the vastus medialis ( vm ) is regarded an important muscle in knee rehabilitation . however , 
 there is limited evidence regarding its physiological function . a systematic review by smith 
 et al.1 discussed how the vm is divided by 
 differentiation of fibre orientation , the presence of a fascial plane that separates vm , and 
 different innervation patterns . the fibres in the distal part of vm run approximately 5055 medially in relation to the 
 long axis of the femur in the frontal plane and insert as fleshy fibres into the medial 
 patellar retinaculum and the upper half of the medial side of the patella2,3,4,5,6 . therefore , the distal part has been termed 
 the vm obliquus ( vmo ) , and its contraction pulls the patella medially and rotates the tibia 
 internally via the patella and the patellar tendon7,8,9 . in contrast , the proximal fibres of vm run approximately 1520 
 medially and insert into the medial margin and anterior surface of the aforementioned 
 aponeurosis , which merges with the aponeurosis of the vastus intermedius muscle2,3,4,5,6 . therefore , the proximal part has been 
 termed the vm longus ( vml ) . although the main action of the vml is to extend the knee , it 
 also stabilizes the patella similar to the vmo3 , 4 , 10 , 11 . there are conflicting reports of specific separate innervation3 , 5 , 12 , 13 , and shared 
 innervation4 , 13 , 14 of the two structures . if the innervation were separate , the timing of contraction in each part would be 
 independently regulated , and the differentiation of muscle activation related to vm 
 contraction could be revealed by electromyographic ( emg ) findings . although there are some 
 emg studies on the subject , muscle activation patterns in the two divisions of vm have not 
 yet been determined10 , 15,16,17,18 . investigating the excitability of spinal motoneuron as the final common pathway that drives 
 the muscles may elucidate the physiological function of vm . the hoffmann reflex ( h - reflex ) 
 amplitude is useful as an indirect measure of the excitability of the spinal motoneurons 
 activating vm19 . however , when electrical 
 stimulation for evoking the h - reflex is delivered to the femoral nerve just below the 
 inguinal ligament , it is not possible to distinguish the branches of the different 
 components in the quadriceps femoris20 . therefore , matching the stimulus intensity for evoking each h - reflex from vmo and vml is 
 difficult . also , the temporal summation of the m - wave and h - reflex is likely to occur in the 
 quadriceps femoris muscle ( especially the h - reflex in vml ) . for these reasons , we also used 
 the compound muscle action potential ( cmap ) evoked by the patellar tendon reflex ( i.e. 
 t - wave ) as an index in addition to the h - reflex . the purpose of this present study was to 
 investigate and compare the h - reflexes and t - waves in vmo and vml . this study was approved by the institutional research ethics committee of kansai university 
 of health sciences . the study procedures , risks , and benefits were explained verbally and in 
 writing to all the participants , who provided their written informed consent prior to their 
 participation . we analysed h - reflex and t - wave characteristics recorded in vmo and vml . twelve healthy male volunteers ( mean age : 21.32.3  years , mean height : 173.73.8  cm ) 
 participated in this study . subjects sat on the seat of a torque machine ( biodex medical systems , inc . , usa ) with 
 their left hips and knees flexed at 30 and 60 ( 0 indicates the extended position ) , 
 respectively . the h - reflex was recorded using a viking quest system ver.9 ( nicolet 
 biomedical , inc . the h - reflex was evoked during knee extension at 10% mvic in order 
 to regularise the appearance of the h - reflex in vm . the cathodal electrode ( 10  mm 
 diameter ) for evoking the h - reflex was positioned over the femoral nerve just below the 
 inguinal ligament and the anodal electrode ( 40  50  mm ) was placed over the greater 
 trochanter . stimulus conditions for evoking h - reflexes were 1.0  ms rectangular wave pulses 
 with a frequency of 0.2  hz . the stimulus intensity was the threshold for the appearance of 
 m - wave . five vmo h - reflexes were recorded , and then a short break was taken before 
 recording five vml h - reflexes within a bandwidth of 20  hz3 khz . lastly , the maximal m - wave was recorded once in each muscle by supramaximal 
 stimulation to the femoral nerve . an exploring electrode ( ag / agcl , 10  mm diameter ) for recording the h - reflex and maximal 
 m - wave was positioned over the vmo motor point . the exploring electrode for vml was 
 positioned on the muscle belly slightly distal to the motor point to avoid sartorius 
 muscle activation because the location of the motor point was close to the sartorius 
 muscle . both motor points were defined as the point where muscle fibre recruitment was 
 obtained with the least amount of current . to locate the motor points , the two divisions 
 of vm were investigated by moving a pentype cathodal probe over the muscle belly as 
 distally or proximally as possible using a recording chronaxie meter cx-3 ( og giken , co. , 
 ltd . the anode was placed over the proximal aspect of the belly of the rectus 
 femoris . the current was a 1.0  ms monophasic square wave that was percutaneously delivered 
 to the muscle at a frequency of 1.0  hz . reference ( ag / agcl , 10  mm diameter ) and ground 
 electrodes were positioned over the patella and the centre of the thigh , respectively . 
