Afferent and efferent control of stance and gait: developmental changes in children

Comparisons were made between the cerebral potentials (CPs) and EMG responses of leg muscles evoked by perturbation impulses during stance and gait in normal children aged from 1 to 10 years. Changes in the efferent arm of the reflex systems during development were reflected in parallel changes with age of the afferent system, expressed in the CP: in the youngest children (1-2 years of age) monosynaptic stretch reflex potentials appeared following perturbations during both stance and gait, together with a reduced level of longer latency EMG responses. The CP, too, had a profile that did not, at this early stage, differ in either condition. In children from 6 to 10 years of age, the adult pattern was reached, with the suppression of monosynaptic stretch reflexes and the early part of the CP during gait perturbation. This is interpreted as an inhibition of group I afferents at both segmental and supraspinal levels, involving suppression of both segmental stretch reflexes and group I signals to supraspinal centres. This control of afferent information had yet to be established in early infancy. The age group from 2 to 6 years showed progressive changes, with an increase in both the level and phasic nature of polysynaptic EMG responses and a corresponding transformation of the latency and shape of the CP. It is suggested that maturation of compensatory EMG responses during gait is achieved by the establishment of descending inhibition of group I afferents and facilitation of polysynaptic spinal reflexes via group II afferents.     

Role of peripheral afferents and spinal reflexes in normal and impaired human locomotion

For many years, electrophysiological investigations of locomotion were restricted to animals, largely the cat. They concentrated on and emphasized the role of spinal interneuronal networks responsible for the generation of the locomotor pattern. Following the introduction of perturbation impulses and electrical nerve stimulation during stance and gait, information became increasingly available concerning the role of the reflex systems involved in the regulation of gait, their afferent pathways and their control by supraspinal motor centres. During gait monosynaptic stretch reflexes are inhibited. From a knowledge of the behaviour of the cerebral potentials evoked during stance and gait, it can be deduced that during gait the signals of group I afferents are blocked at both segmental and supraspinal levels. Polysynaptic reflex responses are mainly responsible for the compensation of perturbations introduced during gait. They are most probably mediated by group II afferents via a spinal pathway closely connected with the spinal locomotor centres. The functioning of these responses depends on an intact supraspinal control. They are suggested to be incorporated in a more complex e.m.g. pattern mainly determined by central mechanisms. In contrast to the gait condition, segmental stretch reflex activity does contribute to activation of extensor muscles of the leg during fast movements, such as running and hopping. In children at an early stage in the development of gait (around 1 to 2 years of age), as well as in patients with spastic paresis, the polysynaptic reflex responses are reduced or absent, and isolated monosynaptic reflex potentials are present. This suggests a reciprocal modulation of mono- and polysynaptic reflex mechanisms, both being dependant on supraspinal control. When this control is either not yet matured (small children) or impaired (spastic paresis), inhibition of monosynaptic stretch reflexes is absent and associated with a reduced facilitation of polysynaptic spinal reflexes. In spastic muscle hypertonia, the tension developed at the Achilles tendon during gait cannot be explained by gastrocnemius activation alone. In patients with spastic hemiparesis gastrocnemius e.m.g. activity is reduced in the spastic leg as compared to the unaffected one. It can be concluded that the paretic muscle undergoes changes in its mechanical properties, secondary to the supraspinal lesion, which results in the development of spastic muscle hypertonia.(ABSTRACT TRUNCATED AT 400 WORDS)     

Tension development and muscle activation in the leg during gait in spastic hemiparesis: independence of muscle hypertonia and exaggerated stretch reflexes.

In 15 patients with spastic hemiparesis the development of tension of calf muscles in relation to their electrical activation and their stretching period was studied on both sides during locomotion. Only in the spastic leg did isolated small biphasic potentials appear in the gastrocnemius E.M.G. with monosynaptic latency at the beginning of the stance phase, while the remaining gastrocnemius activation was reduced compared to the unaffected side. Perturbations of gait were followed in the spastic leg by a large monosynaptic response, while the polysynaptic reflex response was reduced. In the unaffected leg only a strong polysynaptic response appeared, which suggests a reciprocal modulation of monosynaptic and polysynaptic reflex responses. Tension development paralleled the gastrocnemius E.M.G. in the unaffected leg, while in the spastic leg tension was more closely correlated to muscle stretch. It is concluded that in spasticity the exaggerated monosynaptic reflexes represent only a small part of leg extensor activation during gait and that the tension development does not depend on these reflexes.     

