Low-alcohol doses reduce common 10- to 15-Hz input to bilateral leg muscles during quiet standing

The effects of low doses of alcohol on neural synchronization in muscular activity were investigated in ten participants during quiet standing with eyes open or closed. We focused on changes in common input to bilateral motor unit pools as evident in surface electromyographic (EMG) recordings of lower leg extensor and flexor muscles. The extensor muscles exhibited bilateral synchronization in two distinct frequency bands (i.e., 0–5 and 10–15 Hz), whereas synchronization between flexor muscles was minimal. As expected, alcohol ingestion affected postural sway, yielding increased sway at higher blood-alcohol levels. Whereas vision affected bilateral synchronization only at 0–5 Hz, alcohol ingestion resulted in a progressive decrease of synchronization at 10–15 Hz between the EMG activities of the extensor muscles. The decrease in common bilateral input is most likely related to reduced reticulospinal activity with alcohol ingestion.     

Antagonist motor responses correlate with kinesthetic illusions induced by tendon vibration

 In humans, vibration applied to muscle tendons evokes illusory sensations of movement that are usually associated with an excitatory tonic response in muscles antagonistic to those vibrated (antagonist vibratory response or AVR). The aim of the present study was to investigate the neurophysiological mechanisms underlying such a motor response. For that purpose, we analyzed the relationships between the parameters of the tendon vibration (anatomical site and frequency) and those of the illusory movement perceived (direction and velocity), as well as the temporal, spatial, and quantitative characteristics of the corresponding AVRs (i.e., surface EMG, motor unit firing rates and activation latencies). Analogies were supposed between the characteristics of AVRs and voluntary contractions. The parameters of the AVR were thus compared with those of a voluntary contraction with similar temporal and mechanical characteristics, involving the same muscle groups as those activated by vibration. Wrist flexor muscles were vibrated either separately or simultaneously with wrist extensor muscles at frequencies between 30 and 80 Hz. The illusory movement sensations were quantified through contralateral hand-tracking movements. Electromyographic activity from the extensor carpi radialis muscles was recorded with surface and intramuscular microelectrodes. The results showed that vibration of the wrist flexor muscle group induced both a kinesthetic illusion of wrist extension and a motor response in the extensor carpi radialis muscles. Combined vibration of the two antagonistic muscle groups at the same frequency evoked neither kinesthetic illusion nor motor activity. In addition, vibrating the same two antagonistic muscle groups at different frequencies induced both a kinesthetic illusion and a motor response in the muscle vibrated at the lowest frequency. The surface EMG amplitude of the extensor carpi radialis as well as the motor unit activation latency and discharge frequency were clearly correlated to the parameters of the illusory movement evoked by the vibration. Indeed, the faster the illusory sensation of movement, the greater the surface EMG in these muscles during the AVRs and the sooner and the more intense the activation of the motor units of the wrist extensor muscles. Moreover, comparison of the AVR with voluntary contraction showed that all parameters were highly similar. Mainly slow motor units were recruited during the AVR and during its voluntary reproduction. That the AVR is observed only when a kinesthetic illusion is evoked, together with the similarities between voluntary contractions and AVRs, suggests that this vibration-induced motor response may result from a perceptual-to-motor transformation of proprioceptive information, rather than from spinal reflex mechanisms. Received: 21 July 1997 / Accepted: 11 August 1998     

Difference in electromyographic response of finger flexion muscles between tonic vibration reflex and finger flexion reflex induced by finger tip vibration

Vibratory stimulus applied to the skin of the finger tip induced flexion reflex in that finger. By using the cross-correlation function, characteristics of this reflex were compared to those of tonic vibration reflex (TVR). In the cross-correlogram between unitary electromyogram (EMG) activity in the muscle flexor digitorum superficialis and vibratory stimulus with random frequency, one mode was seen in TVR, and two modes in finger flexion reflex. The secondary mode was significantly wider than the primary mode. Thus it may originate from skin mechanoreceptors and manifest via a reflex center involving a long loop.     

