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

Abstract

SUMMARY1. 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 a-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 a-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 I a excitatory pathways which constitute the afferent link of the y-loop.

Electromyographic assessment of muscle fatigue during isometric vibration training at varying frequencies

“…These results seem to be partly due to a neuromuscular phenomenon named tonic vibration reflex (TVR) [9], [10]. This mechanism is ascribed to muscle tuning to vibration damping with preferential recruitment of faster motor units [9], [11]. TVR seems also to be modulated by alterations in spindle sensitivity [9], [12].…”

“…TVR seems also to be modulated by alterations in spindle sensitivity [9], [12]. The increase in neuromuscular response observed with vibration may therefore derive not only from motor unit recruitment strategies but also from alterations in spindle sensitivity through the gamma feedback [9], [12].…”

“…The shape estimate S(k) in (3) is adopted as estimate of the reference signal S(k) in (9). The Nelder-Mead Simplex search method is used to minimize (5) [15].…”

“…Vibration training has been shown to enhance muscle strength and power, mainly due to the neuromuscular demands of such vibrating loads on skeletal muscles [8], [9], [6], [7]. These results seem to be partly due to a neuromuscular phenomenon named tonic vibration reflex (TVR) [9], [10]. This mechanism is ascribed to muscle tuning to vibration damping with preferential recruitment of faster motor units [9], [11].…”

“…Whole body vibration platforms are the most widely adopted devices for vibration training and have been suggested to be an effective modality to exercise the lower limbs [5], [6]; alternative devices have also been proposed for the upper limbs [7]. Vibration training has been shown to enhance muscle strength and power, mainly due to the neuromuscular demands of such vibrating loads on skeletal muscles [8], [9], [6], [7]. These results seem to be partly due to a neuromuscular phenomenon named tonic vibration reflex (TVR) [9], [10].…”