Modulation of long-interval intracortical inhibition and the silent period by voluntary contraction.

Transcranial magnetic stimulation was used to examine the effect of voluntary contraction on the magnitude of long-interval intracortical inhibition (LICI) and the duration of the silent period in intrinsic hand muscles. The magnitude of LICI acting on the first dorsal interosseus (FDI) measured with a paired-pulse protocol with an inter-pulse interval of 100 ms decreased with increasing tonic level of voluntary abduction force generated by the index finger. LICI in abductor pollicis brevis (APB) decreased from the condition in which the index finger was at rest to the conditions in which it was abducted, whereas LICI in abductor digiti minimi (ADM) was unaffected by the level of index finger abduction. During voluntary abduction of the index finger, the magnitude of LICI was least in FDI, intermediate in ADM, and greatest in APB, suggesting that it may be a mechanism by which tonic activation of hand muscles is fractionated. The magnitude of LICI increased with conditioning stimulus intensity, but intensity did not interact with abduction force. The duration of the silent period (SP) in FDI decreased with the level of voluntary index finger abduction and increased with eliciting stimulus intensity. Within-subject correlations showed that the effects of voluntary drive on SP duration and motor-evoked potential amplitude did not covary, implying an indirect effect of voluntary drive on SP duration. It is proposed that whereas voluntary drive directly reduces the magnitude of slow-acting inhibition acting on the active movement representations and near neighbors, sensory feedback from the contracting muscle acts to limit its time course.