Changes in the input-output relation of the Hoffmann reflex recruitment curve after submaximal fatiguing contraction of the antagonist muscles.

PURPOSE: The central nervous system adapts strategies to compensate the decreased motor capacities of a fatigued muscle. However, data on neurophysiological adaptations of muscles other than those under fatigue are scarce. The present study was designed to evaluate the effects of submaximal fatiguing contraction (leading to a task failure) induced in ankle dorsiflexors muscles on the excitability of the Hoffmann reflex (H-reflex) of an ankle plantarflexor (soleus muscle). MATERIALS AND METHODS: Twenty-three physically active males (75.5 ± 8.3 kg; 1.77 ± 0.08 m; 27.0 ± 8.0 years) were asked to maintain the contraction level of the right ankle dorsiflexors at 60% of the maximal isometric voluntary contraction (MIVC). Task failure was defined when the force level dropped below 40% MIVC for 5 consecutive seconds. The input-output relation of the ascending limb of the recruitment curve of the soleus H-reflex was examined at 0 min, 5 min, 10 min, 15 min and 20 min after the task failure. RESULTS: The amplitude parameter representing the first recruited motoneurons (threshold H-reflex - H@th) was significantly higher at 5 min, 10 min, 15 min and 20 min after task failure as compared to control (Hth) (p < 0.05). On the other hand, the parameter that represents the activation of the relatively higher threshold motoneurons (H@100) was reduced (as compared to control - H100), but only at 20 min after the task failure (p < 0.05). CONCLUSIONS: These results suggest differential reflex modulation of the soleus H-reflex after fatigue of the ankle dorsiflexors, that probably reflects neuronal adaptations underlying motor control around the ankle joint.

Therapeutic Elastic Tapes Applied in Different Directions Over the Triceps Surae Do Not Modulate Reflex Excitability of the Soleus Muscle.

CONTEXT: Elastic taping has been widely used for either to facilitate or to inhibit muscle contraction. The efficacy of elastic taping is allegedly ascribed to physiological mechanisms related to subcutaneous tissue and muscle stimulation as a result of tape tension and direction. However, the underlying mechanisms that support the use of elastic taping are still unclear. OBJECTIVE: To investigate changes in electrophysiological responses after 48 hours of tape application in different directions on the calf muscles of healthy individuals. DESIGN: Within-subjects design. SETTING: Research laboratory. PARTICIPANTS: Twenty-seven physically active males (age 18.0 [4.2] y, height 1.65 [0.07] m, body mass 62.3 [10.3] kg) participated. INTERVENTIONS: Soleus H-reflex responses were evoked through stimulation of the tibial posterior nerve with 2- to 4-second interval between stimuli (32 sweeps) for each condition (baseline: without tape; facilitation: tape applied from muscle origin to insertion; inhibition: tape applied from muscle insertion to origin). MAIN OUTCOME MEASURES: The H-reflex amplitude values were normalized by the maximal direct response (Mmax). Parameters were estimated from a sigmoidal fit of the H-reflex recruitment curve (ascending limb). RESULTS: No significant differences were found for the parameters derived from the recruitment curve of the H-reflex among the conditions (P > .05). CONCLUSIONS: The authors' findings showed that, irrespective of the direction of tape application, the elastic tape applied over the triceps surae does not generate any significant alteration on the excitability of the reflex pathway for different subpopulations of motor units. The authors therefore suggest a re-examination of the current recommendations on taping direction in clinical and sports activities.

Bilateral Reflex Fluctuations during Rhythmic Movement of Remote Limb Pairs.

The modulation of spinal cord excitability during rhythmic limb movement reflects the neuronal coordination underlying actions of the arms and legs. Integration of network activity in the spinal cord can be assessed by reflex variability between the limbs, an approach so far very little studied. The present work addresses this question by eliciting Hoffmann (H-) reflexes in both limbs to assess if common drive onto bilateral pools of motoneurons influence spinal cord excitability simultaneously or with a delay between sides. A cross-covariance (CCV) sequence between reflexes in both arms or legs was evaluated under conditions providing common drive bilaterally through voluntary muscle contraction and/or rhythmic movement of the remote limbs. For H-reflexes in the flexor carpi radialis (FCR) muscle, either contraction of the FCR or leg cycling induced significant reduction in the amplitude of the peak at the zero lag in the CCV sequence, indicating independent variations in spinal excitability between both sides. In contrast, for H-reflexes in the soleus (SO) muscle, arm cycling revealed no reduction in the amplitude of the peak in the CCV sequence at the zero lag. This suggests a more independent control of the arms compared with the legs. These results provide new insights into the organization of human limb control in rhythmic activity and the behavior of bilateral reflex fluctuations under different motor tasks. From a functional standpoint, changes in the co-variability might reflect dynamic adjustments in reflex excitability that are subsumed under more global control features during locomotion.

