H-reflex and reciprocal Ia inhibition after fatiguing isometric voluntary contraction in soleus muscle

This study investigated the H-reflex and reciprocal Ia inhibition during fatigue in the human soleus muscle. Ten healthy subjects participated in this study, and performed intermittent isometric voluntary contraction of the ankle plantarflexion at 50% MVC as the fatiguing task. Reciprocal Ia inhibition was evaluated by the degree of H-reflex amplitude depression in the soleus muscle by the test stimulus following conditioning stimulus to the common peroneal nerve. The difference in H-reflex amplitude between before and after fatiguing task was also checked. There was no significant difference in the degree of H-reflex amplitude depression, although the H-reflex amplitude significantly decreased after the fatiguing task (p < 0.01). From the results of this study, it was considered that the decrease in H-reflex amplitude was caused by descending inhibitory input from the supraspinal to alpha-motoneuron, and the excitability of the Ia inhibitory interneuron was not involved. It was suggested that the function of reciprocal Ia inhibition was difficult to modulate during fatigue caused by isometric voluntary contraction in this study.     

Role of Ia afferents in the soleus motoneurones inhibition during a tibialis anterior voluntary contraction in man

Summary Variations of the soleus H-reflex were studied during voluntary isometric or anisometric contractions of the tibialis anterior in man. At the onset of isometric contractions there was a weak inhibition of the soleus H-reflex, which was not related to the force of the tibialis anterior contraction. 110 msec after the onset of the EMG activity, the inhibition became secondarily more marked and was then related to the force of the contraction. This secondary potentiation of the H-reflex inhibition is brought about by group I fibres activity, since it was markedly reduced during is-chemia of the leg. It persisted during local muscular fatigue, this indicating that Ib fibres from tibialis anterior are not involved and that, by the process of elimination, group Ia fibres must be responsible for the supplementary secondary inhibition. It is concluded that the early inhibition is only due to suprasegmental activity, whereas during the secondary part of the inhibition there is a supplementary inhibitory action brought about by Ia fibres from tibialis anterior. The secondary potentiation of the inhibition is therefore likely to be produced via the gamma loop.Attachée de Recherches à l'I.N.S.E.R.M.     

Changes in reciprocal ia inhibition during voluntary contraction in man

Summary Reciprocal Ia inhibition from ankle flexors to extensors was studied during voluntary tonic isometric dorsiflexion and plantar flexion in five normal subjects. The Ia inhibition was examined as the short-latency suppression of the soleus H-reflexes by stimulation of the low-threshold afferents in the common peroneal nerve (Mizuno et al. 1971). At rest, weak Ia inhibition was demonstrated in four subjects out of five, the maximal amount being 14.1 ± 5.0% suppression of the control H-reflex. The absolute amount of inhibition, which was calculated by subtracting the mean size of the conditioned H-reflex from that of the control H-reflex and expressed as a percentage of the maximal M-response, increased during ankle dorsiflexion, and decreased or disappeared during plantar flexion in parallel with the amount of contraction. The neural mechanisms for facilitation of the Ia inhibitory pathway during dorsiflexion were considered to support the hypothesis of --linkage in reciprocal inhibition, i.e. combined facilitatory effects on the Ia inhibitory interneurone from the supraspinal centers directly and indirectly via the motoneurone — Ia afferent route. The mechanism for inhibition of the pathway during plantar flexion was considered to be inhibition of the Ia interneurone of the flexor side by Ia interneurone of antagonist extensors. A quantitative aspect of activity in the reciprocal Ia inhibitory pathway on the performance of voluntary movement is revealed in this study.     

Effects of hip joint angle changes on intersegmental spinal coupling in human spinal cord injury