Perceptual and motor effects of agonist-antagonist muscle vibration in man

Summary Perceptual and motor effects of vibration applied simultaneously to the distal tendons of the Biceps and Triceps muscles, in isometric conditions and without sight of the stimulated arm, have been studied in human volunteers. Motor effects, measured by surface EMG, are inexistent when the flexor and extensor muscles are simultaneously vibrated at the same frequency. However, EMG activity appears in the muscle being vibrated at the lower frequency when simultaneous vibration is applied at different frequencies. The sensations felt by the subjects were reproduced by the nonvibrated arm and recorded by a goniometer. The studies show that the velocity and the amplitude of the ilusory movement is related to the difference in vibration frequency applied to the two muscles. The direction of movement felt (flexion or extension) is that produced by shortening of the muscle being vibrated at the lower frequency. When the two vibration frequencies are the same, there is either no sensation of movement, or a sensation of very slow movement. These results support the notion that the sensation of movement at a joint may be derived from a central processing of the proprioceptive inflow data obtained from flexor and extensor muscles. This interpretation may also be valid for the results obtained earlier by vibration of a single muscle. Furthermore, it is coherent with data on spindle afferent fibres obtained by microneurography in man during passive or active movements.This work was supported by grants from the Ministère de l'Industrie et de la Recherche     

Bilateral motor unit synchronization is functionally organized

To elucidate the neural interactions underlying bimanual coordination, we investigated in 11 participants the bilateral coupling of homologous muscles in an isometric force production task involving fatiguing elbow flexion and extension. We focused on changes in motor unit (MU) synchronization as evident in EMG recordings of relevant muscles. In contrast to a related study on leg muscles, the arm muscles did not exhibit MU synchronization around 16 Hz, consistent with our hypothesis that 16 Hz MU synchronization is linked to balance maintenance. As expected, bilateral MU synchronization was apparent between 8 and 12 Hz and increased with fatigue and more strongly so for extensor than for flexor muscles. MU synchronization in that frequency band is interpreted in terms of common bilateral input and substantiates the idea that common input is functionally organized. Since these findings are consistent with the literature on mirror movements, they suggest that both phenomena may be related.     

Contribution of motor unit activity enhanced by acute fatigue to physiological tremor of finger.

The contribution of motor unit activity to a physiological tremor (hereafter called as tremor) in a middle finger is studied by both a power spectrum and a correlation analysis in which the correlation coefficient and the coherence spectrum are obtained when five kinds of loads, 0, 50, 100, 150, and 200 g, are added to the middle finger for two minutes in a loading experiment on twelve male subjects. A weight of 200 g is applied to the subjects for ten minutes in a fatigue experiment. Throughout both experiments, the middle finger remains stretched from the load of the weight. The tremor is measured by an accelerometer (MT-3T, Nihon Kohden, Japan) attached to the middle finger, and the surface electromyogram (EMG) is measured by bipolar electrodes placed on m. extensor digitorum communis. A power spectrum analysis is carried out on the tremor and EMG, and a correlation analysis is performed on the relationship between the tremor and the demodulated EMG. It is found in the loading experiment that when the weight on the finger increases, the amplitude of the tremor oscillation increases since the activity of the motor units of the muscle is enhanced by the phenomenon of recruitment. Two frequency components of the tremor spectra at 10 Hz and 25 Hz under a no load condition reflect the components of the activity of the motor units of the muscle because the tremor shows a significant correlation in the frequency zone of 10 Hz and 25 Hz with the demodulated EMG. The lower frequency component of the tremor spectrum at 10 Hz results in synchronized activity of the motor units, while the higher frequency at 25 Hz occurs from the stretch reflex loop via the motoneurons of the spinal cord. The shift of the higher frequency component to the lower frequency domain due to the load of the weight originates from the prolongation of the response time of the finger mechanical system because the lag time at the peak of the correlation coefficient increases with the load of the weight. It is found in the fatigue experiment that the amplitude of the tremor oscillation increases with the progress of fatigue. The increase is caused by the recruitment of the motor unit activity of the muscle holding the finger as well as by the synchronization of the firings of the motoneurons. The progress of the synchronization is verified by the fact that the mean power frequency (MPF) of the EMG spectrum decreases and the correlation between the tremor and the demodulated EMG increases with the progress of fatigue. The mechanisms of the increase of the amplitude of the tremor oscillation under the load of the weight to the finger and under the state of fatigue of the finger are elucidated by the analysis of the tremor and EMG.     

