Tonic vibration reflexes elicited during fatigue from maximal voluntary contractions in man

1. In the present study on human foot dorsiflexor muscles we have examined the effects of high-frequency (150 Hz) muscle vibration on weak or moderate voluntary contractions (maintained by constant effort) and on maximal voluntary contractions (MCVs) of (i) non-fatigued muscles, (ii) muscles fatigued by sustained MVCs and (iii) muscles deprived of gamma-fibre innervation by partial anaesthetic nerve block. The motor outcome of the voluntary dorsiflexion efforts was assessed by measuring the firing rates of single motor units in the anterior tibial (TA) muscle, the mean voltage EMG activity from the pretibial muscles and foot dorsiflexion force. 2. With the subject instructed to exert constant effort in maintaining a weak or moderate contraction, superimposed vibration caused an enhancement of EMG activity and contraction force. 3. Previous claims that muscle vibration has no facilitatory effect on motor output in MVCs were found to hold true for non-fatigued but not for fatigued muscles. Thus, the fatigue-induced decline in EMG activity and motor unit firing rates was counteracted by short periods (less than 10-20 s) of superimposed vibration. However, with longer vibration periods it seemed as if the initial facilitation converted into an opposite effect which accentuated the fatigue-induced decline in motor output and contraction force. 4. Like muscle fatigue, a partial anesthetic block of the deep peroneal nerve, supposedly interrupting transmission in gamma-motor fibres, caused a reduction of MVC motor unit firing rates which could be counteracted by muscle vibration. In prolonged MVCs performed during the block, motor unit firing rates did not show the normal progressive decline from an initially high level, but stayed at a relatively constant low level throughout the contraction period. 5. Even though alternative interpretations are possible, the results agree with the hypotheses (i) that in sustained MVCs, fatigue processes occur not only in extrafusal but also in intrafusal muscle fibres, (ii) that the intrafusal fatigue leads to a reduction of the voluntary drive conveyed to the alpha-motoneurones via the gamma-loop and (iii) that vibration-induced activity in group Ia afferents can act as a substitute for the diminished fusimotor drive

Prolonged muscle vibration reducing motor output in maximal voluntary contractions in man

1. We observed in a previous study on the human foot dorsiflexor muscles that the fatigue-induced decline in motor output during sustained maximal voluntary contractions (MVCs) was temporarily counteracted during the initial phase of superimposed high-frequency (150 Hz) muscle vibration, whereas prolonged muscle vibration seemed to accentuate the fatigue-induced decline in gross EMG activity and motor unit firing rates. A more extensive investigation of this late effect of muscle vibration on MVCs was performed in the present study. 2. Prolonged periods of superimposed muscle vibration caused a reduction of EMG activity, motor unit firing rates and contraction force in both intermittent and sustained MVCs. This vibration-induced effect had the following main characteristics: (i) it developed slowly during the course of about 1 min of sustained vibration and subsided within 10-20 s after the end of vibration; (ii) it was much more pronounced in some subjects than in others (not age-dependent) and it was accentuated by preceding muscle exercise; (iii) it affected primarily the subject's ability to generate and/or maintain high firing rates in high-threshold motor units. 3. Since the effect developed while vibration at the same time exerted a tonic excitatory influence on the alpha-motoneurone pool (as evidenced by the presence of a tonic vibration reflex) it is argued that the vibration-induced suppression of motor output in MVCs probably does not depend on alpha-motoneurone inhibition, but on a reduced accessibility of these neurones to the voluntary commands. It is suggested that contributing mechanisms might be vibration-induced presynaptic inhibition and/or 'transmitter depletion' in the group Ia excitatory pathways which constitute the afferent link of the gamma-loop

After-effects on stiffness and strech reflexes of human finger flexor muscles attributed to muscle thixotrophy

1. While the subject maintained a weak contraction in his finger flexor muscles, holding the metacarpophalangeal joints in 45 deg flexion, test torque pulses were applied which caused rapid finger extension movements and electromyographic (EMG) stretch reflex responses. Before each test pulse the fingers were passively flexed or extended (\u2018post\u2010short\u2019 and \u2018post\u2010long\u2019 trials) for about 10 s. The EMG and joint deflection responses in the two types of trial were compared after averaging. 2. In the \u2018post\u2010long\u2019 trials, the EMG reflex response showed a comparative increase in latency, with a reduction of the short\u2010latency (M1) component and an enhancement of the medium\u2010latency (M2) component. 3. The angular deflections were larger, and the turning points of the deflections, which indicated the start of the mechanical reflex responses, occurred later in the \u2018post\u2010long\u2019 trials. These differences were not seen when the torque pulse was immediately preceded by a strong, brief isometric finger flexor contraction in the test position. 4. Immediately following the return to the test position the background finger flexor EMG activity was larger in the \u2018post\u2010long\u2019 trials, a difference which gradually subsided over 15\u201020 s. A strong, brief contraction in the test position also eliminated this inter\u2010trial difference. 5. The results are interpreted as manifestations of thixotropic after\u2010effects in intra\u2010 and extrafusal muscle fibres. It is proposed that the M1 component of the stretch reflex is largely a response to the \u2018initial burst\u2019 of impulses in primary spindle afferents.(ABSTRACT TRUNCATED AT 250 WORDS) \ua9 1995 The Physiological Societ