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

Spectral characterization of human leg EMG signals from an open access dataset for the development of computational models

Large-scale neuromusculoskeletal models have been used for predicting mechanisms underlying neuromuscular functions in humans. Simulations of such models provide several types of signals of practical interest, such as surface electromyographic signals (EMG), which are compared with experimental data for interpretations of neurophysiological phenomena under study. Specifically, realistic characterization of spectral properties of simulated EMG signals is important for achieving powerful inferences, whereas considerations should be taken for myoelectric signals of different muscles. In this study, we characterized spectral properties of surface interference pattern EMG signals and motor unit action potentials (MUAP) acquired from three plantar flexor muscles: Soleus (SO), Medial Gastrocnemius (MG), and Lateral Gastrocnemius (LG); and one dorsiflexor muscle: Tibialis Anterior (TA). Surface EMG signals were acquired from 20 participants using the same convention for electrode placement. Specifically, interference pattern EMG signals were obtained during isometric constant force contractions at 5%, 10% and 20% of maximum voluntary contraction (MVC), whereas surface MUAPs were decomposed from surface EMG signals obtained at low contraction forces. We compared the spectrum median frequency (MDF) estimated from interference pattern EMG signals across muscles and contraction intensities. Additionally, we compared MDF and durations of MUAPs between muscles. Our results showed that MDF of interference pattern EMG signals acquired from TA were higher compared to SO, MG, and LG for all contraction intensities i.e., 5%, 10%, and 20% MVC. Consistently, MUAPs acquired from TA also had higher MDF values and shorter durations compared to the other leg muscles. We provide herein a dataset with the surface MUAPs waveforms and interference pattern EMG signals obtained for this study, which should be useful for implementing and validating the simulation of myoelectrical signals of leg muscles. Importantly, these results indicate that spectral properties of myoelectrical signals should be considered for improving EMG modeling in large-scale neuromusculoskeletal models.

Five weeks of Yuishinkai karate training improves balance and neuromuscular function in older adults: a preliminary study

Background

Martial arts training has shown positive impacts on balance and physiological measurements. Further investigation of the contents and feasibility of an effective therapeutic assessment of martial arts is needed in older adults, mainly for future applications and real-world implementation.

Methods

Sixteen older adults (8 male, 8 female, age 59–90 years), with or without chronic conditions, participated in a preliminary study using 5-weeks of karate training and a triple baseline control procedure. Group and single subject data analyses were conducted for dynamic balance, Timed Up and Go (TUG), hand grip, ankle plantarflexion force, and spinal cord excitability (via the soleus H-reflex) pre- and post-training.

Results

On average, participants completed a total of 2437 steps, 1762 turns, 3585 stance changes, 2047 punches, 2757 blocks, and 1253 strikes. Karate training improved dynamic balance performance such that the group average time was reduced (time to target (−13.6%, p = 0.020) and time to center (−8.3%, p = 0.010)). TUG was unchanged when considering the entire group (p = 0.779), but six participants displayed significant changes. Left handgrip (7.9%, p = 0.037), and plantarflexion force in the right (28.8%, p = 0.045) and left leg (13.3%, p = 0.024) increased for the group. Spinal cord excitability remained unchanged in group data analysis but 5 individuals had modulated H_max/M_max ratios.

Conclusion

5-weeks of karate training delivered in a fashion to mimic generally accessible community-level programs improved balance and strength in older adults. Whole-body movement embodied in karate training enhanced neuromuscular function and postural control. We met the overriding goal of this preliminary study to emphasize and assess feasibility and safety for the generalizability of martial arts interventions to real-world communities to impact health outcomes. Further quantitative work should explore threshold dose and development of martial arts training interventions as potential “exercise is medicine” functional fitness for older adults.