Postural readjustment to body sway induced by vibration in man

Summary Postural readjustment to body sway, induced by vibration applied to leg muscles for 30 or 40 s, was investigated in healthy subjects standing erect on a gravicorder. The vibratory stimulus induced involuntary body sway in the same direction as the vibrated side, and after cessation of vibration, the body swayed in the opposite direction beyond the initial standing position. Body sway consisted of a sustained displacement with gradual onset, upon which were superimposed some oscillatory movements.An increase in discharge of the soleus muscles was not obvious when vibration was applied to the Achilles tendon of a standing subject. However, vibration could provoke a tonic activity in the soleus muscles under the following conditions: (1) when the muscle was maintained in an isometric contraction, or (2) when the muscle was stretched voluntarily by leaning forward. It was found that EMG activity in the soleus muscles correlated well with the position of the center of gravity of the body. During vibration, an enhancement of the EMG activity was roughly proportional to the magnitude of the body inclination. These enhancements can be explained as the tonic vibration reflex (TVR), and body inclination induced by vibration may be partially explained as the TVR phenomenon.An ischemic nerve block of the legs reduced the vibration-induced body sway and the Achilles tendon reflex (ATR). The time courses of suppression and recovery after the release of the nerve block were compared: (a) the backward body inclination induced by the Achilles tendon vibration became obscure after 9–12 min of ischemia, and it took 6–12 min for recovery, and (b) the amplitude of the ATR decreased more gradually than that of body inclination and diminished after 20 min of ischemia, but it recovered very rapidly (within approximately 1 min). The discrepancy between these time courses suggested that afferent fibers which take part in body sway are not only group Ia fibers, but also smaller ones such as group II and cutaneous afferents.Frequency analysis of body sway during postural readjustments to the vibratory stimulus showed: (1) a marked increase in amplitude for the low frequencies (less than 0.2 Hz) that reflected body inclination; and (2) sharp spectral peaks between 0.2 Hz and 0.8 Hz that reflected oscillatory movements.     

Prolonged vibration of the biceps brachii tendon reduces time to failure when maintaining arm position with a submaximal load

Vibration reduces the amplitudes of the tendon jerk response and the Hoffmann and stretch reflexes in the muscle exposed to the vibration, yet does not alter the time to task failure when the task involves exerting a submaximal force against a rigid restraint. Because the amplitude of the stretch reflex is greater when a limb acts against a compliant load than a rigid restraint, the purpose was to determine the influence of prolonged tendon vibration on the time to failure when maintaining limb position with the elbow flexor muscles. Twenty-five healthy men performed the fatiguing contraction by maintaining elbow angle at 1.57 rad until failure while supporting a load equal to 20% of maximal voluntary contraction (MVC) force. The fatiguing contraction was performed on 3 separate days with different levels of vibration applied to the biceps brachii tendon: no vibration, subthreshold for a tonic vibration reflex (TVR), and suprathreshold for a TVR. MVC force before the fatiguing contraction was similar across the three sessions (mean of 3 sessions: 313 +/- 54 N, P = 0.83). Despite a similar decline in MVC force after the fatiguing contraction across conditions (-18.0 +/- 8.0%, P > 0.05), the time to task failure was 3.7 +/- 1.4 min for the suprathreshold TVR condition, 4.3 +/- 2.1 min for the subthreshold TVR condition, and 5.0 +/- 2.2 min for the no-vibration condition (P < 0 0.001). The average EMG of the elbow flexor muscles was similar (P = 0.22) during the fatiguing contractions. However, the fluctuations in limb acceleration at task onset were greater for the suprathreshold TVR condition (P < 0.01), but were not different between the subthreshold TVR and no-vibration conditions (P > or = 0.22). Furthermore, the difference in the SD of limb acceleration between the no-vibration and vibration conditions was correlated with the difference in time to failure for the no-vibration and subthreshold TVR conditions (P = 0.03; r2 = 0.22), but not for the no-vibration and suprathreshold TVR conditions (P = 0.90; r2 = 0.001). These findings indicate that prolonged vibration reduced the time to failure of a sustained contraction when subjects maintained limb position, suggesting that peripheral inputs to the motor neuron pool play a significant role in sustaining a contraction during tasks that require active control of limb position.     

