The effects of transcranial direct current stimulation on objective and subjective indexes of exercise performance: A systematic review and meta-analysis

Objective: To examine the effects of transcranial direct current stimulation (tDCS) on objective and subjective indexes of exercise performance. Design: Systematic review and meta-analysis. Data sources: A systematic literature search of electronic databases (PubMed, Web of Science, Scopus, Google Scholar) and reference lists of included articles up to June 2018. Eligibility criteria: Published articles in journals or in repositories with raw data available, randomized sham-controlled trial comparing anodal stimulation with a sham condition providing data on objective (e.g. time to exhaustion or time-trial performance) or subjective (e.g. rate of perceived exertion) indexes of exercise performance. Results: The initial search provided 420 articles of which 31 were assessed for eligibility. Finally, the analysis of effect sizes comprised 24 studies with 386 participants. The analysis indicated that anodal tDCS had a small but positive effect on performance g = 0.34, 95% CI [0.12, 0.52], z = 3.24, p = .0012. Effects were not significantly moderated by type of outcome, electrode placement, muscles involved, number of sessions, or intensity and duration of the stimulation. Importantly, the funnel plot showed that, overall, effect sizes tended to be larger in studies with lower sample size and high standard error. Summary: The results suggest that tDCS may have a positive impact on exercise performance. However, the effect is probably small and most likely biased by low quality studies and the selective publication of significant results. Therefore, the current evidence does not provide strong support to the conclusion that tDCS is an effective means to improve exercise performance. (C) 2018 Elsevier Inc. All rights reserved.