 before electrode placement , the skin surface at each site was rubbed with an abrasive gel 
 and cleansed with alcohol to remove dead skin cells and oils that might have affected the 
 signal integrity . the amplitudes of the h - reflex and maximal m - wave were measured from the baseline to the 
 negative peak . ten h - reflex amplitudes were averaged and normalised to the maximal m - wave 
 amplitude in each muscle . h - reflex duration was measured as the time of the negative 
 phase , and the 10 h - reflex durations were averaged . the normalised amplitude and duration 
 of the h - reflex in vmo were compared with those in vml using student s t test . statistical 
 analysis was performed using ibm spss statistics version 19 and a significance level of 
 5% . eleven healthy male volunteers ( mean age : 21.41.2  years , mean height : 171.96.2  cm ) with 
 sensitive patellar tendon reflexes participated in this study . none of the subjects had a 
 history of orthopaedic or neurological disease . subjects relaxed on a reclining seat with their left hips and knees flexed at 45 and 
 60 , respectively . the examiner tapped the patellar tendon using an electrical tendon 
 hammer with an emg unit attached ( viking quest system ver.9 , nicolet biomedical , inc . , consequent t - waves were simultaneously recorded from the vmo and vml within a 
 bandwidth of 20  hz3 khz . exploring electrodes ( ag / agcl , 10  mm diameter ) for recording the 
 t - waves were positioned on the motor points of the vmo and vml . the reference and ground 
 electrodes were positioned as described for the h - reflex . the same examiner evoked the t - waves in all subjects and tried to elicit t - waves as large 
 and as stably as possible . four t - waves in each of vmo and vml were recorded for analysis . lastly , the maximal m - wave was recorded once in each muscle by supramaximal electrical 
 stimulation of the femoral nerve . the amplitudes of the t - wave and maximal m - wave were measured from the baseline to the 
 negative peak . four t - wave amplitudes were averaged and normalised to the maximal m - wave 
 amplitude . t - wave duration was measured as the time of the negative phase , and four t - wave 
 durations were averaged . the normalised amplitude and duration in vmo were compared with 
 those in vml as described for experiment 1 . twelve healthy male volunteers ( mean age : 21.32.3  years , mean height : 173.73.8  cm ) 
 participated in this study . subjects sat on the seat of a torque machine ( biodex medical systems , inc . , usa ) with 
 their left hips and knees flexed at 30 and 60 ( 0 indicates the extended position ) , 
 respectively . the h - reflex was recorded using a viking quest system ver.9 ( nicolet 
 biomedical , inc . the h - reflex was evoked during knee extension at 10% mvic in order 
 to regularise the appearance of the h - reflex in vm . the cathodal electrode ( 10  mm 
 diameter ) for evoking the h - reflex was positioned over the femoral nerve just below the 
 inguinal ligament and the anodal electrode ( 40  50  mm ) was placed over the greater 
 trochanter . stimulus conditions for evoking h - reflexes were 1.0  ms rectangular wave pulses 
 with a frequency of 0.2  hz . the stimulus intensity was the threshold for the appearance of 
 m - wave . five vmo h - reflexes were recorded , and then a short break was taken before 
 recording five vml h - reflexes within a bandwidth of 20  hz3 khz . lastly , the maximal m - wave was recorded once in each muscle by supramaximal 
 stimulation to the femoral nerve . an exploring electrode ( ag / agcl , 10  mm diameter ) for recording the h - reflex and maximal 
 m - wave was positioned over the vmo motor point . the exploring electrode for vml was 
 positioned on the muscle belly slightly distal to the motor point to avoid sartorius 
 muscle activation because the location of the motor point was close to the sartorius 
 muscle . both motor points were defined as the point where muscle fibre recruitment was 
 obtained with the least amount of current . to locate the motor points , the two divisions 
 of vm were investigated by moving a pentype cathodal probe over the muscle belly as 
