( neuropack , nihon kohden , tokyo , japan ) . ems was delivered using a portable constant - current stimulator ( torio 300 , ito p&r , 
 tokyo , japan ) via a pair of electrical patches attached to the skin surface on the palm of 
 the right hand over the apb . ems was 
 performed for 120  s at 100  hz with a pulse width of 100 s . we used short - duration 
 high - frequency ems because this type of stimulation has been shown to reduce excessive motor 
 activity and modulate sensorimotor neuronal state13 , 
 14 . the intensity of stimulation was 
 set at just below the motor threshold in consideration of fatigue . this intensity of 
 stimulation elicits no visible thumb movements and no muscle twitch , and was determined 
 prior to the start of the experiment for each participant . tms was delivered to the primary motor cortex ( m1 ) in the left hemisphere using a 
 figure - of - eight coil with an internal wing diameter of 70  mm ( magstim , dyfed , uk ) . the coil 
 was placed tangentially on the scalp with its handle pointing backward and rotated 
 approximately 45 away from the mid - sagittal line to induce a posterior - anterior current in 
 the left m1 . the optimal position for evoking motor evoked potentials ( mep ) from the apb was 
 determined and marked on a swimming cap worn by the subjects with a soft - tip pen to ensure 
 reliable coil placement across trials . the resting motor threshold ( rmt ) was defined as the 
 minimum stimulation intensity needed to evoke an mep of more than 50 v in the apb in five 
 out of 10 trials , and is expressed as the percentage of maximum stimulator output . the rmt 
 and the amplitude of the mep evoked by stimulation at 1.0 , 1.2 and 1.4 times of the baseline 
 rmt were determined at four time points : baseline , immediately after ems ( t0 ) , 10  min after 
 ems ( t1 ) , and 20  min after ems ( t2 ) . the three stimulation intensities were based on the 
 baseline rmt that were administered randomly with an interstimulus interval of 5 to 7  s , and 
 10 stimuli were delivered at each intensity . mep amplitude was measured offline as the 
 peak - to - peak amplitude of the electromyogram . mep amplitude was measured for both the apb 
 and the fdi . the mep amplitude at t0 , t1 and t2 were expressed as ratios of the mean mep 
 amplitude at baseline . one - way repeated - measures analysis of variance ( anova ) of time points ( baseline , t0 , t1 , 
 t2 ) was used to assess the effects of ems on rmt and mep amplitude . anova was performed 
 separately for each stimulation intensity ( 1.0 , 1.2 , and 1.4 rmt ) and each muscle ( apb , 
 fdi ) . the 
 mean rmt at baseline , t0 , t1 and t2 were 49.5  5.2 , 49.9  4.8 , 49.8  5.1 , and 49.6  5.3% 
 (  sd ) , respectively . there was no significant main effect of time point on rmt ( f = 0.793 , 
 p > 0.05 ) , indicating that rmt was not affected by ems . typical mep waveforms in the two muscles at baseline and at each time point after ems are 
 shown in fig . 1fig 1.representative motor evoked potentials waveforms in the abductor pollicis brevis ( a ) 
 and first dorsal interosseous ( b ) muscles recorded from one subject before ( baseline ) 
 and after ( t0t2 ) electrical muscle stimulation . transcranial magnetic stimulation was applied over 
 the motor cortex at 1.0 ( top ) , 1.2 ( middle ) and 1.4 ( bottom ) times resting motor 
 threshold ( rmt ) .. there was a main effect of time on the amplitude of the apb mep elicited by 
 stimulation of 1.2 rmt ( f = 4.981 , p < 0.01 ; table 1table 1.motor evoked potential ratios after electrical muscle stimulationapbfdi1.0 rmt1.2 rmt1.4 rmt1.0 rmt1.2 rmt1.4 rmtbaseline1.00  0.001.00  0.001.00  0.001.00  0.001.00  0.001.00  0.00t00.72  0.510.69  0.25 * 0.89  0.341.47  1.360.84  0.410.87  0.26t10.75  0.660.61  0.26 * 0.88  0.301.36  1.291.01  0.500.90  0.26t20.76  0.570.64  0.35 * 0.83  0.461.36  0.991.01  0.710.78 0.21data are expressed relative to the amplitude at baseline and are the mean  sd . * : 
 significantly different from the baseline measurement ( pairwise comparison , p < 
 0.05 ) . apb : abductor pollicis brevis , fdi : first dorsal interosseous , rmt : resting 
 motor threshold . t0 : immediately after electrical muscle stimulation , t1 : 10  min 
 after electrical muscle stimulation , t2 : 20  min after electrical muscle 
 stimulation . ) . post hoc comparisons revealed that the mep amplitude at each 
 post - ems time point ( t0 , t1 , t2 ) was significantly lower than the mep amplitude at baseline 
 ( p < 0.05 ) . there was no main effect of time on the amplitude of the apb mep elicited by 