Changes in Corticospinal Excitability in the Reactions of Forearm Muscles in Humans to Vibration

Transcranial magnetic stimulation was used to study changes in corticospinal excitability during vibration of the flexor and extensor muscles of the wrist in healthy humans. The ratios of muscle stimulation responses to activity levels in these muscles on contraction associated with vibration (the tonic vibratory reflex, TVR) and after vibration of antagonist muscles in isometric conditions (the antagonist vibratory reflex, AVR) were analyzed. The normalized muscle response in the wrist flexors was found to increase by 66% compared with threshold values in the direct vibratory response (TVR), by 75% in the relayed vibratory response (AVR), and by 18% on voluntary contraction. However, increases in the motor response in vibratory responses as compared with those on voluntary contraction did not reach significance, which contrasted with the responses in the forearm flexors. These results are discussed from the point of view that the motor cortex plays different roles in vibratory responses in the distal and proximal muscles.     

Functional coupling of motor units is modulated during walking in human subjects

Time- and frequency-domain analysis of the coupling between pairs of electromyograms (EMG) recorded from leg muscles was investigated during walking in healthy human subjects. For two independent surface EMG signals from the tibialis anterior (TA) muscle, coupling estimated from coherence measurements was observed at frequencies 相似文献   

Corticomuscular and bilateral EMG coherence reflect distinct aspects of neural synchronization

Using electroencephalography (EEG) and electromyography (EMG), corticomuscular and bilateral motor unit synchronization have been found in different frequency bands and under different task conditions. These different types of long-range synchrony are hypothesized to originate from distinct mechanisms. We tested this by comparing time-resolved EEG–EMG and EMG–EMG coherence in a bilateral precision-grip task. Bilateral EMG activity was synchronized between 7 and 13 Hz for about 1 s when force output from both hands changed from an increasing to a stable force production. In contrast, EEG–EMG coherence was statistically significant between 15 and 30 Hz during stable force production. The disparities in their time–frequency profiles accord with the existence of distinct underlying processes for corticomuscular and bilateral motor unit synchronization. In addition, the absence of synchronization between cortical activity and common spinal input at 10 Hz renders a cortical source unlikely.     

Responses of motor cortical cells to short trains of vibration

The response discharges of precentral motor cortical cells to brief trains of vibration applied to the tendon of biceps brachii were analyzed in two alert but passive monkeys. The activity of 20 phasictonic and 6 tonic cells was analyzed. All had functional linkages with flexor muscles during a preceding flexion task and responded to passive movement of the elbow. Taking as a reference the stereotyped reflex response in the stretched muscle, the effect of changes in the amplitude of a constant frequency vibration (4 vibrations at 58 Hz) was quantified statistically in peristimulus histograms of the cortical cell discharges. All cells were transiently influenced by low vibration amplitudes. Most responses (71 %) were excitatory and occurred at a mean latency of 24 ms, which is consistent with cells activated by input from stretch receptors. Excitatory, reproducible responses to the lowest vibration amplitudes were more frequent in phasictonic than in pure tonic cells. Large-amplitude vibrations always excited the motor cortical cells. The sign of the responses to vibration matched that to passive elbow movements for most cells. These findings show that elbowrelated motor cortical cells are very sensitive to proprioceptive input from primary spindle afferents.     

Differential effect of muscle vibration on intracortical inhibitory circuits in humans

Low amplitude muscle vibration (0.5 ms; 80 Hz; duration 1.5 s) was applied in turn to each of three different intrinsic hand muscles (first dorsal interosseus, FDI; abductor pollicis brevis, APB; and abductor digiti minimi, ADM) in order to test its effect on the EMG responses evoked by transcranial magnetic stimulation (TMS). Recordings were also taken from flexor and extensor carpi radialis (FCR and ECR, respectively). We evaluated the amplitude of motor evoked potentials (MEPs) produced by a single TMS pulse, short interval intracortical inhibition and facilitation (SICI and ICF) and long interval intracortical inhibition (LICI). TMS pulses were applied 1 s after the start of vibration with subjects relaxed throughout. Vibration increased the amplitude of MEPs evoked in the vibrated muscle (162 ± 6 % of MEP with no vibration; mean ± s.e.m .), but suppressed MEPs in the two non-vibrated hand muscles (72 ± 9 %). Compared with no vibration (test response reduced to 51 ± 5 % of control), there was less SICI in the vibrated muscle (test response reduced to 92 ± 28 % of control) and more in the non-vibrated hand muscles (test response reduced to 27 ± 5 % of control). The opposite occurred for LICI: compared with the no vibration condition (test response reduced to 33 ± 6 % control), there was more LICI in the vibrated muscle (test response reduced to 17 ± 3 % control) than in the non-vibrated hand muscles (test response reduced to 80 ± 11 % control) even when the intensity of the test stimulus was adjusted to compensate for the changes in baseline MEP. There was no effect on ICF. Cutaneous stimulation of the index finger (80 Hz, 1.5 s duration, twice sensory threshold) had no consistent differential effect on any of the parameters. We conclude that vibratory input from muscle can differentially modulate excitability in motor cortical circuits.     