 distally or proximally as possible using a recording chronaxie meter cx-3 ( og giken , co. , 
 ltd . , the anode was placed over the proximal aspect of the belly of the rectus 
 femoris . the current was a 1.0  ms monophasic square wave that was percutaneously delivered 
 to the muscle at a frequency of 1.0  hz . reference ( ag / agcl , 10  mm diameter ) and ground 
 electrodes were positioned over the patella and the centre of the thigh , respectively . 
 before electrode placement , the skin surface at each site was rubbed with an abrasive gel 
 and cleansed with alcohol to remove dead skin cells and oils that might have affected the 
 signal integrity . the amplitudes of the h - reflex and maximal m - wave were measured from the baseline to the 
 negative peak . ten h - reflex amplitudes were averaged and normalised to the maximal m - wave 
 amplitude in each muscle . h - reflex duration was measured as the time of the negative 
 phase , and the 10 h - reflex durations were averaged . the normalised amplitude and duration 
 of the h - reflex in vmo were compared with those in vml using student s t test . statistical 
 analysis was performed using ibm spss statistics version 19 and a significance level of 
 5% . eleven healthy male volunteers ( mean age : 21.41.2  years , mean height : 171.96.2  cm ) with 
 sensitive patellar tendon reflexes participated in this study . subjects relaxed on a reclining seat with their left hips and knees flexed at 45 and 
 60 , respectively . the examiner tapped the patellar tendon using an electrical tendon 
 hammer with an emg unit attached ( viking quest system ver.9 , nicolet biomedical , inc . , consequent t - waves were simultaneously recorded from the vmo and vml within a 
 bandwidth of 20  hz3 khz . exploring electrodes ( ag / agcl , 10  mm diameter ) for recording the 
 t - waves were positioned on the motor points of the vmo and vml . the reference and ground 
 electrodes the motor points were identified 
 as in experiment 1 . the same examiner evoked the t - waves in all subjects and tried to elicit t - waves as large 
 and as stably as possible . four t - waves in each of vmo and vml were recorded for analysis . lastly , the maximal m - wave was recorded once in each muscle by supramaximal electrical 
 stimulation of the femoral nerve . the amplitudes of the t - wave and maximal m - wave were measured from the baseline to the 
 negative peak . four t - wave amplitudes were averaged and normalised to the maximal m - wave 
 amplitude . t - wave duration was measured as the time of the negative phase , and four t - wave 
 durations were averaged . the normalised amplitude and duration in vmo were compared with 
 those in vml as described for experiment 1 . in three subjects , the accurate measurement of amplitude and duration of h - reflexes in 
 vml was difficult because the baseline immediately before h - reflex appearance was unstable 
 due to the m - wave . the levels of the exploring electrodes for vmo and vml were 3.10.6  cm and 12.40.9  cm , 
 respectively , from the superior border of the patella along the longitudinal axis of the 
 thigh . the h - reflexes were evoked by a 
 stimulus intensity of 15.6 ma shows typical h - reflexes . the negative phase of the h - reflex in vmo was sharp , 
 while that in vml was slow . the normalised amplitude and duration of h - reflexes in vmo and 
 vml are presented in table 1table 1 . mean and sd of h - reflex and t - wave amplitudes and durationsnormalised amplitudes ( % ) durations ( ms)h - reflexvmo10.86.08.60.8vml9.45.310.31.0*t - wavevmo32.510.311.71.2vml36.611.814.92.4**p<0.001 ; vmo : vastus medialis obliquus ; vml : vastus medialis longus . no significant difference in normalised h - reflex amplitudes was observed 
 between vmo and vml but h - reflex duration was significantly longer in vml 
 ( p<0.001 ) . the h - reflexes were evoked by a 
 stimulus intensity of 15.6 ma * p<0.001 ; vmo : vastus medialis obliquus ; vml : vastus medialis longus the levels of the exploring electrodes for vmo and vml were 2.80.6  cm and 14.41.3  cm , 
 respectively , from the superior border of the patella along the longitudinal axis of the 
 thigh . the vmo and vml traces correspond , e.g. 