Transcutaneous electrical nerve stimulation in the management of calf muscle spasticity in cerebral palsy: A pilot study

This study sets out to evaluate the effectiveness of transcutaneous electrical nerve stimulation (TENS) in the management of calf muscle spasticity in children with cerebral palsy. The study follows a one group pre-test–post-test design involving fifteen children with spastic cerebral palsy, presenting with calf muscle spasticity. Spasticity was assessed before and after a 30 min application of TENS to the bilateral calf muscles. The H-reflex (electromyography) of the calf muscles and Modified Ashworth Scale (MAS) served as a measure of spasticity. A goniometer was used to measure the range of motion (ROM) angles for ankle dorsiflexion. We report here no significant difference (p > 0.05) between the left and right H-reflex responses, MAS scores, and ROM scores recorded at baseline (pre-test). Correlation analysis show no correlation (p > 0.05) between the pre-test HA Max (maximum H-reflex amplitude)/MA Max (maximum M-Wave Amplitude) ratio and MAS scores of both the left and right calf muscles. However, TENS significantly reduced (p < 0.05) the HA of the left calf muscle and MAS scores of the left and right calf muscles. Additionally, TENS significantly increased the ROM scores of the left and right calf muscles. Our findings lend support to existing evidence that TENS is effective in reducing spasticity. The potential mechanism underlying this effect is a reduction in neuron excitability.

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The application of TENS in children with spastic Cerebral palsy is well tolerated.

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H-reflex is an effective tool in measuring spasticity objectively.

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TENS may be effective as an adjunct treatment option in the management of spasticity.

Cross-correlated fractal components of H-wave amplitude fluctuations in medial gastrocnemius and soleus muscles

The time series of the H-wave amplitude in soleus muscle (SOL) shows fractal (long-range) correlation, which is attributed to input from supraspinal centers. However, whether such long-range power-law correlated input also contributes to the synergistic muscles remains unclear. The purpose of this study was therefore to examine the correlation in the fractal components of H-wave amplitude fluctuations between the synergistic muscles used for plantar flexion, i.e., the medial head of the gastrocnemius muscle (MG) and SOL. In eight young male participants, consecutive H-reflexes were recorded almost simultaneously from the MG and SOL at a stimulation frequency of 0.5 Hz for 30 min. We performed detrending moving-average cross-correlation analysis (DMCA) for each of the H- and M-wave amplitude time series between MG and SOL to assess the existence of a common noise input contributing to these long-range correlations. The cross-correlation coefficient ρDMCA (-1 to 1) was calculated to quantify the strength of the correlation between two different time series. The results indicated a significant long-range power-law correlation between H-wave amplitudes in MG and SOL (ρDMCA: 0.50 (0.22) and 0.22 (0.17), mean (standard deviation) for the original and randomly shuffled surrogate data, respectively, P < 0.05). This was not the case for M-wave amplitudes (ρDMCA: 0.29 (0.23) and 0.20 (0.15), P > 0.05). We conclude that there is a common noise input governing these synergistic muscles, possibly due to supraspinal origin, causing long-range power-law correlations in monosynaptic reflexes.

Modulation of H-reflex responses and frequency-dependent depression by repetitive spinal electromagnetic stimulation: From rats to humans and back to chronic spinal cord injured rats

The lack of propagation of signals through survived fibers is among the major reasons for functional loss after incomplete spinal cord injury (SCI). Our recent results of animal studies demonstrate that spinal electromagnetic stimulation (SEMS) can enhance transmission in damaged spinal cord, and this type of modulation depends on the function of NMDA receptors at the neuronal networks below the injury level. Here, our pilot human study revealed that administration of repetitive SEMS induced long‐lasting modulation of H‐responses in both healthy and participants with chronic SCI. In order to understand the mechanisms underlying these effects, we have used an animal model and examined effects of SEMS on H‐responses. Effects of SEMS on H‐responses, frequency‐dependent depression (FDD) of H‐reflex, and possible underlying mechanisms have been examined in both naïve and rats with SCI. Our results demonstrate that consistent with the effects of SEMS on H‐reflex seen in humans, repetitive SEMS induced similar modulation in excitability of peripheral nerve responses in both non‐injured and rats with SCI. Importantly, our results confirmed the reduced FDD of H‐reflex in SCI animals and revealed that SEMS was able to recover FDD in rats with chronic SCI. Using intraspinal injections of the NMDA receptor blocker MK‐801, we have identified NMDA receptors as an important contributor to these SEMS‐induced effects in rats with SCI. These results identify SEMS as a novel non‐invasive technique for modulation of neuro‐muscular circuits and, importantly, modulation of spinal networks after chronic SCI.

