Progressive adaptation of the soleus H-reflex with daily training at walking backward

When untrained subjects walk backward on a treadmill the amplitude of the soleus H-reflex in midswing is equal to or exceeds the value in stance. This is a surprising result because during the swing phase of backward walking the soleus is inactive and its antagonist, the tibialis anterior, is active. We suggested that the high amplitude of the soleus H-reflex in late swing reflects task uncertainties, such as estimating the moment of foot contact with the ground and losing balance. In support of this idea we show that when untrained subjects held on to handrails the unexpected high-amplitude H-reflex during midswing was no longer present. We therefore asked whether daily training at this task without grasping the handrails would adaptively modify the H-reflex modulation pattern. In this event, within 10 days of training for 15 min daily, the anticipatory reflex activity at the beginning of training was gradually abated as the subjects reported gaining confidence at the task. However, when adapted subjects were made to walk backward with their eyes shut, the anticipatory reflex activity in midswing returned immediately. The reflex changes as a result of training were not due to changes in the motor activity or kinematics; they are likely part of the motor program controlling backward walking. This adaptive phenomenon may prove to be a useful model for studying the neural mechanisms of motor learning and adaptive plasticity in humans and may be relevant to rehabilitation programs for neurological patients.     

Differential changes in corticospinal and Ia input to tibialis anterior and soleus motor neurones during voluntary contraction in man

Motor-evoked potentials (MEPs) were recorded in the tibialis anterior and soleus muscles following transcranial magnetic stimulation (TMS) of the motor cortex. In the soleus, the H-reflex amplitude increased with the contraction level to the same extent as that of MEPs, whereas in the tibialis anterior, the H-reflex amplitude increased significantly less than that of MEPs. The latency of the MEPs decreased with contraction, whereas this was not the case of the H-reflexes. In the tibialis anterior, the response probability of single-motor units (SMU) to TMS increased more substantially during voluntary contraction than following stimulation of the peroneal nerve. In the tibialis anterior, the response probability of SMU increased more substantially during voluntary contraction than following stimulation of the peroneal nerve. The short-latency facilitation, presumably monosynaptic of origin, of the soleus H-reflex evoked by subthreshold TMS increased as a function of the plantarflexion force. This was not the case for the heteronymous Ia facilitation of the soleus H-reflex following stimulation of the femoral nerve. It is concluded that the corticospinal input to lower limb motor neurones generated by TMS increases with the level of voluntary contraction, whereas this is true only to a limited extent for the synaptic input from Ia afferents. It is suggested that this reflects changes in the susceptibility of corticospinal cells to TMS during voluntary contraction.     

On the origin of the soleus H-reflex modulation pattern during human walking and its task-dependent differences

Recently, Brooke and colleagues have suggested "that the strong inhibition arising from passive movement about the knee and hip joints, lays down the base for the soleus H-reflex gain modulation seen during human gait." In particular stretch-evoked afferent activity from the quadriceps muscle was emphasized as the most important source of movement-induced inhibition of the H-reflex. To test this hypothesis we examined the kinematics and electromyographic (EMG) activity of the leg during human walking and correlated these with the modulation pattern of the soleus H-reflex. To further test the possible contribution of stretch-evoked quadriceps afferent activity to the soleus H-reflex modulation pattern during walking different walking gaits were studied. In one condition subjects were asked to walk with their knee locked in full extension by a rigid knee brace. In a second condition subjects were asked to walk backwards. During normal walking, the soleus H-reflex modulation pattern is strongly correlated with the EMG events of the soleus and tibialis anterior (TA), but not with hip, knee, or ankle angular displacement or velocity. When subjects walked with the knee locked in full extension, the amplitude of the H-reflex, its modulation pattern, and the task-dependent changes of its amplitude were the same as during normal walking. During backward walking, the H-reflex increases in late swing before activity of the soleus has begun and while the knee is flexing, an observation that highlights central control of the H-reflex amplitude. The effects of imposed flexion of the knee in passive subjects were also reexamined. The knee flexion imposed by the experimenter followed the same trajectory as that which occurred during the swing phase of the subject's step cycle. It was found that imposed knee flexions elicited a burst of TA EMG activity with an average latency of 81.6 ms (SD = 21 ms) in six out of eight subjects. Inhibition of the H-reflex, when it occurred, was associated with the occurrence of this burst. When subjects voluntarily flexed their right knee from an initial quiet standing posture, the inhibition of the soleus H-reflex began before flexion of the knee or that of any other leg segment. Once again the onset of inhibition was closely associated with the onset of activity in the TA. In the discussion section the present observations are examined in light of the predictions made by the movement-induced inhibition hypothesis of Brooke et al. It will be concluded that none of the predictions of this hypothesis were corroborated by present tests done during human walking. In consequence, we suggest that the modulation pattern of the H-reflex observed during normal human walking is centrally determined, as are the task-dependent differences of its amplitude (e.g., standing versus the stance phase of human walking).     

