This study investigated the effects of short-term glucocorticoid administration on voluntary activation and intracortical inhibitory and facilitatory circuits.
Seventeen healthy men participated in a pseudorandomized double-blind study to receive either dexamethasone (8 mg·d−1, n = 9 subjects) or placebo (n = 8 subjects) for 7 d. The ankle dorsiflexion torque, corresponding 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 0.8× of motor threshold and delivered 2 ms (SICI) and 13 ms (ICF) before the test stimulus (1.2× motor threshold).
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 (r 2 = 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), whereas no significant change was observed for ICF. Neither SICI nor ICF changed after treatment when assessed at rest.
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.
1Laboratory of Applied Biology and Neurophysiology, ULB Neuroscience Institute, Université Libre de Bruxelles, Brussels, BELGIUM; 2Division of Endocrinology, Diabetology and Metabolism, Department of Medical Sciences, University of Turin, Turin, ITALY; 3Laboratory for Engineering of the Neuromuscular System, Department of Electronics and Telecommunications, Politecnico di Torino, Torino, ITALY; and 4Division of Physical Medicine and Rehabilitation, Department of Surgical Sciences, University of Turin, Turin, ITALY
Address for correspondence: Stéphane Baudry, Ph.D., Laboratory of Applied Biology, Faculty for Motor Sciences, Université Libre de Bruxelles, 808, Route de Lennik, CP 640, 1070 Brussels, Belgium; E-mail: firstname.lastname@example.org.
Submitted for publication April 2017.
Accepted for publication September 2017.