Following a suprathreshold transcranial magnetic stimulation (TMS) to the primary motor cortex (M1) during voluntary muscle contraction, a motor evoked potential (MEP) occurs in the target muscle followed by a silent period (SP) in the electromyographic (EMG) activities. The present study investigated how short interval intracortical inhibition (SICI) and intracortical facilitation (ICF) change during the SP. The time course of MEP and motor threshold during the SP were examined in the right first dorsal interosseous muscle. Using a triple-pulse protocol, SICI and ICF were tested at different times during the SP. The effects of different intensities of the conditioning stimulation (CS) for SICI and ICF were also investigated during the SP and at rest. During the SP, MEP was inhibited and motor threshold was increased, whereas MEP latency and background EMG level were same as those at rest. SICI decreased during the SP over a wide range of CS intensities. ICF increased at higher CS intensity. We conclude that SICI is suppressed and ICF is facilitated during the SP and the effects are separate from the interruption of voluntary drive.
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