Background: The goal of this study was to determine if 10-min of anodal transcranial direct current stimulation (a-tDCS) to the motor cortex (M1) is capable of modulating quadriceps isometric maximal voluntary contraction (MVC) force or fatigue endurance contralateral or ipsilateral to the stimulation site.

Results: The main finding of this study was a significant interaction effect for stimulation condition x leg tested x time [F(1,60) =7.156, p = 0.010, ηp2 = 0.11] which revealed  significant absolute KE MVC force impairments in the contralateral leg following s-tDCS (p < 0.001, d =1.2) and in the ipsilateral leg following a-tDCS (p < 0.001, d = 1.09). A significant interaction effect for condition x leg tested [F(1,56) = 8.12, p = 0.006, ηp2 = 0.13], showed a significantly lower left quadriceps (ipsilateral to tDCS) relative MVC force with a-tDCS, versus s-tDCS [t(15) = -3.07, p = 0.016, d = -0.77]. There was no significant difference between the relative right quadriceps (contralateral to tDCS) MVC force for a-tDCS and s-tDCS. Although, there was an overall significant [F(1,56) = 8.36, p < 0.001] 12.1% force decrease between the first and twelfth MVC repetitions, there were no significant main or interaction effects for fatigue index force.

Conclusion: Hence, a-tDCS may be ineffective at increasing maximal force or endurance and instead may be detrimental to quadriceps force production.

This study utilized a fully randomized, crossover, repeated measures design, with all participants completing four protocols. The four protocols involved the participant receiving: 1) a-tDCS targeting the left M1, with testing of the contralateral (right) leg, 2) a-tDCS targeting the left M1 with testing of the ipsilateral (left) leg, 3) sham tDCS (s-tDCS) targeting the left M1, with testing of the contralateral (right) leg, and 4) s-tDCS targeting the left M1 with testing of the ipsilateral (left) leg. At least one week of recovery was allocated between each session (protocol) to ensure that no effects from prior stimulations carried over to the next session (Fig 1) [61].

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