Reassessment of Non-Monosynaptic Excitation from the Motor Cortex to Motoneurons in Single Motor Units of the Human Biceps Brachii.

Corticospinal excitation is mediated by polysynaptic pathways in several vertebrates, including dexterous monkeys. However, indirect non-monosynaptic excitation has not been clearly observed following transcranial electrical stimulation (TES) or cervicomedullary stimulation (CMS) in humans. The present study evaluated indirect motor pathways in normal human subjects by recording the activities of single motor units (MUs) in the biceps brachii (BB) muscle. The pyramidal tract was stimulated with weak TES, CMS, and transcranial magnetic stimulation (TMS) contralateral to the recording side. During tasks involving weak co-contraction of the BB and hand muscles, all stimulation methods activated MUs with short latencies. Peristimulus time histograms (PSTHs) showed that responses with similar durations were induced by TES (1.9 ± 1.4 ms) and CMS (2.0 ± 1.4 ms), and these responses often showed multiple peaks with the PSTH peak having a long duration (65.3% and 44.9%, respectively). Such long-duration excitatory responses with multiple peaks were rarely observed in the finger muscles following TES or in the BB following stimulation of the Ia fibers. The responses obtained with TES were compared in the same 14 BB MUs during the co-contraction and isolated BB contraction tasks. Eleven and three units, respectively, exhibited activation with multiple peaks during the two tasks. In order to determine the dispersion effects on the axon conduction velocities (CVs) and synaptic noise, a simulation study that was comparable to the TES experiments was performed with a biologically plausible neuromuscular model. When the model included the monosynaptic-pyramidal tract, multiple peaks were obtained in about 34.5% of the motoneurons (MNs). The experimental and simulation results indicated the existence of task-dependent disparate inputs from the pyramidal tract to the MNs of the upper limb. These results suggested that intercalated interneurons are present in the spinal cord and that these interneurons might be equivalent to those identified in animal experiments.

Neuromechanical interactions between the limbs during human locomotion: an evolutionary perspective with translation to rehabilitation.

During bipedal locomotor activities, humans use elements of quadrupedal neuronal limb control. Evolutionary constraints can help inform the historical ancestry for preservation of these core control elements support transfer of the huge body of quadrupedal non-human animal literature to human rehabilitation. In particular, this has translational applications for neurological rehabilitation after neurotrauma where interlimb coordination is lost or compromised. The present state of the field supports including arm activity in addition to leg activity as a component of gait retraining after neurotrauma.

Regionally distinct cutaneous afferent populations contribute to reflex modulation evoked by stimulation of the tibial nerve during walking.

During walking, cutaneous reflexes in ankle flexor muscle [tibialis anterior (TA)] evoked by tibial nerve (TIB) stimulation are predominantly facilitatory at early swing phase but reverse to suppression at late swing phase. Although the TIB innervates a large portion of the skin of the foot sole, the extent to which specific foot-sole regions contribute to the reflex reversals during walking remains unclear. Therefore, we investigated regional cutaneous contributions from discrete portions of the foot sole on reflex reversal in TA following TIB stimulation during walking. Summation effects on reflex amplitudes, when applying combined stimulation from foot-sole regions with TIB, were examined. Middle latency responses (MLRs; 70-120 ms) after TIB stimulation were strongly facilitated during the late stance to mid-swing phases and reversed to suppression just before heel (HL) strike. Both forefoot-medial (f-M) and forefoot-lateral stimulation in the foot sole induced facilitation during stance-to-swing transition phases, but HL stimulation evoked suppression during the late stance to the end of swing phases. At the stance-to-swing transition, a summation of MLR amplitude occurred only for combined f-M&TIB stimulation. However, the same was not true for the combined HL&TIB stimulation. At the swing-to-stance transition, there was a suppressive reflex summation only for HL&TIB stimulation. In contrast, this summation was not observed for the f-M&TIB stimulation. Our results suggest that reflex reversals evoked by TIB stimulation arise from distinct reflex pathways to TA produced by separate afferent populations innervating specific regions of the foot sole.