Exposure to impacts across a competitive rugby season impairs balance and neuromuscular function in female rugby athletes

Objectives

We used objective assessment tools to detect subtle neurological deficits that accompany repetitive and mild head impacts in contact sport across a season.

Methods

Female participants (n=13, 21±1.8 years old; 167.6±6.7 cm; 72.8±6.1 kg) completed assessments pre and post the varsity rugby season. A commercial balance board was used to assess static balance and response to dynamic postural challenge. Spinal cord excitability via the soleus H-reflex was assessed in both legs. Video analysis was used to identify head impact exposures.

Results

A total of 172 potential concussive events were verified across 11 athletes (15.6±11; 95% CI: 6.5 to 19.8). Balance performance was worse at post-season for total centre of pressure which increased by 26% in the double stance on a stable surface (t(12)=-2.33; p=0.03; d=0.6) and by 140% in the tandem stance on a foam surface (t(12)=-3.43; p<0.01; d=0.9). Despite that, dynamic postural performance was improved after the season (p<0.01). Spinal cord excitability in rugby athletes did not change across the season but deviated from normative values at baseline.

Conclusion

Quantitative measures revealed that exposure to impacts across a competitive rugby season impair balance in two specific stances in female rugby athletes. Tandem-leg stance on an unstable surface and double-leg stance on firm surface are useful assessment conditions when performed over a low-cost balance board, even without clinically diagnosed concussion.

Modulation of spinal cord excitability following remote limb ischemic preconditioning in healthy young men

Remote limb ischemic preconditioning (RIPC) has shown to improve dynamic postural control in humans. However, studies on the underlying adaptations of spinal cord networks have never been performed. The present work addresses this issue by investigating parameters from the soleus H-reflex recruitment curve (RC), presynaptic mechanisms of reflex modulation (presynaptic inhibition-PSI, and post activation depression-PAD), and the excursion of the center of pressure (CP) recorded during 1 min in upright stance over a compliant surface. A sham ischemic protocol (partial obstruction of blood flow) was applied to the contralateral thigh along four consecutive days. The same procedure was repeated with full obstruction (RIPC) three days after ending the sham protocol. Data were collected before and after both sham and RIPC protocols. The follow-up data were collected five days after the last ischemic intervention. Significant reduction was detected for both the fast oscillations of the CP (higher frequency components) and the parameter estimated from the RC corresponding to the high amplitude H-reflexes (p < 0.05). Even though the magnitude of effects was similar, it was washed out within three days after sham, but persisted for at least five days after RIPC. No significant differences were found for PSI and PAD levels across conditions. These findings indicate that RIPC leads to enduring changes in spinal cord excitability for the latest reflexively recruited motoneurons, along with improvement in balance control. However, these adaptations were not mediated by the presynaptic mechanisms currently assessed.

Preferential activation of spinal sensorimotor networks via lateralized transcutaneous spinal stimulation in neurologically intact humans

Transcutaneous spinal stimulation (TSS), a noninvasive technique to modulate sensorimotor circuitry within the spinal cord, has been shown to enable a wide range of functions that were thought to be permanently impaired in humans with spinal cord injury. However, the extent to which TSS can be used to target specific mediolateral spinal cord circuitry remains undefined. We tested the hypothesis that TSS applied unilaterally to the skin ~2 cm lateral to the midline of the lumbosacral spine selectively activates ipsilateral spinal sensorimotor circuitry, resulting in ipsilateral activation of downstream lower extremity neuromusculature. TSS cathodes and anodes were positioned lateral from the midline of the spine in 15 healthy subjects while supine, and the timing of TSS pulses was synchronized to recordings of lower extremity muscle activity and force. At motor threshold, left and right TSS-evoked muscle activity was significantly higher in the ipsilateral leg compared with contralateral recordings from the same muscles. Similarly, we observed a significant increase in force production in the ipsilateral leg compared with the contralateral leg. Delivery of paired TSS pulses, during which an initial stimulus was applied to one side of the spinal cord and 50 ms later a second stimulus was applied to the contralateral side, revealed that ipsilateral leg muscle responses decreased following the initial stimulus, whereas contralateral muscle responses did not decrease, indicating side-specific activation of lateral spinal sensorimotor circuitry. Our results indicate TSS can selectively engage ipsilateral neuromusculature via lumbosacral sensorimotor networks responsible for lower extremity function in healthy humans.NEW & NOTEWORTHY We demonstrate the selectivity of transcutaneous spinal stimulation (TSS), which has been shown to enable function in humans with chronic paralysis. Specifically, we demonstrate that TSS applied to locations lateral to the spinal cord can selectively activate ipsilateral spinal sensorimotor networks. We quantified lumbosacral spinal network activity by recording lower extremity muscle electromyography and force. Our results suggest lumbosacral TSS engages side-specific spinal sensorimotor networks associated with ipsilateral lower extremity function in humans.

