Inhibition of EMG activity in isometrically loaded agonist muscle preceding a rapid contraction.

It was recently suggested that a premotion silent period in isometrically loaded agonist muscle reflects transition process in switching motor program from an isometric to an isotonic condition. To test this hypothesis we investigated changes in EMG activity of a biceps muscle during motor preparatory phase in an entirely isometric paradigm. Five healthy volunteers produced rapid isometric elbow flexions superimposed on a slightly sustained contraction during self-paced and reaction time conditions. In addition to surface EMG, single motor units were recorded in agonist muscle prior to a rapid contraction. Silent period was found in responses of three out of five subjects, whereas inhibition of some but not all tonically active motor units was observed in all studied subjects prior to the phasic EMG burst. Inhibition of motor unit activity occurred more often in self-paced than reaction time contractions. Our results indicate that inhibition is not always powerful enough to produce a complete electrical silence, and weaker inhibitory effects were observed as a declining number of firing motor units. We conclude that the silent period or depression of the EMG activity is associated with changes in motor program from tonic to phasic muscle activation and is not merely a consequence of changes from an isometric to an isotonic condition.