Pathological expression of movement and muscle tone in human upper motor neuron disorders has been partly associated with impaired modulation of spinal inhibitory mechanisms, such as reciprocal or presynaptic inhibition. In addition, input from specific afferent systems contributes significantly to spinal reflex circuits coupled with posture or locomotion. Accordingly, the objectives of this study were to identify the involved afferents and their relative contribution to soleus H-reflex modulation induced by changes in hip position, and to relate these effects with activity of spinal interneuronal circuits. Specifically, we investigated the actions of group I synergistic and antagonistic muscle afferents (e.g. common peroneal nerve, CPN; medial gastrocnemius, MG) and tactile plantar cutaneous afferents on the soleus H-reflex during controlled hip angle variations in 11 motor incomplete spinal cord injured (SCI) subjects. It has been postulated in healthy subjects that CPN stimulation evokes an inhibition on the soleus H-reflex at a conditioning test (C-T) interval of 2–4 ms. This short latency reflex depression is caused mainly by activation of the reciprocal Ia inhibitory pathway. At longer C-T intervals (beyond 30 ms) the soleus H-reflex is again depressed, and is generally accepted to be caused by presynaptic inhibition of soleus Ia afferents. Similarly, MG nerve stimulation depresses soleus H-reflex excitability at the C-T interval of 6 ms, involving the pathway of non-reciprocal group I inhibition, while excitation of plantar cutaneous afferents affects the activity of spinal reflex pathways in the extensors. In this study, soleus H-reflexes recorded alone or during CPN stimulation at either short (2, 3, 4 ms) or long (80, 100, 120 ms) C-T intervals, and MG nerve stimulation delivered at 6 ms were elicited via conventional methods and similar to those adopted in studies conducted in healthy subjects. Plantar skin conditioning stimulation was delivered through two surface electrodes placed on the metatarsals at different C-T intervals ranging from 3 to 90 ms. CPN stimulation at either short or long C-T intervals and MG nerve stimulation resulted in a significant facilitation of the soleus H-reflex, regardless of the hip angle tested. Plantar skin stimulation delivered with hip extended at 10° induced a bimodal facilitation reflex pattern, while with hip flexed (10°, 30°) the reflex facilitation increased with increments in the C-T interval. This study provides evidence that in human chronic SCI, classically key inhibitory reflex actions are switched to facilitatory, and that spinal processing of plantar cutaneous sensory input and actions of synergistic/antagonistic muscle afferents interact with hip proprioceptive input to facilitate soleus H-reflex excitability. These actions might be associated with the pathological expression of neural control of movement in individuals with SCI, and potentially could be considered in rehabilitation programs geared to restore sensorimotor function in these patients.     

Disorder in reciprocal innervation upon initiation of voluntary movement in patients with Parkinson's disease

Summary Reciprocal innervation of the soleus motoneurones upon initiation of voluntary ankle dorsiflexion was investigated in eight patients with Parkinson's disease. H-reflex and visually guided step tracking methods were used for testing moto-neurone excitability and for controlling the timing of movement initiation, respectively. While reciprocal inhibition appeared almost simultaneously with the agonist electromyographic (EMG) onset in normal subjects (Kagamihara and Tanaka 1985), facilitation appeared in the majority of patients under the same onset condition. It increased slowly, reaching a maximum at about 100 ms after the EMG onset. It then subsided slowly at around 200–300 ms, and was replaced thereafter by an inhibitory effect. No coactivation of the soleus muscle was detected electromyographically. The facilitation between the EMG onset and the onset of mechanical contraction was attributed to the direct effect of the descending command from the brain, suggesting a certain disorder in controlling the system for reciprocal innervation.     

Reciprocal inhibition upon initiation of voluntary movement

Phasic changes in reciprocal inhibition of the soleus motoneurons upon initiation of active ankle dorsiflexion was investigated in normal man. H-reflex and reaction time methods were combined. Two phases of reciprocal inhibition were observed. The first inhibition appeared almost simultaneously with the activity of agonist EMG but was so weak as to be cancelled with a strong test stimulus or a slight stretch of the test muscle. The second inhibition developed 100 ms after EMG onset, in the early stage of dynamic contraction, and was strong. These inhibitions increased as the voluntary effort was strengthened. The neural mechanism of these inhibitions is discussed.     