Neural mechanisms that contribute to cyclical modulation of the soleus H-reflex in walking in humans

The amplitude of the Hoffmann reflex (H-reflex) of the human soleus muscle is modulated in a cyclical way during walking. This paper addresses two questions associated with the neural mechanisms that might generate this modulation: (1) Does the amplitude of the H-reflex simply rise and fall as a function of the background excitability of the soleus motoneuron pool? (2) Is the modulation of the H-reflex dependent on events associated with activation of the antagonist muscle? The amplitude of the soleus H-reflex was compared under three conditions: natural walking, walking without activating the tibialis anterior muscle, and walking with activation of the soleus muscle in the swing phase. Human subjects were able to perform these three tasks with minimal training. The results indicated that the soleus H-reflex remained very depressed in the swing phase of walking, even when a voluntary contraction of the soleus muscle was superimposed during this time. Moreover, the presence of tibialis anterior activity had a very minor effect on the amplitude of the soleus H-reflex during walking. It is concluded that modulation of the soleus H-reflex is not simply a reflection of the background excitability of the motoneuron pool, and the modulation is not dependent on activation of the antagonist muscle. Other more powerful mechanisms are acting to modulate the reflex, most likely presynaptic inhibition of the primary afferents.     

Cortical involvement in anticipatory postural reactions in man

All movements are accompanied by postural reactions which ensure that the balance of the body is maintained. It has not been resolved that to what extent the primary motor cortex and corticospinal tract are involved in the control of these reactions. Here, we investigated the contribution of the corticospinal tract to the activation of the soleus (SOL) muscle in standing human subjects (n = 10) in relation to voluntary heel raise, anticipatory postural activation of the soleus muscle when the subject pulled a handle and to reflex activation of the soleus muscle when the subject was suddenly pulled forward by an external perturbation. SOL motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation (TMS) increased significantly in relation to rest −75 ms prior to the onset of EMG in the heel-raise and handle-pull tasks. The short-latency facilitation of the soleus H-reflex evoked by TMS increased similarly, suggesting that the increased MEP size prior to movement was caused at least partly by increased excitability of corticospinal tract cells with monosynaptic projections to SOL motoneurones. Changes in spinal motoneuronal excitability could be ruled out since there was no significant increase of the SOL H-reflex until immediately prior to EMG onset for any of the tasks. Tibialis anterior MEPs were unaltered prior to the onset of SOL EMG activity in the handle-pull task, suggesting that the MEP facilitation was specific for the SOL muscle. No significant increase of the MEPs was observed prior to EMG onset for the external perturbation. These data suggest that the primary motor cortex is involved in activating the SOL muscle as part of an anticipatory postural reaction.     

Phase-dependent inhibition of H-reflexes during walking in humans is independent of reduction in knee angular velocity.