Spinal cord reflexes induced by epidural spinal cord stimulation in normal awake rats

Motor responses in hindlimb muscles to epidural spinal cord stimulation in normal awake rats during bipedal standing were studied. Stimulation at L2 or S1 induced simultaneous and bilateral responses in the vastus lateralis, semitendinosus, tibialis anterior, and medial gastrocnemius muscles. Stimulation at S1 evoked an early (ER), middle (MR) and late (LR) response: stimulation at L2 elicited only a MR and LR. Vibration and double epidural stimulation testing suggests that the ER is a direct motor response, whereas the MR and LR are mediated synaptically. MR has properties of a monosynaptic reflex, i.e., inhibited during vibration and depressed during the second pulse of a double stimulation. Some components of the LR seem to be mediated by afferents associated with the flexor reflex and probably involve group II afferents. During bipedal treadmill stepping, the MR was modulated in extensors, whereas the LR was modulated in flexors. These results show differential modulation of monosynaptic and polysynaptic reflexes in flexor and extensor motor pools during locomotion. Monosynaptic responses to stimulation at either L2 or S1 generally were amplified in extensors during the stance phase and in flexors during the swing phase of the step cycle. No correlation was found between the ER and the EMG background during stepping, whereas both the MR and LR were closely correlated with the changes in the EMG activity level of the corresponding muscle. These data demonstrate the feasibility of using epidural stimulation for examining monosynaptic and polysynaptic pathways to motor pools associated with multiple muscles during movement and over a prolonged period.     

Normal and impaired regulation of muscle stiffness in gait: A new hypothesis about muscle hypertonia

The activation of leg muscles was analyzed in respect to ankle joint movement and the changes in tension produced by the triceps surae muscle during slow gait in spastic adults and children with cerebral palsy. In normal subjects the increase in tension of the triceps surae in the stance phase of gait is mainly due to an increase in gastrocnemius and sole EMG. In spastic patients the abnormally high tension development in triceps surae is due more to passive muscle stretch, for the reciprocally organized leg muscle EMG is reduced. It is concluded that the leg extensor muscles in spastic patients exhibit a pseudostretch-reflex behavior due to their mechanical properties, and that this is mainly responsible for muscle hypertonia. The coactivation of the leg muscles seen in children with cerebral palsy, which also is seen in the stepping of the newborn, suggests impaired maturation of the neuronal locomotor pattern.     

Pathophysiology of gait in children with cerebral palsy

The surface electromyogram (EMG) of leg muscles was recorded together with the changes of the angle at the ankle joint during slow gait in 10 normal children and 10 with cerebral palsy. The characteristic pattern of muscle activity recorded from the spastic legs mainly consisted of a co-activation of antagonistic leg muscles during the stance phase of a gait cycle and a general reduction in amplitude of EMG activity. The tension of the Achilles tendon, measured in 2 hemiparetic children during gait, increased much more steeply in the spastic leg than in the normal one at the beginning of the stance phase, when the electrically almost silent triceps surae was stretched. It is suggested that muscle hypertonia during gait in spastic children is mainly due to changed muscle fibre mechanical properties, as recently discussed also for spastic adults. While in the latter the reciprocal EMG activity of antagonistic leg muscles was preserved it is proposed that muscle co-activation recorded in spastic children is due to an impaired maturation of the locomotor pattern with an early neuronal adaptation to altered muscle fibre mechanical characteristics.     

Spastic paresis: impaired spinal reflexes and intact motor programs.

Leg muscle EMG responses evoked by short treadmill acceleration impulses applied during stance were analysed in patients with spastic hemiparesis. The compensatory reactions on the unaffected side consisted of a diphasic pattern of leg muscle activation. The first response could best be described as a polysynaptic spinal stretch reflex response. This response was absent on the spastic side, except for its later, declining component. This remainder of the first response and the following activation of the antagonistic muscle was identical on both the unaffected and the spastic side. This part of the pattern is assumed to be centrally programmed (at the spinal level) and triggered by the termination of the acceleration impulse.     