The Complexity of H-wave Amplitude Fluctuations and Their Bilateral Cross-Covariance Are Modified According to the Previous Fitness History of Young Subjects under Track Training

The Hoffmann reflex (H-wave) is produced by alpha-motoneuron activation in the spinal cord. A feature of this electromyography response is that it exhibits fluctuations in amplitude even during repetitive stimulation with the same intensity of current. We herein explore the hypothesis that physical training induces plastic changes in the motor system. Such changes are evaluated with the fractal dimension (FD) analysis of the H-wave amplitude-fluctuations (H-wave FD) and the cross-covariance (CCV) between the bilateral H-wave amplitudes. The aim of this study was to compare the H-wave FD as well as the CCV before and after track training in sedentary individuals and athletes. The training modality in all subjects consisted of running three times per week (for 13 weeks) in a concrete road of 5 km. Given the different physical condition of sedentary vs. athletes, the running time between sedentary and athletes was different. After training, the FD was significantly increased in sedentary individuals but significantly reduced in athletes, although there were no changes in spinal excitability in either group of subjects. Moreover, the CCV between bilateral H-waves exhibited a significant increase in athletes but not in sedentary individuals. These differential changes in the FD and CCV indicate that the plastic changes in the complexity of the H-wave amplitude fluctuations as well as the synaptic inputs to the Ia-motoneuron systems of both legs were correlated to the previous fitness history of the subjects. Furthermore, these findings demonstrate that the FD and CCV can be employed as indexes to study plastic changes in the human motor system.

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.

Periodic modulation of repetitively elicited monosynaptic reflexes of the human lumbosacral spinal cord

In individuals with motor-complete spinal cord injury, epidural stimulation of the lumbosacral spinal cord at 2 Hz evokes unmodulated reflexes in the lower limbs, while stimulation at 22-60 Hz can generate rhythmic burstlike activity. Here we elaborated on an output pattern emerging at transitional stimulation frequencies with consecutively elicited reflexes alternating between large and small. We analyzed responses concomitantly elicited in thigh and leg muscle groups bilaterally by epidural stimulation in eight motor-complete spinal cord-injured individuals. Periodic amplitude modulation of at least 20 successive responses occurred in 31.4% of all available data sets with stimulation frequency set at 5-26 Hz, with highest prevalence at 16 Hz. It could be evoked in a single muscle group only but was more strongly expressed and consistent when occurring in pairs of antagonists or in the same muscle group bilaterally. Latencies and waveforms of the modulated reflexes corresponded to those of the unmodulated, monosynaptic responses to 2-Hz stimulation. We suggest that the cyclical changes of reflex excitability resulted from the interaction of facilitatory and inhibitory mechanisms emerging after specific delays and with distinct durations, including postactivation depression, recurrent inhibition and facilitation, as well as reafferent feedback activation. The emergence of large responses within the patterns at a rate of 5.5/s or 8/s may further suggest the entrainment of spinal mechanisms as involved in clonus. The study demonstrates that the human lumbosacral spinal cord can organize a simple form of rhythmicity through the repetitive activation of spinal reflex circuits.