 the other stimulation intensities ( 1.0 rmt : f = 0.257 , p = 0.855 ; 1.4 rmt : f = 1.191 , p = 
 0.330 ) , and no main effect of time on the amplitude of the fdi mep for any stimulation 
 intensity ( 1.0 rmt : f = 0.994 , p = 0.409 ; 1.2 rmt : f = 0.606 , p = 0.616 ; 1.4 rmt : representative motor evoked potentials waveforms in the abductor pollicis brevis ( a ) 
 and first dorsal interosseous ( b ) muscles recorded from one subject before ( baseline ) 
 and after ( t0t2 ) electrical muscle stimulation . transcranial magnetic stimulation was applied over 
 the motor cortex at 1.0 ( top ) , 1.2 ( middle ) and 1.4 ( bottom ) times resting motor 
 threshold ( rmt ) . data are expressed relative to the amplitude at baseline and are the mean  sd . * : 
 significantly different from the baseline measurement ( pairwise comparison , p < 
 0.05 ) . apb : abductor pollicis brevis , fdi : first dorsal interosseous , rmt : resting 
 motor threshold . t0 : immediately after electrical muscle stimulation , t1 : 10  min 
 after electrical muscle stimulation , t2 : 20  min after electrical muscle 
 stimulation . the present study demonstrates that the excitability of the corticospinal tract was 
 inhibited by short - duration high - frequency ems applied to the apb , and that this effect 
 continued for 20  min . furthermore , the effect was observed only in the apb , which was the 
 target muscle of ems , there being no change in the mep elicited in the fdi . these results 
 support the hypothesis that short - duration high - frequency ems modulates the excitability of 
 the corticospinal tract . the electrical stimulation parameters used in the present study 
 changed the excitability of the corticospinal tract , even though electrical stimulation was 
 only applied for a short duration , and the effect was sustained for 20  min , suggesting that 
 it could be clinically useful . the results of this study differ from many previous studies in which several tens of 
 minutes or a few hours of electrical stimulation were required before effects were 
 observed4,5,6,7,8 . somatosensory inputs , such 
 as electrical stimulation , influence the sensorimotor network within the cerebral cortex . in 
 particular , it has been reported that sensory input from a muscle excites the 
 cortex - striatum and cortex - cerebellum circuits3 . uehara et al.14 
 reported that thenar muscle stimulation facilitated functional connectivity in 
 cortico - subcortical motor circuits , and improved performance of fine hand motor skills . it is believed that there are two reasons why short - duration 
 high - frequency ems inhibits the excitability of the corticospinal tract . many previous studies using low - frequency ( 1030  hz ) electrical 
 stimulation have shown increased excitability of the corticospinal tract4,5,6,7,8 , whereas a few studies using high - frequency 
 ( 90100  hz ) electrical stimulation have shown decreased excitability17 , 18 . mima et 
 al.18 reported that the apb mep 
 decreased after the thenar muscle was stimulated at 90  hz for 30  min . thus , it is possible 
 that stimulation at about 100  hz inhibits excitability of the corticospinal tract . in the 
 present study a stimulation of 100  hz was used , and even though the duration of stimulation 
 was short , inhibition of the corticospinal tract was observed , indicating that the 
 stimulation parameters were appropriate . sugawara et al.19 investigated the influence of stimulation frequency by comparing 
 the effects of stimulation at 1  hz , 10  hz , 100  hz , and 1,000  hz . although the excitability 
 of the corticospinal tract tended to increase after stimulation at the higher frequencies , 
 there were no statistically significant differences . further studies of the relation between 
 the frequency of electrical stimulation and its effects on the corticospinal tract are 
 needed . it is 
 possible that high - frequency stimulation of a muscle induces fatigue ; however , in the 
 present study the stimulation was below the motor threshold . when muscle fatigue is induced 
 by voluntary contraction , the corticospinal tract is inhibited and intracortical inhibition 
 is disinhibited20 . when muscle fatigue 
 was induced by electrical stimulation above the motor threshold , as opposed to voluntary 
 contraction , the excitability of the corticospinal tract initially increased before 
 decreasing a few minutes later21 . it is 
 considered that the corticospinal tract is inhibited by muscle fatigue that is caused by 
 voluntary contraction . however , there have been few studies of the effects of muscle fatigue 