The purpose of this investigation was to investigate whether reduction in impulses arising from stretch of the quadriceps by restricting rapid knee flexion in early swing would affect inhibition of the H-reflex during swing. The contribution of afferent input arising from knee angular velocity to phase-dependent modulation of short-latency responses in the soleus was studied by simultaneously measuring joint velocity and soleus H-reflex responses at midstance and midswing phases of treadmill walking in 15 normal subjects. Stimulus strength was varied so that both maximal M and H waves were identified in each subject at midswing and midstance with the knee unrestricted (UK) and with knee movement restricted (RK), using a full leg bivalved cast to immobilize the knee joint. All subjects exhibited short-latency reflex responses in the soleus muscle. The H/M ratio at midswing was significantly reduced compared with midstance under both UK and RK walking conditions (P < 0.0001). When compared with UK walking, knee joint angular velocity during RK walking was significantly reduced at midswing (P < 0.001) and midstance (P < 0.005) compared with UK. There were, however, no significant differences in H/M ratios at midswing and midstance between UK and RK walking tests. Inhibition of the H-reflex in the soleus muscle during swing was not affected by significant reduction in knee angular velocity. These results indicate that the sensory input from changes in angular velocity at the knee does not lay the inhibitory foundation of phase-related reflex modulation in the ankle extensors during walking as suggested by Brooke and colleagues.     

Task-specific depression of the soleus H-reflex after cocontraction training of antagonistic ankle muscles

Ballet dancers have small soleus (SOL) H-reflex amplitudes, which may be related to frequent use of cocontraction of antagonistic ankle muscles. Indeed, SOL H-reflexes are depressed during cocontraction compared with plantarflexion at matched background EMG level. We investigated the effect of 30-min training of simultaneous activation of ankle dorsi- and plantarflexor muscles (cocontraction task) on the SOL H-reflex in 10 healthy volunteers. Measurements were taken during cocontraction. After training, there was a significant improvement in the ability of the subjects to perform a stable cocontraction. SOL H-reflex recruitment curves and H-max/M-max ratios were decreased after cocontraction training but not after 30 min of static dorsi or plantarflexion. The decreased H-reflex size correlated with improved motor performance. No changes in SOL and tibialis anterior (TA) EMG activity or EMG power were observed, suggesting that increased presynaptic inhibition of Ia afferents is a likely mechanism for H-reflex depression. In different sessions we measured SOL and TA motor-evoked potentials (MEPs) by using transcranial magnetic stimulation (TMS), TMS-elicited suppression of SOL EMG, and coherence between electroencephalographic (EEG) activity (Cz) and TA and SOL EMG. SOL and TA MEPs were depressed, whereas TMS-elicited suppression of SOL EMG and coherence were increased after training. Decreased excitability of corticospinal neurons due to increased intracortical inhibition seems a likely explanation of these observations. Our results indicate that the depression in H-reflex observed during a cocontraction task can be trained and that repeated performance of tasks involving cocontraction may lead to prolonged changes in reflex and corticospinal excitability.     

Differential regulation of crossed cutaneous effects on the soleus H-reflex during standing and walking in humans

Although sensory inputs from the contralateral limb strongly modify the amplitude of the Hoffmann (H-) reflex in a static posture, it remains unknown how these inputs affect the excitability of the monosynaptic H-reflex during walking. Here, we investigated the effect of the electrical stimulation of a cutaneous (CUT) nerve innervating the skin on the dorsum of the contralateral foot on the excitability of the soleus H-reflex during standing and walking. The soleus H-reflex was conditioned by non-noxious electrical stimulation of the superficial peroneal nerve in the contralateral foot. Significant crossed facilitation of the soleus H-reflex was observed at conditioning-to-test intervals in a range of 100–130 ms while standing, without any change in the background soleus electromyographic (EMG) activity. In contrast, the amplitude of the soleus H-reflex was significantly suppressed by the contralateral CUT stimulation in the early-stance phase of walking. The background EMG activity of the soleus muscle was equivalent between standing and walking tasks and was unaffected by CUT stimulation alone. These findings suggest that the crossed CUT volleys can affect the presynaptic inhibition of the soleus Ia afferents and differentially modulate the excitability of the soleus H-reflex in a task-dependent manner during standing and walking.     