Enhanced D1 and D2 inhibitions induced by low-frequency trains of conditioning stimuli: differential effects on H- and T-reflexes and possible mechanisms

Mechanically evoked reflexes have been postulated to be less sensitive to presynaptic inhibition (PSI) than the H-reflex. This has implications on investigations of spinal cord neurophysiology that are based on the T-reflex. Preceding studies have shown an enhanced effect of PSI on the H-reflex when a train of ~10 conditioning stimuli at 1 Hz was applied to the nerve of the antagonist muscle. The main questions to be addressed in the present study are if indeed T-reflexes are less sensitive to PSI and whether (and to what extent and by what possible mechanisms) the effect of low frequency conditioning, found previously for the H-reflex, can be reproduced on T-reflexes from the soleus muscle. We explored two different conditioning-to-test (C-T) intervals: 15 and 100 ms (corresponding to D1 and D2 inhibitions, respectively). Test stimuli consisted of either electrical pulses applied to the posterior tibial nerve to elicit H-reflexes or mechanical percussion to the Achilles tendon to elicit T-reflexes. The 1 Hz train of conditioning electrical stimuli delivered to the common peroneal nerve induced a stronger effect of PSI as compared to a single conditioning pulse, for both reflexes (T and H), regardless of C-T-intervals. Moreover, the conditioning train of pulses (with respect to a single conditioning pulse) was proportionally more effective for T-reflexes as compared to H-reflexes (irrespective of the C-T interval), which might be associated with the differential contingent of Ia afferents activated by mechanical and electrical test stimuli. A conceivable explanation for the enhanced PSI effect in response to a train of stimuli is the occurrence of homosynaptic depression at synapses on inhibitory interneurons interposed within the PSI pathway. The present results add to the discussion of the sensitivity of the stretch reflex pathway to PSI and its functional role.

Spinal segment-specific transcutaneous stimulation differentially shapes activation pattern among motor pools in humans

Transcutaneous and epidural electrical spinal cord stimulation techniques are becoming more valuable as electrophysiological and clinical tools. Recently, we observed selective activation of proximal and distal motor pools during epidural spinal stimulation. In the present study, we hypothesized that the characteristics of recruitment curves obtained from leg muscles will reflect a relative preferential activation of proximal and distal motor pools based on their arrangement along the lumbosacral enlargement. The purpose was to describe the electrophysiological responses to transcutaneous stimulation in leg muscles innervated by motoneurons from different segmental levels. Stimulation delivered along the rostrocaudal axis of the lumbosacral enlargement in the supine position resulted in a selective topographical recruitment of proximal and distal leg muscles, as described by threshold intensity, slope of the recruitment curves, and plateau point intensity and magnitude. Relatively selective recruitment of proximal and distal motor pools can be titrated by optimizing the site and intensity level of stimulation to excite a given combination of motor pools. The slope of the recruitment of particular muscles allows characterization of the properties of afferents projecting to specific motoneuron pools, as well as to the type and size of the motoneurons. The location and intensity of transcutaneous spinal electrical stimulation are critical to target particular neural structures across different motor pools in investigation of specific neuromodulatory effects. Finally, the asymmetry in bilateral evoked potentials is inevitable and can be attributed to both anatomical and functional peculiarities of individual muscles or muscle groups.

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.

Absence of effects of contralateral group I muscle afferents on presynaptic inhibition of Ia terminals in humans and cats

Crossed effects from group I afferents on reflex excitability and their mechanisms of action are not yet well understood. The current view is that the influence is weak and takes place indirectly via oligosynaptic pathways. We examined possible contralateral effects from group I afferents on presynaptic inhibition of Ia terminals in humans and cats. In resting and seated human subjects the soleus (SO) H-reflex was conditioned by an electrical stimulus to the ipsilateral common peroneal nerve (CPN) to assess the level of presynaptic inhibition (PSI_control). A brief conditioning vibratory stimulus was applied to the triceps surae tendon at the contralateral side (to activate preferentially Ia muscle afferents). The amplitude of the resulting H-reflex response (PSI_conditioned) was compared to the H-reflex under PSI_control, i.e., without the vibration. The interstimulus interval between the brief vibratory stimulus and the electrical shock to the CPN was -60 to 60 ms. The H-reflex conditioned by both stimuli did not differ from that conditioned exclusively by the ipsilateral CPN stimulation. In anesthetized cats, bilateral monosynaptic reflexes (MSRs) in the left and right L(7) ventral roots were recorded simultaneously. Conditioning stimulation applied to the contralateral group I posterior biceps and semitendinosus (PBSt) afferents at different time intervals (0-120 ms) did not have an effect on the ipsilateral gastrocnemius/soleus (GS) MSR. An additional experimental paradigm in the cat using contralateral tendon vibration, similar to that conducted in humans, was also performed. No significant differences between GS-MSRs conditioned by ipsilateral PBSt stimulus alone and those conditioned by both ipsilateral PBSt stimulus and contralateral tendon vibration were detected. The present results strongly suggest an absence of effects from contralateral group I fibers on the presynaptic mechanism of MSR modulation in relaxed humans and anesthetized cats.

