CHANGES IN CORTICOMOTOR EXCITATION AND INHIBITION DURING PROLONGED SUBMAXIMAL MUSCLE CONTRACTIONS

Changes in motor evoked potential (MEP) amplitude, posts-MEP silent pe riod duration, and interpolated twitch torque were measured using tran scranial magnetic (TMS) and electrical (TES) stimulation during a 20% maximum voluntary contraction of the elbow flexors sustained to exhaus tion. TMS-and TES-induced MEP amplitude increased progressively over t he contraction period up until the point of exhaustion. The TMS-induce d silent period was prolonged only during the second half of the contr action period, the time course being different from that of the MEP re sponses, whereas the TES-induced silent period did not change. The fin dings indicate that corticomotor excitability increases during a susta ined submaximal voluntary contraction and that, as fatigue develops, t here is a progressive buildup of intracortical inhibition. This may re present a mechanism whereby corticomotor output is maintained at an ap propriate level to preserve optimal motor unit firing frequencies duri ng a fatiguing contraction. (C) 1997 John Wiley & Sons, Inc.