Excitability of the soleus H reflex during graded walking in humans

The excitability of the soleus Hoffmann (H) reflex was measured in five healthy male subjects during graded treadmill walking. Uphill and downhill walking at an 8% grade as well as level walking were used to vary the demands for lengthening and shortening contractions of the soleus muscle. These changes were assumed to cause differences in control of the afferent input in the spinal cord and the voluntary output to the soleus muscle. The H reflex was strongly modulated in all three walking conditions, high during the stance phase and low or absent during the swing phase. The shape of the modulations was, however, different. At uphill walking the reflex increased gradually during the whole stance phase and seemed to follow the soleus electromyogram (EMG) pattern closely. In the downhill condition the reflex excitability increased rapidly at heel strike like the soleus EMG and co-contraction of the anterior tibial muscle was observed. At level walking a fast rise in reflex excitability was seen just after heel strike with low or absent soleus EMG. Mean soleus EMG was lower during downhill than during uphill or level walking, but the mean H reflex amplitude was similar in all three conditions. However, when the H reflex was related directly to the EMG activity by linear regression the reflex gain was lower during uphill walking than in the two other conditions. Furthermore, the ratio between H reflex and EMG amplitude was high during the first half of the stance phase at level walking indicating an elevated reflex excitability independent of the voluntary motor output. It is therefore concluded that the modulation of reflexes during walking cannot be interpreted in terms of the idea of automatic gain compensation. The reflexes must be controlled specifically and independently during the different phases of the motor output to meet the mechanical requirements of the movement task. Most explicitly this was seen during downhill walking, where an elevated reflex excitability together with co-contraction at the ankle joint seem to provide increased joint stiffness and security, when the kinetic energy of the body has to be brought under control at heel strike.     

Corticospinal Facilitation Studied During Voluntary Contraction of Human Abdominal Muscles

Transcranial magnetic stimulation (TMS) of the human motor cortex was used to study facilitation of motor-evoked potentials (MEPs) in the rectus abdominis (RA) muscle, a trunk flexor, during voluntary activation. MEPs could be produced in the relaxed RA muscles of all six normal subjects studied. The MEPs had short latencies (18-22 ms) which are consistent with other studies suggesting a fast corticospinal input to the trunk muscles. Marked facilitation was observed in the MEPs when subjects were asked to produce graded levels of voluntary contractions. The two tasks used to produce voluntary contractions were a forced expiration during a breath-holding task (FEBH) and bilateral trunk flexion (BTF). Maximal voluntary EMG activity during the BTF task produced around 4.2 times more integrated EMG than during the FEBH task. Similarly the MEP amplitude at MVC was 2.3 times greater during BTF than FEBH. The pattern of MEP facilitation with increasing voluntary EMG was not linear and a maximal MEP amplitude was observed at a level of voluntary contraction around 30 % MVC in both tasks. There were some subtle differences in the pattern of facilitation in the two tasks. When TMS was applied to the right cortex only, MEPs were seen in both left and right RA muscles suggesting some ipsilateral corticospinal innervation. The latency of the right (ipsilateral) response was approximately 2 ms longer than the left. Comparison with studies in hand and leg muscles suggests that the facilitation pattern in RA may reflect a substantial degree of corticospinal innervation. Experimental Physiology (2001) 86.1, 131-136.     