A method to estimate EMG crosstalk between two muscles based on the silent period following an H-reflex

The crosstalk phenomenon consists in recording the volume-conducted electromyographic activity of muscles other than that under study. This interference may impair the correct interpretation of the results in a variety of experiments. A new protocol is presented here for crosstalk assessment between two muscles based on changes in their electrical activity following a reflex discharge in one of the muscles in response to nerve stimulation. A reflex compound muscle action potential (H-reflex) was used to induce a silent period in the muscle that causes the crosstalk, called here the remote muscle. The rationale is that if the activity recorded in the target muscle is influenced by a distant source (the remote muscle) a silent period observed in the electromyogram (EMG) of the remote muscle would coincide with a decrease in the EMG activity of the target muscle. The new crosstalk index is evaluated based on the root mean square (RMS) values of the EMGs obtained in two distinct periods (background EMG and silent period) of both the remote and the target muscles. In the present work the application focused on the estimation of the degree of crosstalk from the soleus muscle to the tibialis anterior muscle during quiet stance. However, the technique may be extended to other pairs of muscles provided a silent period may be evoked in one of them.

Bilateral motor resonance evoked by observation of a one-hand movement: role of the primary motor cortex

In humans, observation of movement performed by others evokes a subliminal motor resonant response, probably mediated by the mirror neurone system, which reproduces the motor commands needed to execute the observed movement with good spatial and temporal fidelity. Motor properties of the resonant response were here investigated with the ultimate goal of understanding the principles operating in the transformation from observation to internal reproduction of movement. Motor resonance was measured as the modulation of excitability of spinal motoneurones, evoked by the observation of a cyclic flexion-extension of one hand. The first two experiments showed that the observation of a one-hand movement always evoked a bimanual resonant response independent of which hand was observed and that these bilateral responses were consistently phase-linked. H-reflexes simultaneously recorded in right and left flexor carpi radialis muscles were always modulated 'in-phase' with each other. The goal of the third experiment was to define the role of primary motor cortex in the bilateral resonant response. Bilateral H-reflexes were recorded during a temporary inactivation induced by transcranial magnetic stimulation over the left cortical hand motor area of observers. The finding that such cortical depression abolished the H-reflex modulation of only the right flexor carpi radialis motoneurones, leaving it unchanged on the left side, suggested that both primary motor areas were activated by the premotor cortex and transmit the resonant activation through crossed corticospinal pathways. The data provide further evidence that the subliminal activation of motor pathways induced by movement observation is organized according to general rules shared with the control of voluntary movement.

Spinal Source for the Synchronous Fluctuations of Bilateral Monosynaptic Reflexes in Cats

Successive stimuli of constant intensity applied to Ia afferents produce spinal monosynaptic reflexes (MSRs) of variable amplitude. We recorded simultaneous MSRs in the left and right L7 (or L6) ventral roots of anesthetized cats. We analyzed the cross-covariance (CCV) between the amplitudes of bilateral MSRs. Long-time series (5 to 8 h) of these bilateral MSRs exhibited transitory changes in their covariations (as measured by the zero-lag peak of their CCV), thus suggesting the existence of certain neural sources contributing to produce these changes. The aim of the present study was to show that spinal centers producing negative spontaneous cord dorsum potentials (nSCDPs) contribute to maintain correlations in the amplitude of bilateral MSRs. After spinal cord transection at the L1 segment, no significant changes were observed in the correlation between the amplitude of bilateral nSCDPs versus the amplitude of bilateral MSRs. However, this correlation, as well as the peak at zero lag in the CCV between bilateral MSRs and the CCV between bilateral nSCDPs, respectively, were abolished after a subsequent longitudinal bisection at the L1–S2 spinal segments. These results suggest that lumbar spinal neurons (bilaterally interconnected) contribute to maintain the synchronous fluctuations of bilateral MSRs.