Stumbling reactions in man: influence of corticospinal input

The aim of this study was to evaluate the degree of contribution of supraspinal input to the generation of the compensatory leg muscle activation following stance perturbation. Therefore, evoked motor response (EMR) input–output relations of two different motor tasks were compared at 3 distinct periods: (1) the basic period of muscular activity during standing, i.e. when no additional cortical or spinal activity due to the different tasks is to be expected, (2) the pre-movement period with low background activity, when different spinal and cortical inputs to the motoneuronal pool can be assumed and (3) the period of plateau EMG activity of compensatory and voluntary motor task. Transcranial magnetic stimulation (TMS) just below the motor threshold was applied randomly at 19 different time-intervals before and during the onset of stance perturbation and for comparison during an equivalent voluntary foot-dorsiflexion task. Recordings of electromyographic (EMG) activity from the tibialis anterior (TA) and corresponding ankle-joint movements were made from both legs. Forward-directed displacements were induced by randomly-timed ramp impulses of constant acceleration upon a moveable platform. For comparison, leg muscle EMG was recorded during isometric foot dorsiflexion during stance while leaning back against a support. The stance perturbations were followed by a compensatory response (CR) in the TA with a mean onset time of 81 ms. During the basic period of muscular activity and the period of plateau EMG activity there was no significant difference of the input–output relation between stance perturbation and the voluntary motor task. However, in the voluntary task compared with the CR, there was significantly greater input–output relation (facilitation) of the EMR in the TA following TMS, which may be related to an increased cortical influence. In contrast to this result of the CR following stance perturbation, a facilitation of the EMR was described for hand muscles under corresponding conditions of automatic compensation for muscle stretch, suggesting a transcortical reflex loop. This difference in the results from upper and lower extremity muscles favors the assumption of a predominantly spinal generation of the TA-CR following stance perturbation.     

Maturation of set-modulation of lower extremity EMG responses to postural perturbations.

To analyse the influence of different "postural sets" on stance stabilizing EMG responses in children, EMG responses to toe-up tilt perturbations were recorded in 70 children between the age of 9 months and 10 years, as well as in a control group of 10 adults under different postural set conditions, using either bilateral destabilization with eyes opened, eyes closed, or introducing additional minute upper extremity support. Recordings were also made with the children seated in front of the platform with the ankle joint angle being identical to that in the standing condition. Also recordings were made after unilateral destabilization in bilateral lower leg muscles, to determine if there is a generalization of EMG response patterns to the mechanically not disturbed side. Across all age-groups the principal modulation of EMG response changes according to postural conditions was identical. Long latency (LL) EMG responses were down-regulated when additional upper extremity support was provided. LL-responses were abolished in the sitting condition. With unilateral destabilizations throughout all age-groups short latency responses were restricted to the perturbed side, whereas long latency responses could be obtained symmetrically. The proximal to distal gradient of recruitment of muscle groups, remained identical across all age-groups. The data indicate that the basic organizational principle of stance stabilizing EMG responses and their modification by postural sets remains invariant across development. This indicates that the involved organizational principles are present as soon as a child is able to stand upright and are not subject to further shaping by motor learning.     

Selective effects of vibration on monosynaptic and late EMG responses in human soleus muscle after stimulation of the posterior tibial nerve or a tendon tap.

In normal subjects it was possible to evoke tendon and Hoffman reflexes which were followed by late EMG responses with a latency of 150-250 ms after the reflex stimuli. Analysis of the covariations of reflexes and late responses revealed that muscle spindle sensitivity and strength of the preceding twitch are not essential factors in determining the occurrence of the late responses as opposed to excitability changes within the spinal cord. Inhibition of monosynaptic reflexes and facilitation of late EMG responses to vibration indicate a difference in central pathways. A polysynaptic pathway may be involved in the late responses.     

Inhibitory effect of acupuncture on the vibration-induced finger flexion reflex in man