Evidence suggesting that a transcortical reflex pathway contributes to cutaneous reflexes in the tibialis anterior muscle during walking in man

Stimulation of cutaneous foot afferents has been shown to evoke a facilitation of the tibialis anterior (TA) EMG-activity at a latency of 70–95 ms in the early and middle swing phase of human walking. The present study investigated the underlying mechanism for this facilitation. In those subjects in whom it was possible to elicit a reflex during tonic dorsiflexion while seated (6 out of 17 tested), the facilitation in the TA EMG evoked by stimulation of the sural nerve (3 shocks, 3-ms interval, 2.0–2.5× perception threshold) was found to have the same latency in the swing phase of walking. The facilitation observed during tonic dorsiflexion has been suggested to be – at least partly – mediated by a transcortical pathway. To investigate whether a similar mechanism contributes to the facilitation observed during walking, magnetic stimulation of the motor cortex (1.2× motor threshold) was applied in the early swing phase at different intervals in relation to the cutaneous stimulation in 17 subjects. In 13 of the subjects, the motor potentials evoked by the magnetic stimulation (MEPs) were more facilitated by prior sural-nerve stimulation (conditioning-test intervals of 50–80 ms) than the algebraic sum of the control MEP and the cutaneous facilitation in the EMG when evoked separately. In four of these subjects, a tibialis anterior H-reflex could also be evoked during walking. In none of the subjects was an increase of the H-reflex similar to that for the MEP observed. In five experiments on four subjects, MEPs evoked by magnetic and electrical cortical stimulation were compared. In four of these experiments, only the magnetically induced MEPs were facilitated by prior stimulation of the sural nerve. We suggest that a transcortical pathway may also contribute to late cutaneous reflexes during walking. Received: 24 September 1997 / Accepted: 2 June 1998     

Functional reorganization of soleus H-reflex modulation during stepping after robotic-assisted step training in people with complete and incomplete spinal cord injury

Body weight–supported (BWS) robotic-assisted step training on a motorized treadmill is utilized with the aim to improve walking ability in people after damage to the spinal cord. However, the potential for reorganization of the injured human spinal neuronal circuitry with this intervention is not known. The objectives of this study were to determine changes in the soleus H-reflex modulation pattern and activation profiles of leg muscles during stepping after BWS robotic-assisted step training in people with chronic spinal cord injury (SCI). Fourteen people who had chronic clinically complete, motor complete, and motor incomplete SCI received an average of 45 training sessions, 5 days per week, 1 h per day. The soleus H-reflex was evoked and recorded via conventional methods at similar BWS levels and treadmill speeds before and after training. After BWS robotic-assisted step training, the soleus H-reflex was depressed at late stance, stance-to-swing transition, and swing phase initiation, allowing a smooth transition from stance to swing. The soleus H-reflex remained depressed at early and mid-swing phases of the step cycle promoting a reciprocal activation of ankle flexors and extensors. The spinal reflex circuitry reorganization was, however, more complex, with the soleus H-reflex from the right leg being modulated either in a similar or in an opposite manner to that observed in the left leg at a given phase of the step cycle after training. Last, BWS robotic-assisted step training changed the amplitude and onset of muscle activity during stepping, decreased the step duration, and improved the gait speed. BWS robotic-assisted step training reorganized spinal locomotor neuronal networks promoting a functional amplitude modulation of the soleus H-reflex and thus step progression. These findings support that spinal neuronal networks of persons with clinically complete, motor complete, or motor incomplete SCI have the potential to undergo an endogenous-mediated reorganization, and improve spinal reflex function and walking function with BWS robotic-assisted step training.     

Corticospinal excitability is lower during rhythmic arm movement than during tonic contraction

Humans perform rhythmic, locomotor movements with the arms and legs every day. Studies using reflexes to probe the functional role of the CNS suggest that spinal circuits are an important part of the neural control system for rhythmic arm cycling and walking. Here, by studying motor-evoked potentials (MEPs) in response to transcranial magnetic stimulation (TMS) of the motor cortex, and H-reflexes induced by electrical stimulation of peripheral nerves, we show a reduction in corticospinal excitability during rhythmic arm movement compared with tonic, voluntary contraction. Responses were compared between arm cycling and tonic contraction at four positions, while participants generated similar levels of muscle activity. Both H-reflexes and MEPs were significantly smaller during arm cycling than during tonic contraction at the midpoint of arm flexion (F = 13.51, P = 0.006; F = 11.83, P = 0.009). Subthreshold TMS significantly facilitated the FCR H-reflex during tonic contractions, but did not significantly modulate H-reflex amplitude during arm cycling. The data indicate a reduction in the responsiveness of cells constituting the fast, monosynaptic, corticospinal pathway during arm cycling and suggest that the motor cortex may contribute less to motor drive during rhythmic arm movement than during tonic, voluntary contraction. Our results are consistent with the idea that subcortical regions contribute to the control of rhythmic arm movements despite highly developed corticospinal projections to the human upper limb.     