Short-term plasticity of human spinal inhibitory circuits after isometric and isotonic ankle training

The purpose of this study was to determine to what extent one session of isotonic and isometric ankle dorsi and plantar flexion training induces changes in the frequency-dependent depression of the soleus H-reflex. Further, adaptation of reciprocal Ia inhibition exerted from tibialis anterior flexor group I afferents on soleus motoneurons, and presynaptic inhibition of Ia afferent terminals induced by a conditioning afferent volley following stimulation of the antagonist nerve were established with subjects seated before and after training. The soleus H-reflexes evoked at the inter-stimulus intervals of 1, 2, 3, 5, and 8 s were normalized to the mean amplitude of the H-reflex evoked every 10 s. Conditioned H-reflexes were normalized to the associated control H-reflex evoked with subjects seated before and after training. Twenty-six subjects were randomly assigned to one or more of the 4 exercise groups. Isometric ankle dorsi flexion training decreased the reciprocal and presynaptic inhibition, while isotonic ankle dorsi flexion had no significant effects. Isotonic plantar flexion training decreased only the reciprocal inhibition, whilst isometric plantar flexion had no significant effects on the reciprocal or presynaptic inhibition. None of the training exercise protocols affected the amount of homosynaptic depression of the soleus H-reflex. Our findings support the notion that plastic changes of reciprocal and presynaptic inhibition due to exercise are transferrable to a resting state, and that homosynaptic depression remains unaltered after a single session of ankle training. Further research is needed to outline the time-course of plastic changes of spinal inhibitory mechanisms in humans.     

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).     

Modulation of motor unit discharge rate and H-reflex amplitude during submaximal fatigue of the human soleus muscle

Declining motor unit discharge rates and H-reflex amplitude have been observed in separate experiments during fatiguing submaximal contractions in humans. The purpose of this experiment was to investigate motor unit discharge rate, H-reflex amplitude, and twitch contractile properties concurrently during a fatiguing submaximal isometric contraction of the ankle plantarflexors. Eleven healthy subjects performed fatiguing contractions of low force (25% maximal voluntary contraction (MVC)) or high force (42–66% MVC). Hoffmann (H)-reflexes, muscle compound action potentials (M-waves), twitch contractile properties, and motor unit discharges were recorded from the soleus muscle. In the low-force fatigue task, motor unit firing rate increased gradually over time, whereas the resting H-reflex was significantly depressed at 15% of endurance time and remained quasiconstant for the rest of the task. This suggests that the processes mediating the resting H-reflex depression are relatively independent of those modulating the motor unit firing rate during a low-force fatigue task. In the high-force fatigue task, a decline in the average motor unit discharge rate was accompanied by a decrease in the resting H-reflex amplitude and a prolongation of the twitch half-relaxation time (HRT) at the completion of the fatigue task. Overall, motor unit firing rate was modulated in parallel with changes in the twitch HRT, consistent with the muscle wisdom hypothesis.     

Ia afferents of the antagonist are inhibited presynaptically before the onset of a ballistic muscle contraction in man

Summary The aim of the present investigation was to study whether the pre-movement inhibition of the H reflex in the antagonist of a ballistic voluntary contraction was due to a reduced activity of the motoneuronal pool of the antagonist, or to a reduced excitatory effect of the afferent volley reaching these motoneurons. Human subjects performed visually conditioned ballistic dorsal flexions of the ankle. The inhibition of the H reflex in the antagonist (soleus muscle) was similar if the muscle was initially relaxed or if there was a preexisting level of motor discharge. Since the soleus muscle was inhibited before movement onset in parallel with the H reflex inhibition, the relation between the level of a background activity and the size of superimposed H reflexes was studied. The finding that H reflexes were only slightly reduced in size with decreasing steady EMG levels could not explain the pre-movement inhibition, and it was concluded than an increased presynaptic inhibition of Ia terminals was the source of the H reflex inhibition.     

Temporal depression of the soleus H-reflex during passive stretch

Synaptic efficacy associated with muscle spindle feedback is regulated via depression at the Ia-motoneurone synapse. The inhibitory effects of repetitive Ia afferent discharge on target motoneurones of different sizes were investigated during a passive stretch of ankle extensors in humans. H-reflex recruitment curves were collected from the soleus muscle for two conditions in ten subjects. H-reflexes were elicited during passive stretch at latencies of 50, 100, 300, and 500 ms after a slow (20°/s) dorsiflexion about the right ankle (from 100 to 90°). Control H-reflexes were recorded at corresponding static (without movement) ankle angles of 99, 98, 94, and 90° of flexion. The slope of the H-reflex recruitment curves (Hslp) was then calculated for both conditions. H-reflex values were similar for the static and passive stretch conditions prior to 50–100 ms, not showing the early facilitation typical of increased muscle spindle discharge rates. However, the H-reflex was significantly depressed by 300 ms and persisted through 500 ms. Furthermore, less than 300 ms into the stretch, there was significantly greater H-reflex depression with a lower stimulus intensity (20 % Mmax) versus a higher stimulus intensity (Hmax), though the effects begin to converge at later latencies (>300 ms). This suggests there is a distinct two-stage temporal process in the depression observed in the Ia afferent pathway for all motoneurones during a passive stretch. Additionally, there is not a single mechanism responsible for the depression, but rather both heterosynaptic presynaptic inhibition and homosynaptic post-activation depression are independently influencing the Ia-motoneurone pathway temporally during movement.     