Combined effects of preceding muscle vibration and contraction on the tonic vibration reflex

As a result of intrafusal thixotropy, muscle contraction at a short length followed by passive lengthening enhances the subsequent tonic vibration reflex (TVR). We studied the effects of muscle vibration, contraction, and their combination on the subsequent TVR in the left biceps in 20 healthy men. The preceding vibration (20 or 80 Hz) conditioning at a short or long length was applied to the muscle belly with and without a contraction. After conditioning, distal tendon vibration (80 Hz) was used to elicit the TVR at the test length. The strength of the TVR was measured by surface electromyography. Conditioning with 80-Hz vibration at a short length followed by passive lengthening enhanced the subsequent TVR, which was greater in the presence than in the absence of a conditioning contraction. These results suggest that vibration and contraction work synergistically to develop intrafusal thixotropy.     

The inhibitory effect of acupuncture on the tonic vibration reflex (TVR) in man

The effect of acupuncture on the tonic vibration reflex (TVR) has been examined in healthy men. Vibrations (100 Hz) were applied over the muscle bellies of either finger flexion muscles or extension muscles, while finger flexions and extensions were measured by a strain attached to the middle finger. A stainless steel acupuncture needle was inserted for 10 min into an acupuncture point named "Chu-Chih (LI-II)". After the application of acupuncture, TVRs in both flexion and extension muscles were significantly less than those observed before the application of acupuncture. The inhibitory effect of acupuncture almost disappeared 10 min after removing the needle. Acupuncture transiently inhibits TVR in extension and flexion muscles in man.     

Frequency-dependent effects of muscle tendon vibration on corticospinal excitability: a TMS study

The aim of the present study was to investigate the effects of muscle tendon vibration at different frequencies on corticospinal excitability by means of transcranial magnetic stimulation (TMS). A second objective was to describe whether the observed modulations in motor evoked potentials (MEPs), as a function of vibration frequency, reflect the behavior of Ia afferents during and after vibration. In ten subjects, muscle tendon vibration (duration 30 s) was applied to the flexor carpi radialis (FCR) muscle at three different frequencies (20, 75 and 120 Hz). MEPs following single-pulse TMS were recorded from the targeted muscle during a previbration, vibration, and postvibration period. Muscle tendon vibration at 75 Hz increased the MEP amplitude significantly during vibration, whereas a smaller but still significant effect was observed during 120 Hz vibration. No significant MEP changes could be observed during 20 Hz vibration and during the postvibration period for each frequency. Our findings indicate that muscle tendon vibration exerts a frequency-dependent effect on corticospinal excitability. Furthermore, evidence is provided for the notion that the excitatory effect of muscle tendon vibration on the primary motor cortex is mediated by Ia afferent input.     

Reinforcement of tonic vibration reflex and exteroceptive vibration-induced flexion reflex in finger flexion muscles by forced respiration

The effects of forced respiration on tonic vibration reflex (TVR) and extteroceptive vibration-induced flexion reflex (VFR) were studied in healthy man. Vibration (100 Hz) applied to finger flexion muscles or the volar side of the middle finger induced a slowly augmenting facilitation of the TVR, which has mono- and polysynaptic components, and VFR, which is polysynaptic. Forced inspiration and forced expiration facilitated these slowly augmenting processes in both reflexes. Respiration reinforces both proprioceptive and exteroceptive reflexes elicited by vibration.     