Phase-dependent modulation of the soleus H-reflex during rhythmical arm swing in humans

The purpose of this study is to investigate modulation of the soleus H-reflex during rhythmic arm swing in humans. Significant depression of the soleus H-reflex was observed when subjects swung their ipsilateral arms or both arms reciprocally during testing. The degree of soleus H-reflex depression appeared directly proportional to the speed of the arm swing. This depression was observed in the conditioning-testing intervals of 400, 500, and 600 msec during the ipsilateral backward arm swing and at the onset of the ipsilateral arm forward swing. This phase of depression partially overlapped the phase of depression of the soleus H-reflex during walking. However, the pattern of modulation during arm swing was not exactly the same as that during walking. Therefore, we concluded that the ipsilateral arm swing may partially affect the depression of the soleus H-reflex during the arm swing phase of walking but is not responsible for depression of the soleus H-reflex throughout the entire walking cycle.     

Cortical control of human soleus muscle during volitional and postural activities studied using focal magnetic stimulation

The surface-recorded electromyographic (EMG) responses evoked in the ankle musculature by focal, transcranial, magnetic stimulation of the motor cortex were studied in healthy human subjects. Such soleus evoked motor responses (EMRs) were characterised over a wide range of background levels of motor activity and using different stimulus intensities. EMRs were recorded during predominantly (1) volitional and (2) postural tasks. In the former task subjects were seated and voluntarily produced prescribed levels of soleus activation by reference to a visual monitor of EMG. In the latter task subjects assumed standing postures without EMG feedback. Comparison of the EMRs of soleus, traditionally considered a slow anti-gravity extensor muscle, during these tasks was used to evaluate its cortical control in primarily volitional versus primarily postural activities. The form of soleus EMRs produced by single magnetic cortical stimuli comprised an initial (approx. 30 ms) increase and subsequent (approx. 50 ms) depression of EMG. Cortical stimulation could elicit substantial excitatory soleus EMG responses; for example, responses evoked by mild, magnetic stimuli (125% threshold for inducing a response in the relaxed muscle) as subjects exerted full voluntary plantarflexor effort averaged almost 20% of the maximum M-wave which could be elicited by an electrical stimulus to the posterior tibial nerve. Excitatory EMRs could be elicited in the voluntarily relaxed soleus muscle of the majority of subjects during sitting. The amplitude of soleus responses, induced by threshold stimuli for the relaxed state or approximately 125% threshold intensity, increased approximately linearly with background EMG over a wide range of volitional contraction levels. By contrast, there was no systematic change in the latency of excitatory soleus EMRs with increasing voluntary effort. The excitatory responses evoked in the voluntarily relaxed soleus of seated subjects by magnetic stimulation were regularly facilitated by incremental, voluntary contraction of the contralateral ankle extensors in a graded manner. However, such facilitation of responses was not observed when subjects voluntarily activated the muscle in which EMRs were elicited. The pattern of the responses elicited in soleus by magnetic stimulation during the postural task generally resembled that found during the volitional task. The amplitudes of excitatory soleus EMRs at a given stimulus intensity, obtained when subjects stood quietly, leaned forwards or stood on their toes to produce differing levels of ankle extensor contraction, increased with background EMG. Overall, the relationship between the size of cortically evoked soleus responses and the tonic level of motor activity, observed in individual subjects at matched stimulus intensities, did not consistently differ between postural and volitional tasks. The present results suggest that the motor cortex is potentially capable of exerting rapid regulation of the soleus muscle, and presumably other ankle extensors, not only when the muscle participates in volitional tasks but also when it is engaged in postural maintenance.     