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.     

Effects of cortical stimulation on reciprocal inhibition in humans

We attempted to demonstrate convergence onto human spinal Ia inhibitory interneurons from Ia afferents and from fast conducting corticospinal axons. Stimulation of the common peroneal nerve at or below the threshold of the alpha motoneuron axons resulted in inhibition of the soleus H-reflex, attributed to reciprocal inhibition. Magnetic stimulation over the contralateral motor cortex resulted in complex modulations of the soleus H-reflex, including a short latency-inhibition. To test for convergence, the two stimuli were given together so that the two inhibitions coincided.When each stimulus alone produced clear inhibition, the inhibition produced by both stimuli was less than expected, implying an interaction between the two volleys, for example, occlusion occurring in interneurons or motoneurons.When the H-reflex was relatively unaffected by one or other conditioning volley, the inhibition produced by the combined stimulation was greater than expected, as might be expected with convergence onto a common pool of interneurons.     

Changes in reciprocal inhibition across the ankle joint with changes in external load and pedaling rate during bicycling

The purpose of this study was to investigate the role of reciprocal inhibition in the regulation of antagonistic ankle muscles during bicycling. A total of 20 subjects participated in the study. Reciprocal inhibition was induced by stimulation of the peroneal nerve (PN) at 1.2 times threshold for the M-response in the tibialis anterior muscle (TA) and recorded as a depression of the rectified soleus (SOL) EMG. Recordings were made during tonic plantar flexion and during bicycling on an ergometer bicycle. During tonic contraction, the amount of inhibition in the SOL EMG was linearly correlated to the amount of background EMG. This linear relation was used to calculate the expected amount of reciprocal inhibition at corresponding EMG levels during bicycling. During the early phase of down-stroke of bicycling at 60 revolutions per minute (RPM) and an external load of 1.0 kg, the amount of recorded reciprocal inhibition was significantly smaller than that calculated from the linear relation during tonic contraction. In nine subjects, the SOL H-reflex was used to evaluate the amount of inhibition. At a short conditioning test interval (2-3 ms), the PN stimulation depressed the SOL H-reflex when the subjects were at rest. This short latency inhibition was absent during downstroke, but appeared during upstroke just prior to and during TA activation. A positive linear relation was found between the level of SOL background EMG in early downstroke and the external load (0.5-2.5 kg) as well as the rate of pedaling (30-90 RPM at 1.0 kg external load). The amount of inhibition in the SOL EMG when expressed as a percentage of the background EMG activity decreased significantly with increasing load. During increased pedaling rate, a similar decrease was seen, but it did not reach a statistically significant level. The data illustrate that reciprocal inhibition of the soleus muscle is modulated during bicycling being small in downstroke when the SOL muscle is active and large in upstroke where the muscle is inactive and its antagonist becomes active. The depression of the inhibition in relation to increased load and pedaling rate likely reflects the need of reducing inhibition of the SOL motoneurons to ensure a sufficient activation of the muscle.     

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.     

The effect of the ankle joint angle in the level of soleus Ia afferent presynaptic inhibition

The factors that are responsible for the relationship between motoneuron excitability and muscle length may have both mechanical and/or neurophysiologic origins. The aim of the study was to investigate the changes in the level of presynaptic inhibition, as measured with a soleus H-reflex conditioning protocol, and muscle length. Ten healthy volunteers were measured at three different ankle angles: 30 degrees plantar flexion, neutral position (0 degrees) and 15 degrees dorsiflexion. At each position the soleus H-reflex and the maximum M-wave were measured while the limb was relaxed. The H-reflex was conditioned by a stimulation of the common peroneal nerve, 100 ms prior to the tibial nerve stimulation. The results revealed that the level of presynaptic inhibition was higher at the neutral position in comparison to the dorsiflexed or plantarflexed positions. Additionally, the HMAX/MMAX ratio was significantly decreased when the joint position was set at dorsiflexion. Further, there was a significant correlation, independent of ankle joint angle, between presynaptic inhibition levels and the HMAX/MMAX ratio. The above findings support the concept that peripheral feedback from passive, static modifications in the joint angle and consequently in muscle length, can modify the input/output threshold of the motoneurons on a presynaptic level.     

