Fatigue-induced changes in short-interval intracortical inhibition and the silent period with stimulus intensities...

During fatiguing exercise, previous studies have employed transcranial magnetic stimulation (TMS) paradigms eliciting either maximal or submaximal short-interval intracortical inhibition (SICI) and silent period (SP) durations. However, the effect of using either approach on the change in these variables with fatigue is unknown. This study examined the effects of using conditioning stimulus (CS, experiment A) and single-pulse TMS intensities (experiment B) that elicit maximal and submaximal SICI and SP duration (MaxSICI vs. SubmaxSICI in experiment A, MaxSP vs. SubmaxSP in experiment B) on the change in these measures with fatigue. In both experiments, participants performed a 10-min sustained isometric knee-extension contraction at a constant level of EMG, with measurements taken with maximal and submaximal intensities at baseline and every 2.5 min throughout the task. Immediately after the 10-min contraction (i.e., without recovery), responses were also measured at the same absolute force level as at baseline. In experiment A, no change in SICI was observed with either CS intensity throughout the EMG task (P > 0.05). However, an 18% decrease in SICI (i.e., less inhibition) was observed at the same absolute force only with the MaxSICI CS intensity (P < 0.01), with no change in SubmaxSICI (P = 0.72). In experiment B, the magnitude of increase in SP with fatigue was similar for both stimulus intensities (stimulus × time interaction: P = 0.44). These results suggest that CS intensities eliciting maximum SICI are more sensitive in detecting fatigue-induced reductions in SICI, whereas increases in SP are detectable with TMS intensities evoking maximal or submaximal SPs.NEW & NOTEWORTHY This study compared the change in silent period (SP) and short-interval intracortical inhibition (SICI) with conditioning stimulus and single-pulse transcranial magnetic stimulation (TMS) intensities (for SICI and SP, respectively) eliciting maximal and submaximal SICI and SP during fatiguing exercise. The results showed that changes in SICI were only detectable with intensities evoking maximal responses, with no difference between intensities for SP. These findings highlight the importance of maximizing SICI with appropriate intensities before measuring SICI during fatiguing exercise.