Frequency peaks of tremor, muscle vibration and electromyographic activity at 10 Hz, 20 Hz and 40 Hz during human finger muscle contraction may reflect rhythmicities of central neural firing

 The output from the central nervous system to muscles may be rhythmic in nature. Previous recordings investigating peripheral manifestations of such rhythmic activity are conflicting. This study attempts to resolve these conflicts by employing a novel arrangement to measure and correlate rhythms in tremor, electromyographic (EMG) activity and muscle vibration sounds during steady index finger abduction. An elastic attachment of the index finger to a strain gauge allowed a strong but relatively unfixed abducting contraction of the first dorsal interosseous (1DI). An accelerometer attached to the end of the finger recorded tremor, surface electrodes over 1DI recorded EMG signals and a heart-sounds monitor placed over 1DI recorded vibration. This arrangement enabled maintenance of a constant overall muscle contraction strength while still allowing measurement of the occurrence of tremulous movements of the finger. Ten normal subjects were studied with the index finger first extended at rest and then contracting 1DI to abduct the index finger against three different steady forces up to 50% of maximal voluntary contraction (MVC). Power spectral analysis of tremor, EMG activity and muscle vibration signals each revealed three frequency peaks occurring together at around 10 Hz, 20 Hz and 40 Hz. Coherence analysis showed that the same three peaks were present in the three signals. Phase analysis indicated a fixed time lag of tremor behind EMG of around 6.5 ms. This is compared with previous measurements of electromechanical delay. Other experiments indicated that the three peaks were of central nervous origin. Introducing mechanical perturbations or extra loading to the finger and making recordings under partial anaesthesia of the hand and forearm demonstrated preservation of all the peaks, suggesting that they did not originate from mechanical resonances or peripheral feedback loop resonances. It is concluded that, at least for a small hand muscle, there exist not one but a number of separate peak frequencies of oscillation during active contraction, and that these oscillations reflect synchronization of motor units at frequencies determined within the central nervous system. It is proposed that the multiple oscillations may be a means of frequency coding of motor commands. Received: 23 April 1996 / Accepted: 8 October 1996     

Vibration-evoked reciprocal inhibition between human wrist muscles

Summary Reciprocal inhibition of the voluntarily contracting wrist extensor (extensor carpi radialis, ECR) evoked by proprioceptive afferent input from the flexor (flexor carpi radialis, FCR), was studied in healthy human subjects. Vibration of the FCR tendon was used to elicit Ia-dominated afferent discharge whilst inhibition of ECR was assessed as the reduction in asynchronous, on-going EMG. A small early phase of inhibition (I1) was evident in 25% of trials. The latency (ca. 25 ms) of this component suggested that it was mediated by an Ia oligosynaptic, possibly classical disynaptic, inhibitory pathway. A later and apparently separate phase of reduced activity (12, ca. 40 ms) was, however, far more consistently observed (96% of trials) and of greater magnitude. The 12 component was usually followed, some 20 ms later, by a phase of elevated activity (El, 72% trials). Reductions in simultaneously recorded net extensor torque commenced at about 60 ms following the onset of flexor tendon vibration, i.e. some 20 ms after the main I2 EMG component. These mechanical responses must have almost exclusively resulted from reciprocal inhibition of extensor EMG since vibration of the relaxed FCR evoked minimal excitatory flexor activity. The reflex pattern, in any individual subject, was relatively unaffected by altering the duration of the vibration train between one and nineteen cycles (125 Hz). This suggests that the entire response complex resulted largely from the initial afferent volley. The sizes of both the I1 and I2 reductions in ECR activity increased with increasing voluntary extensor contraction so that their depths remained constant proportions of background EMG. Very similar results were obtained when reciprocal inhibition of FCR was produced by vibration of the belly of ECR. Thus, reciprocal inhibition between wrist muscles is mainly expressed as a rather stereotyped, short duration reduction in EMG whose depth is determined by the pre-existing level of motor activity. Some functional implications of this form of reflex behaviour are discussed.     

Synchronisation of motor firing by vibration during stretch evoked responses of the human wrist flexors

Summary The wrist was rapidly dorsiflexed while the flexor muscles were being vibrated at 120 Hz. During both the M1 and the M2 responses the EMG showed a series of waves at the vibration frequency. When the vibration was turned off after overlapping with the stretch for only three to four cycles these EMG waves ceased about 30 ms later. Spinal reflexes are thus shown to continue to influence the discharging motoneurones throughout the human M2 long-latency response.Supported by BCHCRF and NSERC