 the first traces in vmo and vml were simultaneously recorded following a patellar 
 tendon tap displays typical t - waves . the negative phase of the t - wave in vmo was sharp , while 
 that in vml was slow . the normalised amplitudes and durations of t - waves in vmo and vml 
 are presented in table 1 . no significant 
 difference was observed in the normalised t - wave amplitude between vmo and vml . t - wave 
 duration was significantly longer in vml than in vmo ( p<0.001 ) . the vmo and vml traces correspond , e.g. 
 the first traces in vmo and vml were simultaneously recorded following a patellar 
 tendon tap finally , each maximal m - wave configuration recorded from vmo and vml was similar to those 
 observed for the h - reflex and t - wave . in three subjects , the accurate measurement of amplitude and duration of h - reflexes in 
 vml was difficult because the baseline immediately before h - reflex appearance was unstable 
 due to the m - wave . the levels of the exploring electrodes for vmo and vml were 3.10.6  cm and 12.40.9  cm , 
 respectively , from the superior border of the patella along the longitudinal axis of the 
 thigh . the h - reflexes were evoked by a 
 stimulus intensity of 15.6 ma shows typical h - reflexes . the negative phase of the h - reflex in vmo was sharp , 
 while that in vml was slow . the normalised amplitude and duration of h - reflexes in vmo and 
 vml are presented in table 1table 1 . mean and sd of h - reflex and t - wave amplitudes and durationsnormalised amplitudes ( % ) durations ( ms)h - reflexvmo10.86.08.60.8vml9.45.310.31.0*t - wavevmo32.510.311.71.2vml36.611.814.92.4**p<0.001 ; vmo : vastus medialis obliquus ; vml : vastus medialis longus . no significant difference in normalised h - reflex amplitudes was observed 
 between vmo and vml but h - reflex duration was significantly longer in vml 
 ( p<0.001 ) . the h - reflexes were evoked by a 
 stimulus intensity of 15.6 ma * p<0.001 ; vmo : vastus medialis obliquus ; vml : vastus medialis longus the levels of the exploring electrodes for vmo and vml were 2.80.6  cm and 14.41.3  cm , 
 respectively , from the superior border of the patella along the longitudinal axis of the 
 thigh . the vmo and vml traces correspond , e.g. 
 the first traces in vmo and vml were simultaneously recorded following a patellar 
 tendon tap displays typical t - waves . the negative phase of the t - wave in vmo was sharp , while 
 that in vml was slow . the normalised amplitudes and durations of t - waves in vmo and vml 
 are presented in table 1 . no significant 
 difference was observed in the normalised t - wave amplitude between vmo and vml . t - wave 
 duration was significantly longer in vml than in vmo ( p<0.001 ) . the vmo and vml traces correspond , e.g. 