Soleus Hoffmann reflex amplitudes are specifically modulated by cutaneous inputs from the arms and opposite leg during walking but not standing.

Electrical stimulation of cutaneous nerves innervating heteronymous limbs (the arms or contralateral leg) modifies the excitability of soleus Hoffmann (H-) reflexes. The differences in the sensitivities of the H-reflex pathway to cutaneous afferents from different limbs and their modulation during the performance of motor tasks (i.e., standing and walking) are not fully understood. In the present study, we investigated changes in soleus H-reflex amplitudes induced by electrical stimulation of peripheral nerves. Selected targets for conditioning stimulation included the superficial peroneal nerve, which innervates the foot dorsum in the contralateral ankle (cSP), and the superficial radial nerve, which innervates the dorsum of the hand in the ipsilateral (iSR) or contralateral wrist (cSR). Stimulation and subsequent reflex assessment took place during the standing and early-stance phase of treadmill walking in ten healthy subjects. Cutaneous stimulation produced long-latency inhibition (conditioning-test interval of ~100 ms) of the H-reflex during the early-stance phase of walking, and the inhibition was stronger following cSP stimulation compared with iSR or cSR stimulation. In contrast, although similar conditioning stimulation significantly facilitated the H-reflex during standing, this effect remained constant irrespective of the different conditioning sites. These findings suggest that cutaneous inputs from the arms and contralateral leg had reversible effects on the H-reflex amplitudes, including inhibitions with different sensitivities during the early-stance phase of walking and facilitation during standing. Furthermore, the differential sensitivities of the H-reflex modulations were expressed only during walking when the locations of the afferent inputs were functionally relevant.

Phase-dependent reversal of the crossed conditioning effect on the soleus Hoffmann reflex from cutaneous afferents during walking in humans.

We previously demonstrated that non-noxious electrical stimulation of the cutaneous nerve innervating the contralateral foot modified the excitability of the Hoffmann (H-) reflex in the soleus muscle (SOL) in a task-dependent manner during standing and walking in humans. To date, however, it remains unclear how the crossed conditioning effect on the SOL H-reflex from the contralateral foot is modified during the various phases of walking. We sought to answer this question in the present study. The SOL H-reflex was evoked in healthy volunteers by an electrical test stimulation (TS) of the right (ipsilateral) posterior tibial nerve at five different phases during treadmill walking (4 km/h). A non-noxious electrical stimulation was delivered to the superficial peroneal nerve of the left (contralateral) ankle ~100 ms before the TS as a conditioning stimulation (CS). This CS significantly suppressed the H-reflex amplitude during the early stance phase, whereas the same CS significantly facilitated the H-reflex amplitude during the late stance phase. The CS alone did not produce detectable changes in the full-wave rectified electromyogram of the SOL. This result indicates that presynaptic mechanisms driven by the activation of low-threshold cutaneous afferents in the contralateral foot play a role in regulating the transmission between the Ia terminal and motoneurons in a phase-dependent manner. The modulation pattern of the crossed conditioning effect on the SOL H-reflex may be functionally relevant for the left-right coordination of leg movements during bipedal walking.

Cutaneous stimulation of discrete regions of the sole during locomotion produces "sensory steering" of the foot.