Experiments were performed on 12 young and healthy subjects. The palmar side of the index finger was vibrated with a triangular wave form at randomly changed inter-stimulus intervals. This vibratory stimulation elicited a flexion movement of the index finger, which was depressed by acupuncture at 'Wai-Kuan.' It was generally observed that the depressive effect developed slowly and lasted for a long time. The depressive effect of acupuncture on this reflex was analyzed using a cross-correlation between the vibratory stimuli and motor unit spikes recorded from the flexor digitorum communis muscle. Cross-correlograms revealed two types of reflex responses. One of them showed a peak of time-locked spikes to the vibratory stimuli at a latency of 25 msec, which indicates monosynaptic activity of the motoneuron, and also showed time-unlocked spikes distributed in both sides of the peak, which implies polysynaptic activity. This type of response was frequently observed and was supposed to be elicited by the proprioceptive tonic vibration reflex. The other revealed no peaks, which indicates that only time-unlocked spikes to the stimuli are generated, and that these spikes are evoked via polysynaptic pathway. This type of response was supposed to be elicited by so-called exteroceptive vibration-induced finger flexion reflex. Since both types of reflex responses, i.e., mono- and polysynaptic ones, were equally depressed during acupuncture, it was concluded that the excitability of the motoneuron, a common output of this reflex, is lowered by acupuncture.     

The role of preparation in tuning anticipatory and reflex responses during catching

The pattern of muscle responses associated with catching a ball in the presence of vision was investigated by independently varying the height of the drop and the mass of the ball. It was found that the anticipatory EMG responses comprised early and late components. The early components were produced at a roughly constant latency (about 130 msec) from the time of ball release. Their mean amplitude decreased with increasing height of fall. Late components represented the major build-up of muscle activity preceding the ball's impact and were accompanied by limb flexion. Their onset time was roughly constant (about 100 msec) with respect to the time of impact (except in wrist extensors). This indicates that the timing of these responses was based on an accurate estimate of the instantaneous values of the time-to-contact (time remaining before impact). The mean amplitude of the late anticipatory responses increased linearly with the expected momentum of the ball at impact. The reflex responses evoked by the ball's impact consisted in a short-latency coactivation of flexor and extensor muscles at the elbow and wrist joints. Their mean amplitude generally increased with the intensity of the perturbation both in the stretched muscles and in the shortening muscles. We argue that both the anticipatory and the reflex coactivation are centrally preset in preparation for catching and are instrumental for stabilizing limb posture after impact. A model with linear, time-varying viscoelastic coefficients was used to assess the neural and mechanical contributions to the damping of limb oscillations induced by the ball's impact. The model demonstrates that (1) anticipatory muscle stiffening and anticipatory flexion of the limb are synergistic in building up resistance of the hand to vertical displacement and (2) the reflex coactivation produces a further increment of hand stiffness and viscosity which tends to offset the decrement which would result from the limb extension produced by the impact.     

Phase-dependent modulation of cutaneous reflexes in tibialis anterior muscle during passive stepping

The purpose of this study was to determine whether the cutaneous reflex elicited in the tibialis anterior (TA) muscle would be modulated in a phase-dependent manner while human subjects were passively stepping on a treadmill (treadmill stepping) or in the air (air stepping). The passive stepping was produced by a robotic gait trainer, Lokomat. The cutaneous reflexes following electric stimulation to the distal tibial nerve were recorded at ten different phases of a step cycle under the condition of tonic dorsiflexion [10% of maximum electromyography activity (EMGmax)]. Cutaneous reflex EMG responses with peak latencies of 70-120 ms [middle latency responses (MLR)] were then analysed. The results showed that there were no visible differences in the background EMG activities at the ten phases or two passive stepping conditions. During treadmill stepping, however, the magnitude of the facilitatory reflex responses between the late stance and the early swing phase was strongly enhanced, whereas no clear modulation of the MLR during air stepping was observed. These results suggest that the load-related afferent information plays a key role in the modulation of the cutaneous reflex during human walking.     

Discharge pattern of single motor units in the tonic vibration reflex of human triceps surae.

Using a single fibre EMG electrode the firing pattern of 46 motor units in the triceps surae has been studied during vibration of the Achilles tendon at frequencies of 25--200 Hz. Potentials activated in the tonic vibration reflex (TVR) were phase-locked to the vibration cycle but tended to become somewhat less so with continued vibration. The firing pattern of voluntarily activated motor units became locked to the waveform by the application of the vibrator. The discharges of 21 motor units were studied during low threshold (sub-M wave) tetanic stimulation of the tibial nerve at 25--100 Hz. No evidence was found of synchronization of potentials activated in the resulting tonic contraction. During weak voluntary contractions, stimulation also failed to regularize voluntarily activated motor units. The findings can be reconciled by postulating that, in normal man, vibration activates monosynaptic and polysynaptic pathways, the latter circuit being adequate to generate reflex contraction, while the former merely affects the temporal patterning of the motor outflow.     

