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Fatigue-induced changes in short-interval intracortical inhibition and the silent period using stimulus intensities evoking maximal versus submaximal responses

Authors: Callum G Brownstein, Loïc Espeit, Nicolas Royer, Thomas Lapole, Guillaume Y Millet

Published: 25 June 2020 - J Appl Physiol .

ABSTRACT

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) which elicit maximal and submaximal SICI and SP duration (MaxSICI vs SubmaxSICI - Experiment A, MaxSP vs SubmaxSP - 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 measures taken using maximal and submaximal intensities at baseline and every 2.5 min throughout the task. Immediately following the 10 min contraction (i.e. without recovery), responses were also measured at the same absolute force level as with baseline. In Experiment A, no change in SICI was observed with either CS intensity throughout the EMG task (P>0.05). However, a 18% decrease in SICI (i.e. less inhibition) was observed at the same absolute force only when using 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 using CS intensities eliciting maximum SICI are more sensitive in detecting fatigue-induced reductions in SICI, while increases in SP are detectable using TMS intensities evoking maximal or submaximal SPs.

Keywords: Fatigue; Inhibition; Short-interval intracortical inhibition; Silent period; Transcranial magnetic stimulation.

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