BACKGROUND: While the neural and mechanical effects of whole nerve cutaneous stimulation on human locomotion have been previously studied, there is less information about effects evoked by activation of discrete skin regions on the sole of the foot. Electrical stimulation of discrete foot regions evokes position-modulated patterns of cutaneous reflexes in muscles acting at the ankle during standing but data during walking are lacking. Here, non-noxious electrical stimulation was delivered to five discrete locations on the sole of the foot (heel, and medial and lateral sites on the midfoot and forefoot) during treadmill walking. EMG activity from muscles acting at the hip, knee and ankle were recorded along with movement at these three joints. Additionally, 3 force sensing resistors measuring continuous force changes were placed at the heel, and the medial and lateral aspects of the right foot sole. All data were sorted based on stimulus occurrence in twelve step-cycle phases, before being averaged together within a phase for subsequent analysis. METHODS: Non-noxious electrical stimulation was delivered to five discrete locations on the sole of the foot (heel, and medial and lateral sites on the midfoot and forefoot) during treadmill walking. EMG activity from muscles acting at the hip, knee and ankle were recorded along with movement at these three joints. Additionally, 3 force sensing resistors measuring continuous force changes were placed at the heel, and the medial and lateral aspects of the right foot sole. All data were sorted based on stimulus occurrence in twelve step-cycle phases, before being averaged together within a phase for subsequent analysis. RESULTS: The results demonstrate statistically significant dynamic changes in reflex amplitudes, kinematics and foot sole pressures that are site-specific and phase-dependent. The general trends demonstrate responses producing decreased underfoot pressure at the site of stimulation. CONCLUSIONS: The responses to stimulation of discrete locations on the foot sole evoke a kind of "sensory steering" that may promote balance and maintenance of locomotion through the modulation of limb loading and foot placement. These results have implications for using sensory stimulation as a therapeutic modality during gait retraining (e.g. after stroke) as well as for footwear design and implementation of foot sole contact surfaces during gait.

Convergence in reflex pathways from multiple cutaneous nerves innervating the foot depends upon the number of rhythmically active limbs during locomotion.

Neural output from the locomotor system for each arm and leg influences the spinal motoneuronal pools directly and indirectly through interneuronal (IN) reflex networks. While well documented in other species, less is known about the functions and features of convergence in common IN reflex system from cutaneous afferents innervating different foot regions during remote arm and leg movement in humans. The purpose of the present study was to use spatial facilitation to examine possible convergence in common reflex pathways during rhythmic locomotor limb movements. Cutaneous reflexes were evoked in ipsilateral tibialis anterior muscle by stimulating (in random order) the sural nerve (SUR), the distal tibial nerve (TIB), and combined simultaneous stimulation of both nerves (TIB&SUR). Reflexes were evoked while participants performed rhythmic stepping and arm swinging movement with both arms and the leg contralateral to stimulation (ARM&LEG), with just arm movement (ARM) and with just contralateral leg movement (LEG). Stimulation intensities were just below threshold for evoking early latency (<80 ms to peak) reflexes. For each stimulus condition, rectified EMG signals were averaged while participants held static contractions in the stationary (stimulated) leg. During ARM&LEG movement, amplitudes of cutaneous reflexes evoked by combined TIB&SUR stimulation were significantly larger than simple mathematical summation of the amplitudes evoked by SUR or TIB alone. Interestingly, this extra facilitation seen during combined nerve stimulation was significantly reduced when performing ARM or LEG compared to ARM&LEG. We conclude that locomotor rhythmic limb movement induces excitation of common IN reflex pathways from cutaneous afferents innervating different foot regions. Importantly, activity in this pathway is most facilitated during ARM&LEG movement. These results suggest that transmission in IN reflex pathways is weighted according to the number of limbs directly engaged in human locomotor activity and underscores the importance of arm swing to support neuronal excitability in leg muscles.

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.

After stroke bidirectional modulation of soleus stretch reflex amplitude emerges during rhythmic arm cycling.

OBJECTIVES: after stroke a typical presentation is exaggerated stretch reflexes (SRs) on the more affected (MA) side. The present study evaluated the contribution of presynaptic inhibition (PSI) induced by arm cycling and homosynaptic depression (HD) to the modulation of hyperreflexia at the ankle after stroke. Possible asymmetry of these effects between the MA and less affected (LA) legs was also assessed. METHODS: soleus SR was conditioned by: arm cycling at 1 Hz (to increase Ia PSI); or, a preceding conditioning tendon tap applied 1 s before the test stimulus (to induce HD). The extent of conditioning effects was compared between the MA and the LA legs. RESULTS: for both MA and LA legs, rhythmic arm movement induced a bidirectional effect in different participants, either increasing or decreasing SR amplitude (p < 0.05). HD had a significant effect in both legs (p < 0.05), however, the effect of both a previous muscle stretch and arm cycling was not different between the MA and the LA legs. CONCLUSION: our data reveal a bidirectional reflex modulation induced by arm cycling that produced facilitation in some and suppression in other participants after stroke. Relative SR amplitude modulation did not differ between the LA and MA legs. We speculate that alterations in SR amplitude modulation after stroke may reflect specific changes in both presynaptic afferent transmission mechanisms and fusimotor control. SIGNIFICANCE: the present findings open new perspectives on the characterization of pathophysiology of stroke during the performance of functionally relevant motor tasks.

