Acute effects of anodal transcranial direct current stimulation on maximal voluntary force, repeated maximal isometric contractions and corticospinal excitability in healthy young adults

The aim of this triple-blind, placebo-controlled study was to determine the effects of anodal transcranial direct current stimulation (a-tDCS) applied over the primary motor cortex (M1) and the dorsolateral prefrontal cortex (DLPFC) on maximal voluntary force, the ability to repeat maximal handgrip contractions, cortical and corticospinal excitability measured with transcranial magnetic stimulation (TMS), and rating of perceived effort. Forty-two healthy participants (19 women; 1 left-handed; 21.5 ± 1.4 years) completed three identical sessions differing only in the a-tDCS condition: 20 min of 2 mA a-tDCS over the M1, the DLPFC, and a sham session (30 s of real stimulation) in a randomized order. Maximal voluntary force, corticospinal excitability and intracortical inhibition and facilitation were assessed before (pre) and after (post) a-tDCS. Additionally, the number of maximal contractions performed until a 30% force loss in handgrip strength, and the overall perceived effort of the task were recorded following a-tDCS application. The results showed no significant main effects or interactions in any behavioural, neurophysiological, or perceptual variables, beyond a small but consistent reduction from pre to post measurements in maximal voluntary force (-3.2%) across all sessions. These findings indicate that a-tDCS over M1 or DLPFC does not improve maximal voluntary force, endurance during repeated maximal contractions, perceived effort or associated cortical and corticospinal excitability in healthy young adults (all results p > 0.05). Accordingly, in healthy young adults, the present results do not support recommending a-tDCS as a performance-enhancing intervention for maximal or repeated maximal muscle efforts.