The Influence of Inter-Individual Variability on the Acute Effects of Anodal Transcranial Direct Current Stimulation on Training Volume During Velocity-Based Back Squat Exercise

This study investigated the acute effects of anodal transcranial direct current stimulation (a-tDCS) applied over the dorsolateral prefrontal cortex (DLPFC) and primary motor cortex (M1) on neuromuscular performance during a velocity-based back squat exercise. Fifteen recreationally trained men participated in a randomized, double-blind, crossover design, completing three experimental conditions (SHAM, DLPFC, and M1 stimulation) consisting of 20 min of 2 mA a-tDCS followed by a squat protocol performed to a 15% velocity loss threshold. Total repetitions, repetitions per set, mean concentric velocity, and rating of perceived exertion (RPE) were recorded. No significant differences between stimulation conditions were observed for any outcome variable. However, two individuals showed reversed responses, consistent with previously reported inter-individual variability in response to tDCS. Given the high inter-individual variability in response to a-tDCS, we additionally performed a post hoc sensitivity analysis based on response direction relative to SHAM. This analysis indicated that a-tDCS over M1 and DLPFC resulted in a significantly greater total number of repetitions compared with SHAM, whereas repetitions per set, mean velocity, and RPE were not different between conditions. Accordingly, a systematic and individualized approach may be needed to address inter-individual variability in response to tDCS to optimize its effect on fatigue tolerance.