Neural mechanisms influencing interlimb coordination during locomotion in humans: presynaptic modulation of forearm H-reflexes during leg cycling.

Presynaptic inhibition of transmission between Ia afferent terminals and alpha motoneurons (Ia PSI) is a major control mechanism associated with soleus H-reflex modulation during human locomotion. Rhythmic arm cycling suppresses soleus H-reflex amplitude by increasing segmental Ia PSI. There is a reciprocal organization in the human nervous system such that arm cycling modulates H-reflexes in leg muscles and leg cycling modulates H-reflexes in forearm muscles. However, comparatively little is known about mechanisms subserving the effects from leg to arm. Using a conditioning-test (C-T) stimulation paradigm, the purpose of this study was to test the hypothesis that changes in Ia PSI underlie the modulation of H-reflexes in forearm flexor muscles during leg cycling. Subjects performed leg cycling and static activation while H-reflexes were evoked in forearm flexor muscles. H-reflexes were conditioned with either electrical stimuli to the radial nerve (to increase Ia PSI; C-T interval  = 20 ms) or to the superficial radial (SR) nerve (to reduce Ia PSI; C-T interval  = 37-47 ms). While stationary, H-reflex amplitudes were significantly suppressed by radial nerve conditioning and facilitated by SR nerve conditioning. Leg cycling suppressed H-reflex amplitudes and the amount of this suppression was increased with radial nerve conditioning. SR conditioning stimulation removed the suppression of H-reflex amplitude resulting from leg cycling. Interestingly, these effects and interactions on H-reflex amplitudes were observed with subthreshold conditioning stimulus intensities (radial n., ∼0.6×MT; SR n., ∼ perceptual threshold) that did not have clear post synaptic effects. That is, did not evoke reflexes in the surface EMG of forearm flexor muscles. We conclude that the interaction between leg cycling and somatosensory conditioning of forearm H-reflex amplitudes is mediated by modulation of Ia PSI pathways. Overall our results support a conservation of neural control mechanisms between the arms and legs during locomotor behaviors in humans.

Lower limb electromygraphy and kinematics of neuropathic diabetic patients during real-life activities: stair negotiation.

INTRODUCTION: In this study we investigate the influence of diabetic neuropathy (DN) on lower limb electromyography (EMG) and kinematics during stair negotiation. METHODS: Forty-six adults (healthy and DN) performed stair ascent and descent tasks. Kinematic and EMG data were assessed unilaterally. RESULTS: DN patients had lower ankle dorsiflexion while ascending and plantarflexion while descending. This reduced dorsiflexion compromises proper ankle and knee positions necessary for an efficient lifting action by the vastus lateralis (VL). The mechanical disadvantage of VL at the beginning of the stance triggered prolonged VL activation at the end of stair ascent. In stair descent, DN patients showed lower tibialis anterior activity in the early phase that can potentially impair the mechanism of impact absorption when the forefoot contacts the step. CONCLUSIONS: Our results reveal an adaptive motor strategy in DN patients to overcome the challenge of stair ascent, which promoted more biomechanical deficits.

Interlimb coupling from the arms to legs is differentially specified for populations of motor units comprising the compound H-reflex during "reduced" human locomotion.

