Caffeine combined with excitatory neuromodulation based on transcranial Direct Current Stimulation (tDCS) enhances performance in a time-trial CrossFit® workout: a randomized, placebo-controlled, double-blind study

Abstract

Background: Caffeine (CAF) and transcranial direct current stimulation (tDCS) are ergogenic strategies with potential benefits for performance, yet their combined effects remain underexplored, particularly in high-intensity functional training contexts such as CrossFit®. Objective: This randomized, double-blind, placebo-controlled crossover study aimed to investigate the impact of tDCS, with and without CAF, on performance time in the Clean & Jerk (C&J) during the benchmark WOD GRACE among competitive CrossFit® athletes. Secondarily, we aimed to compare the RPE across the different experimental conditions, as well as to establish the relationship between personal record (PR) values adjusted for body mass and the execution time of the WOD GRACE, considering different athletes’ classification levels (RX Elite and RX Intermediate). Methods: Twenty participants completed four experimental conditions: CAF ingestion (400 mg) combined with anodal tDCS (CAF + a-tDCS), CAF with Sham tDCS (CAF + Sham-tDCS), placebo (PLA) with a-tDCS (PLA + a-tDCS), and PLA with Sham tDCS (PLA + Sham-tDCS). Results: The results indicated that the combination of CAF + a-tDCS significantly improved performance, reducing execution time (205.5 ± 58.0 s) compared to CAF + Sham-tDCS (218.3 ± 61.2 s; p = 0.034), PLA + a-tDCS (231.7 ± 64.1 s; p = 0.012), and PLA + Sham-tDCS (240.9 ± 66.4 s; p = 0.002). However, no significant differences were observed between CAF + Sham-tDCS and PLA + a-tDCS (p = 0.690), CAF + Sham-tDCS and PLA + Sham-tDCS (p = 0.352), or PLA + a-tDCS and PLA + Sham-tDCS (p = 0.595). The responder analysis revealed that 45% of participants improved performance with isolated tDCS, while 60% responded positively to CAF. No significant differences were found in RPE scores among conditions (p = 0.145). Additionally, no correlations were identified between PR values adjusted for body mass and execution time in both RX Elite (r = 0.265; p = 0.526) and RX Intermediate (r = 0.049; p = 0.901) groups, nor between training experience and performance across interventions. Conclusions: These findings suggest that tDCS, when combined with CAF, may serve as an effective ergogenic aid for improving performance in high-intensity functional training, whereas its isolated use does not yield meaningful benefits.