 the first traces in vmo and vml were simultaneously recorded following a patellar 
 tendon tap finally , each maximal m - wave configuration recorded from vmo and vml was similar to those 
 observed for the h - reflex and t - wave . amplitude alteration of the h - reflex and t - wave can be explained by variations in 
 motoneuron excitability and the amount of neurotransmitter released by the afferent 
 terminals21 , 22 . therefore , it is considered that the alteration of their 
 amplitudes reflects spinal reflex arc excitability . although we could not confirm the innervation modes that drive vmo and vml in this study , 
 we were careful to ensure that each exploring electrode location matched the vmo and vml 
 areas in which the femoral nerve branches are specifically illustrated by lieb and 
 perry3 , and were photographed by ono et 
 al12 . emg studies have not previously shown functional differentiation between the two vm 
 divisions10 , 17 , 18 . rainoldi et al.15 and botter et al.16 investigated emg differentiation of the vmo , vml , and 
 vastus lateralis ( vl ) during a fatiguing condition using linear array of electrodes . in both 
 reports , the differences were greater between vl and the two vm divisions than between vmo 
 and vml . the lack of significant differences for h - reflex and t - wave amplitudes between vmo and vml 
 described in our present study may support previous emg research10 , 15,16,17,18 . however , the configurations of the h - reflex and t - wave in 
 vmo and vml are clearly different , and the durations of both parameters were significantly 
 longer in vml than in vmo . moreover , the maximal m - wave configurations in vmo and vml were 
 also clearly different . the h - reflex and t - wave are components of the late - response cmap . cmaps are the summation of nearly synchronous muscle fibre action potentials recorded from a 
 muscle , and are commonly produced by stimulating the nerve supplying the muscle either 
 directly or indirectly . the 
 impulses of slow - conducting fibres lag behind those of the fast - conducting fibres23 . when the difference in conduction 
 velocity between the fastest and slowest is high , the summation of muscle fibre action 
 potentials is asynchronously recorded . travnik et al.24 
 investigated the histochemical and morphometric characteristics of vml and vmo , and reported 
 that the proportions of type i and iib fibres were significantly higher and lower , 
 respectively , also , the diameters of type i and iia fibres were significantly smaller in vml 
 than in vmo . the analysed portions of vmo and vml were near the superomedial border of the 
 patella and approximately 15.0  cm proximal to the base of patella , which generally matches 
 the regions investigated in our study . based on the above data , we consider that the 
 different configurations of the h - reflex and t - wave were related to muscle fibre composition 
 in the two vm divisions examined in our study . although the h - reflex and t - wave are known to 
 be composed of slow motor units that are recruited first25 , we consider that there is a difference of muscle fibre properties 
 in the slow units . also , the earliest components of the h - reflex and t - wave are 
 predominantly comprised of monosynaptic ia effects , and the later portions probably include 
 contributions from the oligosynaptic ia pathway22 . this synaptic factor may relate to h - reflex and t - wave 
 configurations . in conclusion , although the excitability of spinal reflex arcs corresponding to vmo and vml 
 were similar , the differentiation of configurations and durations of the h - reflex and t - wave 
 were detectable on emg . on the basis of the present findings , vmo and vml training may be 
 addressed differently during knee rehabilitation because they are two electrophysiologically 
 distinct parts .
[ purpose ] the gross morphology of the vastus medialis ( vm ) muscle has been thoroughly 
 described . however , there is insufficient evidence of physiological differentiation 
 between the vm obliquus ( vmo ) and vm longus ( vml ) . to elucidate spinal reflex arc 
 excitability in two divisions of the vm , we compared h - reflexes and t - waves in vmo and 
 vml . 
 [ subjects ] twenty - three healthy male volunteers participated in this study . [ methods ] 
 the h - reflex was evoked from the vmo and vml by electrical stimulation of the femoral 
 nerve during knee extension at 10% maximal voluntary isometric contraction . 
 also , the 
 patellar tendon was tapped by an examiner using an electrical tendon hammer , and a 
 component of the compound muscle action potential ( t - wave ) was recorded . 
 [ results ] the 
 configurations of the h - reflex and t - wave were sharp and slow in vmo and vml , 
 respectively . 
 no significant differences in the amplitudes of the h - reflexes and t - waves 
 were observed between vmo and vml . 
 the durations of vml h - reflexes and t - waves were 
 significantly longer than those in vmo . 
 [ conclusion ] spinal reflex arc excitability 
 corresponding to vmo and vml was similar . 
 however , the configurations and durations of the 
 h - reflex and t - wave were differentiated with electromyography . on the basis of these 
 findings , we suggest that vmo and vml are electrophysiologically distinct entities .