Recent experiments have identified neuromechanical interactions between the arms and legs during human locomotor movement. Previous work reported that during the rhythmic movement of all four limbs, the influence of the arms on reflex expression in the legs was superimposed on the dominant effect of the legs. This evidence was based upon studies using cutaneous and H-reflex modulation as indices of neuronal activity related to locomotion. The earlier H-reflex study was restricted to one phase of movement and to only a fixed H-reflex amplitude. Also, all four limbs were actively engaged in locomotor movement, and this led to the speculation that the effect from the arms could be underestimated by "swamping" of the conditioning during movement of the test limb. Work from the cat suggests that descending locomotor drive may be differentially specified for different motor unit populations in the hindlimb. Accordingly, details of interlimb coordination between the arms and legs in humans require further characterization and an examination of different populations of motor units as can be obtained from H-reflex recruitment curve (RC) parameters. Using modulation of H-reflex amplitudes across the entire ascending limb as neural probes for interlimb coupling, the present study evaluated the separated influences of rhythmic activity of the arms and leg on neuronal excitability of a stationary "test leg". This three-limb "reduced" locomotion approach was applied using a stepping ergometer during the performance of three rhythmic movement tasks: arms (A); contralateral leg (L); and arms and contralateral leg (AL). Data were sampled at four different phases of the stepping cycle (using the moving leg as reference): start power (SP); end power (EP); start recovery (SR); and end recovery (ER). The main result was a large and significant influence of rhythmic AL activity on RC parameters of the H-reflex at EP and SP phases. However, the parameters (and thus motor unit populations) were differentially affected at each phase and task. For instance, a significant contribution of arms movement was noticed for H (max) (largest motor units) at EP phase (P < 0.05), but no changes was observed for other parameters related to lower reflex amplitude (e.g., H-reflex evoked with an input that elicited 50% of maximum reflex response during static condition; H@50%). On the other hand, at SR phase, the parameter H@50% was significantly affected during AL compared to L. It is suggested that the remote effect from arms rhythmic activity has been differentially manifested across motor unit populations for each phase of movement. These findings provide definitive evidence for interlimb coupling between cervical and lumbar oscillators in gating the excitability of reflex pathways to a leg muscle for different populations of motorneurons within the pool. This further supports the contention of similar functional organization for locomotor networks in the human when compared to other animals. Additionally, these data provide additional confirmation of the significant role of the output of neural control for rhythmic arm movement in modulating reflex excitability of the legs that is specifically adjusted according to the phase and task.

Bilateral soleus H-reflexes in humans elicited by simultaneous trains of stimuli: symmetry, variability, and covariance.

Experiments using electrical and mechanical activation of spinal reflexes have contributed important results toward the understanding of neuronal and synaptic dynamics involved in spinal neural circuits as well as their response to different inputs. In this work, data obtained from the simultaneous stimulation of both legs are analyzed to provide information on the degree of symmetry of the respective spinal reflex circuits and on the characteristics of reflex variability. H-reflexes recorded from relaxed muscles show a frequency-dependent amplitude depression when elicited by a train of stimuli. This effect has been attributed to homosynaptic depression. Soleus H-reflexes were recorded in response to trains of simultaneous stimuli applied to both legs in right-handed subjects that were sitting in a relaxed state. The first objective was to verify the existence of asymmetries in H-reflex parameters obtained from the two legs. We measured the mean, variance, and coefficient of variation of the depressed H-reflex amplitudes and the time constant of decay toward the depressed plateau. The second objective was the analysis of the time correlation of subsequent H-reflex amplitudes in a long train of responses recorded from a given leg. The statistical dependence of H-reflex amplitudes in the long trains recorded from both legs was also investigated. Data obtained from preliminary experiments showed that there was no effect of a given stimulus on the contralateral leg applied simultaneously or 1 s before, therefore validating the simultaneous stimulation paradigm. Paired t-tests indicated that several parameters measured bilaterally from soleus H-reflex trains of right-handed subjects were not statistically different in the overall, although individually there were statistically significant asymmetries, toward either the right or left leg. Sequences of H-reflex amplitudes, as measured by the auto-covariance, were either white or had a memory ranging from 2 up to 50 s. This indicates that the random fluctuations in presynaptic inhibition and/or postsynaptic inputs to motoneurons may have either fast or slow time courses. The average auto-covariance sequences of the right and left legs, computed from all subjects, were practically superposable. The cross-covariance between the bilateral H-reflex amplitudes showed a statistically significant peak at zero lag in some experiments, suggesting a correlation between the synaptic inputs to the Ia-motoneuron systems of the soleus muscles of both legs.