Reflex inhibition during muscle fatigue in endurance-trained and sedentary individuals

Reflex inhibition of the motoneuron pool following fatiguing contractions may be mediated by the build-up of byproducts of fatigue. Endurance training is accompanied by neuromuscular adaptations that would alter the production and/or clearance of metabolic substrates. The purpose of the study was to determine the extent of reflex inhibition during and after fatigue in endurance-trained individuals compared to sedentary controls. Subjects produced isometric ankle plantarflexion contractions at 30% of maximal voluntary contraction (MVC) until their MVC torque declined by 30%. H-reflexes were measured during a brief rest period every 3 min as well as superimposed upon the contraction every minute. Both groups of subjects experienced a similar amount of reflex inhibition by the end of the fatiguing protocol, although the endurance time was twice as long for the endurance-trained subjects. The endurance-trained subjects showed a greater reduction in H-reflex amplitude early in the fatiguing protocol compared to the sedentary subjects. These experiments have demonstrated that the neuromuscular processes associated with fatigue-related reflex inhibition must be multi-faceted and cannot be explained solely by small-diameter afferents responding to the byproducts of muscle contraction. Electronic Publication     

Anti-opioid effects of neuropeptide FF receptors in the ventral tegmental area

The aim of this study was to establish the modulation pattern of the soleus H-reflex and excitability changes of interneurones mediating presynaptic inhibition from tibialis anterior to soleus Ia afferents when the right foot approached and withdrew from a step in standing humans. The task was conducted at 40 beats per minute, and this tempo corresponded to the rhythm of a half full movement cycle. Each subject was instructed not to load and not to move forward. Soleus H-reflexes were elicited once in every full movement cycle that lasted approximately 3s. The ipsilateral knee joint angle and activity of leg muscles were recorded through a twin-axis goniometer and surface EMG, respectively. In all subjects, the soleus H-reflex was modulated in a phase-dependent pattern. The H-reflex was significantly depressed during the approach phase of the motor task and when the foot was on the step, and facilitated during the withdrawal phase and when the foot was on the ground. The soleus H-reflex conditioned by common peroneal nerve stimulation at tibialis anterior motor threshold at a long conditioning test interval was increased during the withdrawal phase or while the foot was on the ground suggesting that presynaptic inhibition was decreased. In most subjects, knee extensor activity was small, while ankle flexors and extensors were active in a reciprocal pattern. This study provides evidence that the soleus H-reflex is modulated in a phasic pattern during one-legged foot reaching and withdrawal by changes occurring at a premotoneuronal level.     

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.     

Recurrent inhibition in the soleus motor pool of elderly and young adults

Recurrent inhibition in the spinal cord has been suggested to serve as a variable gain regulator to allow for optimal muscle force control, to influence alpha-motoneuron firing rate, and to contribute to task related motor synergies between muscles at the same or different joints. The purpose of this study was to examine the resting recurrent inhibition levels in the soleus motoneuron pool of 20 elderly and 21 young adult subjects. To assess recurrent inhibition, a conditioning electrical stimulus was used to activate group Ia afferent fibers and elicit a reflex response in some of the a-motoneurons innervating the soleus muscle; producing both activation of Renshaw interneurons excited by those involved soleus a-motoneurons via a recurrent branch of the a-motoneuron axon, and an H-reflex response in the soleus muscle. A H' test reflex elicited by a successive supramaximal stimulus to the same nerve 10 ms after the conditioning stimulus evaluated the resulting inhibitory effect. There was no difference in the H' test reflex amplitude when comparing the young and elderly adult subjects. This result was found following two different methods employed to control for a possible effect on the H' test reflex amplitude of a smaller maximum H-reflex amplitude in the elderly subjects. These results indicate that the level of recurrent inhibition in the motoneuron pool of the resting soleus muscles of the young and elderly adults examined was not significantly different.