Soleus H-reflex modulation during body weight support treadmill walking in spinal cord intact and injured subjects

The soleus H-reflex modulation pattern was investigated in ten spinal cord intact subjects during treadmill walking at varying levels of body weight support (BWS), and nine spinal cord injured (SCI) subjects at a BWS level that promoted the best stepping pattern. The soleus H-reflex was elicited by tibial nerve stimulation with a single 1-ms pulse at an intensity that the M-waves ranged from 4 to 8% of the maximal M-wave (M_max). During treadmill walking, the H-reflex was elicited every four steps, and stimuli were randomly dispersed across the gait cycle which was divided into 16 equal bins. EMGs were recorded with surface electrodes from major left and right hip, knee, and ankle muscles. M-waves and H-reflexes at each bin were normalized to the M_max elicited at 60–100 ms after the test reflex stimulus. For every subject, the integrated EMG area of each muscle was established and plotted as a function of the step cycle phase. The H-reflex gain was determined as the slope of the relationship between H-reflex and soleus EMG amplitudes at 60 ms before H-reflex elicitation for each bin. In spinal cord intact subjects, the phase-dependent H-reflex modulation, reflex gain, and EMG modulation pattern were constant across all BWS (0, 25, and 50) levels, while tibialis anterior muscle activity increased with less body loading. In three out of nine SCI subjects, a phase-dependent H-reflex modulation pattern was evident during treadmill walking at BWS that ranged from 35 to 60%. In the remaining SCI subjects, the most striking difference was an absent H-reflex depression during the swing phase. The reflex gain was similar for both subject groups, but the y-intercept was increased in SCI subjects. We conclude that the mechanisms underlying cyclic H-reflex modulation during walking are preserved in some individuals after SCI.     

Effect of clenching levels on heteronymous H-reflex in human temporalis muscle

 Selective stimulation of the masseteric nerve has been shown to elicit a heteronymous H-reflex in the ipsilateral temporalis muscle during voluntary clenching. However, the relation between the electromyographic (EMG) activity of the temporalis muscle and the amplitude of the H-reflex has not been previously described. In the present study, the hypothesis was tested that there would be a positive relationship between the level of EMG activity and the amplitude of the H-reflex. The direct motor response (M-response) in the masseter muscle and the heteronymous H-reflex in the anterior temporalis muscle were successfully elicited by electrical stimulation of the masseteric nerve in 12 of 13 subjects. A new automatic system was used to control the on-line EMG activity and to trigger the stimulus. In a random order, two series of 20 stimuli were delivered at each of four clenching levels (0, 25, 50, and 75% of maximal voluntary contraction). The analysis showed that both the masseteric M-response and the temporalis H-reflex were reproducible within and between series. The amplitude of the temporalis H-reflex increased significantly at higher clenching levels (ANOVA: P=0.003). Clenching at 50% and 75% of the maximal voluntary contraction caused significantly larger amplitudes of the H-reflex than clenching at 25% of the maximal voluntary contraction; at rest, no H-reflex could be recorded. There was a significant correlation between the background EMG activity in the ipsilateral temporalis muscle and the amplitude of the H-reflex (Pearson: r=0.313, P=0.008). These data indicate that the heteronymous H-reflex can be reliably elicited by means of an automatic system for stimulus delivery and that the amplitude of the H-reflex is dependent on the preceding activity of the motoneuron pool. Received: 27 November 1998 / Accepted: 16 February 1999     

Reflex response and control of the human soleus and gastrocnemius muscles during walking and running at increasing velocity