 caused by electrical stimulation . in this study , we only measured the mep amplitude , which 
 is thought to reflect the excitability of m1 . however , the excitability of the spinal cord 
 will also influence the amplitude of the mep . therefore , the mep amplitude reflects the 
 excitability of the whole corticospinal tract , indicating it will be necessary to measure 
 m - waves to confirm the influence of muscle fatigue . in this study mep were recorded from the apb , which was the target muscle for ems , and from 
 the neighboring fdi muscle . the mep amplitude in the apb decreased after ems , and although 
 there was a tendency of decrease , there was no statistically significant decrease in the mep 
 amplitude in the fdi . we suggest that this is related to the innervating nerve : the apb is 
 innervated by the median nerve whereas the fdi is innervated by the ulnar nerve . a previous 
 study4 reported that the mep recorded 
 from the abductor digiti minimi muscle increased after 2  h of pns to the ulnar nerve , but 
 the mep recorded from the apb muscle did not change . this indicates the afferent 
 somatosensory input from a muscle or a nerve is related to the innervating nerve . a limitation of this study is that we used only mep amplitude to quantify the effect of 
 ems . mep amplitude reflects the excitability of the entire corticospinal tract , including 
 the cortex and the spinal cord . to exclude the effect of spinal cord excitability on mep 
 amplitude it would be necessary to measure the amplitude of the h reflex . however , it has 
 previously been reported that electrical stimulation of a peripheral nerve does not 
 influence the excitability of the monosynaptic spinal cord reflex22 . although there was inhibition of corticospinal tract 
 excitability after ems , we could not attribute this to a change in the excitability of m1 . therefore , it will be necessary to study the mechanism of the observed effects in more 
 detail . when interpreting the results of this study , we could not exclude the influence of 
 the placebo effect , nor discount the possibility that electrical stimulation increased 
 attention on the stimulated part . moreover , as plasticity in the injured brain differs from 
 that in healthy subjects , the neurophysiological and behavioral effects of ems need to be 
 examined in patient populations . the results of the study have clinical implications for the rehabilitation of stroke 
 survivors receiving rehabilitation training utilizing unilateral afferent inputs , since ems 
 administered to an unaffected limb might modulate the undamaged m1 . depending on the 
 impairment levels of motor and sensory functions , an intervention for the unaffected limb 
 including ems might be a more efficient method of stroke rehabilitation than an intervention 
 for the affected limb . furthermore , the advantage of ems is that it more easily permits 
 delivery of electrical stimulation in conjunction with task - oriented training and other 
 exercises23,24,25 . in conclusion , this study showed that a short - duration high - frequency ems inhibited the 
 excitability of the corticospinal tract to the apb . this finding provides a basis for the 
 development of individual stimulation protocols in neurorehabilitation .
[ purpose ] afferent input caused by electrical stimulation of a peripheral nerve or a 
 muscle modulates corticospinal excitability . 
 however , a long duration of stimulation is 
 required to induce these effects . 
 the purpose of this study was to investigate the effect 
 of short - duration high - frequency electrical muscle stimulation ( ems ) on corticospinal 
 excitability through the measurement of motor evoked potentials ( mep ) in young healthy 
 subjects . [ subjects ] eleven healthy right - handed subjects participated in this study . 
 [ methods ] ems was applied to the abductor pollicis brevis ( apb ) muscle at 100  hz with a 
 pulse width of 100 s for 120 s. the intensity of stimulation was just below the motor 
 threshold . 
 transcranial magnetic stimulation was applied over the motor cortex , and mep 
 were recorded from the apb before , and immediately , 10 , and 20  min after ems . [ results ] in 
 the apb muscle , the mep amplitude significantly decreased after ems , and this effect 
 lasted for 20  min . 
 [ conclusion ] the excitability of the corticospinal tract decreased 
 after short - duration high - frequency ems , and the effect lasted for 20  min . these results 
 suggest that even short duration ems can change the excitability of the corticospinal 
 tract .