Purpose: In the current study, we measured EMG suppression induced by subthreshold transcranial magnetic stimulation (TMS) to investigate the effects of sustained cycling exercise on intracortical inhibition.
Methods: Sixteen subjects cycled at 75% of their maximum workload (Wmax) for 30 min, during which subthreshold TMS was applied at a defined crank angle where vastus lateralis (VL) EMG amplitude was increasing and approximately 50% of its recorded maximum. Subthreshold TMS was also applied during nonfatiguing control cycling bouts at 75% and 37.5% of Wmaxbefore sustained cycling.
Results: Although EMG in VL during control cycling at 37.5% Wmax was approximately half that during cycling at 75% Wmax (P ≤ 0.05), the amount of EMG suppression was not different between workloads (P > 0.05). EMG amplitude in VL recorded in the last 5 min of sustained cycling was not different from the first 5 min (P > 0.05), whereas the amount of EMG suppression at the end of the sustained cycling was significantly greater than that at the start (P ≤ 0.05).
Conclusions: The increase in TMS-evoked EMG suppression during sustained cycling implies an increase in the excitability of the intracortical inhibitory interneurons during the exercise. The observed increase in intracortical inhibition is similar to that observed during sustained single joint contractions, suggesting that changes in the responsiveness of intracortical inhibitory interneurons are similar during locomotor exercise and static single joint contractions.