Neuronal coordination of arm and leg movements during human locomotion

We aimed to study the neuronal coordination of lower and upper limb muscles. We therefore evaluated the effect of small leg displacements during gait on leg and arm muscle electromyographic (EMG) activity in walking humans. During walking on a split-belt treadmill (velocity 3.5 km/h), short accelerations or decelerations were randomly applied to the right belt during the mid or end stance phase. Alternatively, trains of electrical stimuli were delivered to the right distal tibial nerve. The EMG activity of the tibialis anterior (TA), gastrocnemius medialis (GM), deltoideus (Delt), triceps (Tric) and biceps brachii (Bic) of both sides was analysed. For comparison, impulses were also applied during standing and sitting. The displacements were followed by specific patterns of right leg and bilateral arm muscle EMG responses. Most arm muscle responses appeared with a short latency (65-80 ms) and were larger in Delt and Tric than in Bic. They were strongest when deceleration impulses were released during mid stance, associated with a right compensatory TA response. A similar response pattern in arm muscles was obtained following tibial nerve stimulation. The arm muscle responses were small or absent when stimuli were applied during standing or sitting. The arm muscle responses correlated more closely with the compensatory TA than with the compensatory GM responses. The amplitude of the responses in most arm muscles correlated closely with the background EMG activity of the respective arm muscle. The observations suggest the existence of a task-dependent, flexible neuronal coupling between lower and upper limb muscles. The stronger impact of leg flexors in this interlimb coordination indicates that the neuronal control of leg flexor and extensor muscles is differentially interconnected during locomotion. The results are compatible with the assumption that the proximal arm muscle responses are associated with the swinging of the arms during gait, as a residual function of quadrupedal locomotion.     

Spindle activity and monosynaptic reflex excitability during foreperiod

Healthy volunteers were instructed to perform an isometric plantar foot flexion as quickly as possible after a foreperiod (FP) of 1000 msec defined by two clicks (warning signal (WS) and response signal (RS). In 6 volunteers the H reflex was evoked in triceps surae muscle and recorded by surface electrodes (stimulus intensity 30% of maximum). The H reflex was elicited at WS and RS as well as during FP at intervals of 100 msec. H reflex amplitudes were taken as a sign of monosynaptic reflex excitability (MSRE). Amplitudes during FP were compared with the average control values at rest. Relaxation of lower limb muscles before and during FP was controlled by EMG. MSRE was increased in the first part of FP with a maximum at 300 msec after WS and decreased in the second part, with a minimum at 800 msec after WS. In a second series of experiments, in 10 volunteers, single fiber activity from primary muscle spindle afferents was recorded with tungsten electrodes from deep peroneal nerve (6 records) and from tibial nerve (3 records). The activity of primary spindle afferents before and during the FP was calculated by instantaneous discharge frequency and histograms of spike distribution. The EMG was taken from sural triceps and anterior tibial muscles with needle electrodes; a mechanogram of tendon deflection was taken by an appropriate strain gauge. In 5 primary afferents without spontaneous activity at rest and during FP, discharge started with a delay of 10-15 msec after the onset of EMG activity during the motor reaction.(ABSTRACT TRUNCATED AT 250 WORDS)     

Soleus muscle length, stretch reflex excitability, and the contractile properties of muscle in children and adults: a study of the functional joint angle

The influence of the joint angle on stretch reflex excitability of the soleus muscle at the ankle has been studied in 22 children aged 3.9 to 13.6 years and 9 adults aged 19 to 70 years. For all subjects, reflex EMG and mechanical twitch torque gain were trivial at resting plantar flexion. The reflex EMG gain reached a maximum between-15^o and-10^o of plantar flexion beyond the neutral angle, 0^o, denned as the foot at right angle to the tibia, diminishing steeply with further dorsiflexion. The reflex mechanical gain rose to a peak between 0^o and +10^o of dorsiflexion beyond neutral, declining steeply thereafter. By contrast, axonally stimulated muscle twitch torque increased serially up to +30^o dorsiflexion beyond neutral. For the soleus muscle, the optimal reflex neuromechanical angle lies approximately midway between the angle for optimal reflex EMG gain (in mild plantar flexion, at which the largest and strongest motor units can be activated) and the optimal muscle mechanical angle (at the extreme of soleus muscle dorsiflexion). These studies confirm that the excitability of the spinal alpha motor neuron pool in vivo is strongly influenced by muscle length and explain the variability in reflex excitability within and between subjects, if the joint angle is not controlled. They also indicate how posture influences movement, agreeing with the known function of the soleus muscle in the stance phase of gait and the modulation of motor unit recruitment during voluntary alternating movements at the ankle. Soleus muscle twitch characteristics show a fivefold to eightfold increase in peak force associated with a tenfold reduction in compliance in the first two decades of life and an apparent speeding up of twitch time in the first decade.     