We measured the soleus and the gastrocnemius H-reflex modulation in seven subjects during walking at 4.5 km/h and during running at 8, 12 and 15 km/h. The recordings in the medial gastrocnemius were corrected for cross-talk from the soleus muscle. The gastrocnemius H-reflex was in general lower than the soleus H-reflex. In both muscles the H-reflex increased significantly from walking to running but also with increasing running speed. The peak of EMG activity increased in both muscles with increasing speed. The V-wave of both muscles was absent or rather low during walking, but it increased significantly from walking to running with increasing running speed in the soleus but not in the medial gastrocnemius. In both muscles the V-wave was highest just prior to heel strike. It is suggested that this was due to a high firing frequency of the motoneurones in this phase of the movement. It is concluded that a shift towards the faster gastrocnemius at higher running speeds on behalf of the soleus muscle did not occur. The fact that the physiological cross-sectional area of the soleus is much larger than that of the lumped gastrocnemii is most probably the reason why the soleus is important also at higher running velocities.     

Reciprocal inhibition of soleus motor output in humans during walking and voluntary tonic activity

1. The extent to which an active, human motoneuron pool can be inhibited via short-latency inhibitory pathways was studied by stimulating the common peroneal nerve and recording the inhibition of on-going soleus electromyographic (EMG) activity. The responses were compared at the same EMG level during walking and tonic voluntary activity to determine whether the inhibition was task dependent. 2. In both tasks the amount of inhibition (measured as the depression in rectified, filtered, and averaged EMG activity) increased approximately linearly with the amount of motor activity, as determined from the mean EMG level before stimulation (correlation coefficient greater than or equal to 0.9). No difference in the amount of inhibition was found between the two tasks at the same stimulus and EMG levels. 3. Previously published studies based on the H-reflex method have reported that the amount of inhibition decreases with the amount of motor activity. On the contrary, single-unit studies and the present results suggest that segmental inhibitory reflexes retain their capacity to mediate a rapid reduction of motoneuronal discharge during voluntary activity. This inhibition may be important in regulating the amount of activity early in the stance phase of walking and during the transition from stance to the swing phase. 4. Analytic results are derived in an APPENDIX that should be of general interest in interpreting the inhibition of motor units from a peristimulus time histogram (PSTH). The linear correlation between inhibition and level of voluntary activity can be explained if newly recruited units are strongly inhibited by the stimulus, whereas previously active motor units are inhibited relatively less, as their firing rate increases with increasing background activity.     

Focal depression of cortical excitability induced by fatiguing muscle contraction: a transcranial magnetic stimulation study

Motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation (TMS) and transcranial electrical stimulation (TES) of the motor cortex were recorded in separate sessions to assess changes in motor cortex excitability after a fatiguing isometric maximal voluntary contraction (MVC) of the right ankle dorsal flexor muscles. Five healthy male subjects, aged 37.4±4.2 years (mean±SE), were seated in a chair equipped with a load cell to measure dorsiflexion force. TMS or TES was delivered over the scalp vertex before and after a fatiguing MVC, which was maintained until force decreased by 50%. MEPs were recorded by surface electrodes placed over quadriceps, hamstrings, tibialis anterior (TA), and soleus muscles bilaterally. M-waves were elicited from the exercised TA by supramaximal electrical stimulation of the peroneal nerve. H-reflex and MVC recovery after fatiguing, sustained MVC were also studied independently in additional sessions. TMS-induced MEPs were significantly reduced for 20 min following MVC, but only in the exercised TA muscle. Comparing TMS and TES mean MEP amplitudes, we found that, over the first 5 min following the fatiguing MVC, they were decreased by about 55% for each. M-wave responses were unchanged. H-reflex amplitude and MVC force recovered within the 1st min following the fatiguing MVC. When neuromuscular fatigue was induced by tetanic motor point stimulation of the TA, TMS-induced MEP amplitudes remained unchanged. These findings suggest that the observed decrease in MEP amplitude represents a focal reduction of cortical excitability following a fatiguing motor task and may be caused by intracortical and/or subcortical inhibitory mechanisms.