The Effect of Anodal Transcranial Direct Current Stimulation on Quadriceps Maximal Voluntary Contraction, Corticospinal Excitability, and Voluntary Activation Levels.

ABSTRACT: Kristiansen, M, Thomsen, MJ, Nørgaard, J, Aaes, J, Knudsen, D, and Voigt, M. The effect of anodal transcranial direct current stimulation on quadriceps maximal voluntary contraction, corticospinal excitability, and voluntary activation levels. J Strength Cond Res 36(6): 1540-1547, 2022-Anodal transcranial direct current stimulation (a-tDCS) has previously been shown to improve maximal isometric voluntary contraction (MVIC), possibly through an upregulation of corticospinal excitability. Because muscle strength is an essential part of the performance of many sports, any ergogenic effect of a-tDCS on this parameter could potentially increase performance outcomes. The purpose of this study was to investigate the effect of a-tDCS on MVIC, voluntary activation levels (VALs), and corticospinal excitability, assessed by eliciting motor-evoked potentials (MEPs), in untrained subjects. Thirteen subjects completed 2 test sessions in which they received either a-tDCS or sham stimulation for 3 consecutive intervals of 10 minutes, separated by 5-minute breaks. Before and after each stimulation session, transcranial magnetic stimulation was used to elicit MEPs, and femoral nerve stimulation was used to assess VAL by measuring twitch torque during an MVIC test and in a relaxed state. Two-way analyses of variance with statistical significance set at p ≤ 0.05 were used to test for differences. A significant main effect was identified, as the MVIC pre-test (271.2 ± 56.6 Nm) was on average 4.1% higher compared to the post-test (260.6 ± 61.4 Nm) (p = 0.05). No significant differences were found in MEP, MVIC, or VAL as a result of stimulation type or time. In healthy subjects, the potential for improvement in corticospinal excitability may be negligible, which may in turn explain the lack of improvements in MEP, MVIC, and VAL after a-tDCS. The small decrease in MVIC for both conditions and nonsignificant changes in MEP and VAL do not justify the use of a-tDCS in combination with sporting performance in which the intent is to increase maximal isometric strength performance in the quadriceps muscle of healthy subjects.

Anodal transcranial direct current stimulation increases corticospinal excitability, while performance is unchanged.

Anodal transcranial direct current stimulation (a-tDCS) has been shown to improve bicycle time to fatigue (TTF) tasks at 70-80% of VO2max and downregulate rate of perceived exertion (RPE). This study aimed to investigate the effect of a-tDCS on a RPE-clamp test, a 250-kJ time trial (TT) and motor evoked potentials (MEP). Twenty participants volunteered for three trials; control, sham stimulation and a-tDCS. Transcranial magnetic stimulation was used to determine the corticospinal excitability for 12 participants pre and post sham stimulation and a-tDCS. The a-tDCS protocol consisted of 13 minutes of stimulation (2 mA) with the anode placed above the Cz. The RPE-clamp test consisted of 5 minutes ergometer bicycling at an RPE of 13 on the Borg scale, and the TT consisted of a 250 kJ (∼10 km) long bicycle ergometer test. During each test, power output, heart rate and oxygen consumption was measured, while RPE was evaluated. MEPs increased significantly by 36% (±36%) post a-tDCS, with 8.8% (±31%) post sham stimulation (p = 0.037). No significant changes were found for any parameter at the RPE-clamp or TT. The lack of improvement may be due to RPE being more controlled by afferent feedback during TT tests than during TTF tests. Based on the results of the present study, it is concluded that a-tDCS applied over Cz, does not enhance self-paced cycling performance.