Descending supraspinal effect in tetanus intoxication of the spinal cord

In experiments performed on cats under Nembutal-chloralose anesthesia it was established that in development of local tentanus there occurs a strengthening of facilitatory and weakening of inhibitory descending effects on the side of toxin injection. At stimulation of facilitatory structures of the medulla oblongata (parvocellular and ambigus nuclei) strengthening of extensor and flexor monosynaptic reflexes and appearance of polysynaptic activity were observed. Stimulation of structures which in the normal state evokes inhibition (gigantocellular and raphe nuclei) in local tetanus produced facilitation instead of inhibition of extensor and flexor monosynaptic reflexes and appearance of polysynaptic activity on the side of toxin injection. On the contralateral side at stimulation of the same nuclei usual inhibitory effects are observed. The intracellular recording of motoneurones potentials showed abolishment of IPSP in tetanus at stimulation of different brain stem structures and their replacement with EPSP. The study of temporal course of suprasegmental inhibition of monosynaptic and early polysynaptic reflexes in normal state and on the side of toxin injection has shown that there are two components. The first short-lasting component (20–60 msec) and the second-later with a maximum about 160 msec. In local tetanus the short-lasting inhibition which is apparently of postsynaptic origin was abolished. The late inhibition was maintained, but slightly weakened. Inhibition of late polysynaptic relfexes belonging by their characteristics to spinobulbo-spinal ones were not abolished but even strengthened in local tetanus. The possible mechanism of appearance of excitatory responses instead of inhibitory ones in local tetanus are discussed.     

The influence of vibration on the excitability of alpha motoneurones

Transcranial magnetic brain stimuli were delivered to 6 healthy subjects at different time intervals after the beginning of muscle vibration. Vibration of 6 sec duration in 4 subjects and of 100 msec duration in 6 subjects was applied to the right abductor digiti minimi muscle using an electromagnetic mechanical stimulator. The responses to brain stimuli were enhanced in this muscle when vibration began 9 msec before the transcranial stimulus, i.e., when the descending volley and the monosynaptic afferent Ia volley arrived simultaneously at the anterior horn cell. With long lasting vibration an enhancement of responses to brain stimuli was seen, which began after 120 msec and continued for up to 5 sec after the onset of vibration. This is consistent with a tonic, probably polysynaptic, excitatory Ia influence on homonymous alpha motoneurones, as well as the well known monosynaptic effect.     

Inhibition of hand muscle motoneurones by peripheral nerve stimulation in the relaxed human subject. Antidromic versus orthodromic input

In active muscle, a supramaximal conditioning stimulus to peripheral nerve produces a classic silent period in the EMG. The present experiments examined the effect of this type of conditioning stimulus on motoneurone excitability in relaxed muscle.EMG responses evoked by transcranial magnetic stimulation of the brain were recorded from the first dorsal interosseus muscle (FDI) in 10 healthy subjects and 5 patients with sensory neuropathy. These responses (motor evoked potentials) were conditioned by supramaximal peripheral nerve stimuli given 0–150 msec beforehand. In the normal subjects, the classic silent period in the FDI lasted about 100 msec. The same conditioning stimulus only abolished motor evoked potentials when the conditioning-test interval was so short that the antidromic peripheral nerve volley collided with the orthodromic volley set up by magnetic brain stimulation. At longer conditioning-test intervals, although remarkably inhibited (65% mean suppression between 10 and 40 msec), the test motor potential was never completely abolished and gradually recovered by 100 msec.Inhibition of cortically evoked motor potentials did not depend upon activity set up by the conditioning stimulus in peripheral nerve sensory fibres. The patients with complete peripheral sensory neuropathy had the same extent and time-course of inhibition as the normal subjects. We conclude that in relaxed subjects the inhibitory effect of peripheral conditioning results almost exclusively from the motoneuronal inhibitory mechanisms consequent to antidromic invasion.