Effect of anodal transcranial direct current stimulation on voluntary activation levels and maximal voluntary contractions in the quadriceps muscle

INTRODUCTION: Anodal transcranial direct current stimulation (a-tDCS) has previously been investigated as a potential ergogenic aid in sports, capable of increasing cortical excitability as well as maximal voluntary contraction (MVC) (1,2). Changes in voluntary activation levels (VAL) have been suggested as a possible explanation for these increases (1). The aim of this study was therefore to investigate the effects of a-tDCS on cortical excitability, MVC, and VAL. METHODS: Nine untrained male participants were recruited for this randomized sham-controlled experiment. The participants reported to the lab on two separate days, with at least seven days in between, where they received either a-tDCS or sham-stimulation. At each test day: baseline cortical excitability, MVC, and VAL were assessed prior to receiving either a-tDCS or sham-stimulation. Ten minutes poststimulation cortical excitability, MVC and VAL were assessed again to evaluate the effects of the two types of stimulation. Transcranial magnetic stimulation was used to assess the cortical excitability of the cortical representation at M1 of the right rectus femoris, by averaging 20 motor evoked potentials elicited with 120% of the resting motor threshold stimulation intensity. Femoral nerve stimulation was applied to determine the VAL. Anodal-tDCS and sham-stimulation was applied through saline-soaked sponge electrodes (5x7 cm) with the anode placed over the motor representation of the right rectus femoris and the cathode centered over the contralateral deltoid muscle. During the a-tDCS condition, the electrical current amplitude was ramped up over 30 seconds to 2 mA and kept constant for 9 minutes and ramped down again over 30 seconds, this procedure was repeated three times separated by two 5-minute breaks. During the sham-stimulation the current was ramped up to 2 mA over 30 seconds, and then ramped down over 30 seconds to zero again followed by 9 min without stimulation. This procedure was repeated three times with two 5- minute breaks. RESULTS: The results of the present study showed that the cortical excitability increased significantly following a-tDCS (54.8 ± 61.3 (%), p = 0.03), while no significant change occurred following sham stimulation (-4.0 ± 21.9%, p > 0.05). The increase in cortical excitability following a-tDCS did not lead to any significant changes in MVC (pre: 1000.7 ± 253.4 to post: 986.8 ± 271.9 (N) p = 0.72) or VAL (pre: 84.5 ± 8.7 to post: 84.8 ± 9.5 (%), p = 0.33). No significant changes occurred following sham stimulation in MVC (pre: 1043.7 ± 251.2 to post: 1016.4 ± 243.2 (N) p > 0.05) or VAL (pre: 87.0 ± 5.0 to post: 84.2 ± 9.6 (%), p > 0.05). CONCLUSION: In conclusion, a total of 27 minutes of anodal transcranial direct current stimulation at 2mA applied to the cortical representation of M1 resulted in increased cortical excitability, but no significant changes in either MVC or VAL. (1) Krishnan et al., Brain stimul, 2014. (2) Vargas et al., J Strength Cond Res, 2017