Anodal transcranial direct current stimulation prolongs the cross-education of strength and corticomotor plasticity

Purpose This study aimed to assess the efficacy of applying anodal transcranial direct-current stimulation (a-tDCS) to the ipsilateral motor cortex (iM1) during unilateral strength training to enhance the neurophysiological and functional effects of cross-education. Methods Twenty-four healthy volunteers were randomly allocated to perform either of the following: strength training during a-tDCS (ST + a-tDCS), strength training during sham tDCS (ST + sham), or a-tDCS during rest (a-tDCS) across 2 wk. Strength training of the right biceps brachii involved four sets of six repetitions at 80% of one-repetition maximum three times per week. Anodal tDCS was applied to the iM1 at 1.5 mA for 15 min during each strength training session. Outcome measures included one-repetition maximum strength of the untrained biceps brachii, corticomotoneuronal excitability, cross-activation, and short-interval intracortical inhibition (SICI) of the iM1 determined by transcranial magnetic stimulation. Results Immediately after the final training session, there was an increase in strength for both the ST + a-tDCS (12.5%, P < 0.001) and the ST + sham group (9.4%, P = 0.007), which was accompanied by significant increases in corticomotoneuronal excitability and decreases in SICI for both groups. After a 48-h retention period, strength increase was maintained in the ST + a-tDCS (13.0%, P = 0.001) group, which was significantly greater than the ST + sham group (7.6%, P = 0.039). Similarly, increases in corticomotoneuronal excitability and decreases in SICI were maintained in the ST + a-tDCS group but not in the ST + sham group. No main effects were reported for the a-tDCS group (all P > 0.05). Conclusions The addition of a-tDCS to the iM1 during unilateral strength training prolongs the benefits of cross-education, which may have significant implications to enhancement of rehabilitation outcomes after a single-limb injury or impairment.