[SHORT REPORTS]Relationship between the Voluntary Excitation Threshold and Premovement Cessation in Motor Unit Activity Prior to Rapid Contraction

概要

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When a weak steady contraction is maintained before a rapid voluntary contraction, a silent period in EMG activity is observed just before the phasic EMG discharge. However, EMG activity is not always completely absent during this pause. Susceptibility of motor units to premovement cessation in activity was examined in relation to each unit's voluntary excitation threshold in isometric plantar flexion. When a motor unit showed no firing just before the phasic EMG discharge for a significantly longer interval than tha the mean discharge interval recorded during sustained contractions, the motor units was regarded as a ceased firing, and was labeled a silenced unit. The excitation threshold in these silenced units was compared with that of other motor unit (the nonsilenced nuits) which fired during the same inhibitory period as the silenced units. The majority of the silenced units had a higher excitation threshold than the non-silenced higher excitation thresholds than the non-silenced units, suggesting that susceptibilityto premovement silence is higher in motor units with higher excitation thresholds. Possible neural mechanisms reponsible for the premovement silence are discussed.
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