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Randomized Controlled Trial
. 2025 Apr 3;17(7):1261.
doi: 10.3390/nu17071261.

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

Alberto Souza Sá Filho  1 , Thiago Albernaz-Silva  1 , Pedro Augusto Inacio  1 , Vicente Aprigliano  2 , Iransé Oliveira-Silva  1 , Gaspar R Chiappa  1   3 , Rodolfo P Vieira  1 , Antônio Sérgio Nakao de Aguiar  1 , Raphael Martins Cunha  1 , James Oluwagbamigbe Fajemiroye  1   4 , Marcelo Magalhães Sales  5
Affiliations
Randomized Controlled Trial

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

Alberto Souza Sá Filho et al. Nutrients. .

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®. 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).

Conclusions: 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. 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.

Keywords: CrossFit®; HIFT; HIIT; high-intensity functional training; performance; tDCS.

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Conflict of interest statement

The authors declare no conflicts of interest.

Figures

Figure 1
Figure 1
Design of the study. CAF = ingestion of 400 mg of caffeine; a-tDCS = anodal transcranial direct current stimulation; PLA = placebo substance (calcium carbonate); Sham = Sham-tDCS (placebo) condition; C&J = Clean and Jerk.
Figure 2
Figure 2
The tDCS electrodes assembly pattern. In the left image (A), the initial process for segmentation according to the 10/20 EEG reference system is illustrated; in the central image (B), the procedure to identify the reference area corresponding to the left dorsolateral prefrontal cortex (left DLPFC) is presented; in the right image (C), the placement of both electrodes over the targeted stimulation sites is shown.
Figure 3
Figure 3
Clean & Jerk (C&J) lifting movement pattern.
Figure 4
Figure 4
Participant inclusion and exclusion process. C&J = Clean & Jerk.
Figure 5
Figure 5
Differences among the experimental procedures. CAF + a-tDCS = ingestion of 400 mg of caffeine plus anodal transcranial direct current stimulation; PLA + a-tDCS = ingestion of 400 mg of placebo substance (calcium carbonate) plus anodal transcranial direct current stimulation; CAF + Sham-tDCS = ingestion of 400 mg of caffeine plus placebo condition for transcranial direct current stimulation; PLA + Sham-tDCS = ingestion of 400 mg of placebo substance (calcium carbonate) plus placebo condition for transcranial direct current stimulation. (*) Differences between the CAF + a-tDCS interventions and the other interventions.
Figure 6
Figure 6
Magnitude of differences between CAF + a-tDCS and other interventions. Effect size (ES) for the condition combining caffeine supplementation and anodal transcranial direct current stimulation (CAF + a-tDCS) compared to the other interventions (PLA + a-tDCS, PLA + Sham-tDCS, and CAF + Sham-tDCS). Effect sizes were calculated based on Z-scores from the non-parametric analysis. Symbols represent the ES for each comparison: circle (PLA + a-tDCS), square (PLA + Sham-tDCS), and triangle (CAF + Sham-tDCS). Dashed vertical lines indicate the thresholds for ES classification, with values considered large (ES ≤ −0.60) or medium (−0.60 < ES ≤ −0.40). Negative values indicate greater effectiveness of the CAF + a-tDCS intervention.
Figure 7
Figure 7
Responders and non-responders to the tDCS and CAF interventions; (A) represents responders to tDCS intervention; (B) represents responders to CAF intervention. CAF = ingestion of 400 mg of caffeine; a-tDCS = anodal transcranial direct current stimulation; PLA= placebo substance (calcium carbonate); Sham = Sham-tDCS (placebo) condition.
Figure 8
Figure 8
The median data of the RPE scale across interventions.
See this image and copyright information in PMC

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