Purpose: This study investigated the effects of short-term glucocorticoid administration on voluntary activation and intracortical inhibitory and facilitatory circuits.
Methods: Seventeen healthy men participated in a pseudo randomized double-blind study to receive either dexamethasone (8 mg[middle dot]d-1, n = 9 subjects) or placebo (n = 8 subjects) for 7 days. The ankle dorsiflexion torque, corresponding electromyography (EMG) of the tibialis anterior, and voluntary activation assessed by the interpolated twitch method using transcranial magnetic stimulation (TMS) were measured during a maximal voluntary contraction (MVC). Short-latency intracortical inhibition (SICI) and intracortical facilitation (ICF) were assessed at rest and during submaximal contraction (50% MVC torque) by paired-pulse TMS with the conditioning stimulus set at 0.8x of motor threshold (0.8x MT) and delivered 2 ms (SICI) and 13 ms (ICF) prior to the test stimulus (1.2x MT).
Results: The MVC torque (+14%), tibialis anterior EMG (+31%) and voluntary activation (+3%) increased after glucocorticoid treatment (p<0.05). The increase in voluntary activation was associated with the gain in MVC torque (r2 = 0.56; p = 0.032). The level of SICI and the duration of the EMG silent period that followed the test TMS decreased (-18.6% and -13.5%, respectively) during the 50% MVC after treatment (p<0.05) while no significant change was observed for ICF. Neither SICI nor ICF changed after treatment when assessed at rest.
Conclusion: Short-term dexamethasone treatment induced specific decrease in the excitability of intracortical inhibitory circuits that likely contributed to the increase in the voluntary activation and associated MVC torque.
(C) 2017 American College of Sports Medicine