The mixed nerve silent period is prolonged during a submaximal contraction sustained to failure

The purpose of this study was to determine the effect of a sustained contraction of vastus lateralis on the silent period (SP) in the surface electromyogram (EMG) following direct neural stimulation. Five men and 5 women performed isometric knee extension at 30% maximal voluntary contraction (MVC) to the limit of endurance. During the contraction, EMG increased, and superimposed twitch amplitude and time to peak tension decreased, but the SP duration did not change. After 10 min of recovery, MVC had returned to its initial value, and the potentiated twitch amplitude was 70% of initial value, but the SP was now 11% shorter. Based on these results, we hypothesize that during a sustained contraction of 30% MVC, the increase in central drive may have been offset by inhibitory input from the periphery, but after 10 min of recovery the SP was shortened because of increased central drive. This aspect of the SP's behavior should be taken into account whenever it is employed as a diagnostic tool.