Acute Responses to Creatine Monohydrate and L-Arginine, Alone and Combined, on Repeated-Sprint Power, Countermovement Jump Height, and Stroop Reaction Time in Recreationally Active Men: A Randomized Double-Blind Crossover Trial

Background/Objectives: Creatine monohydrate (CR) and L-arginine (LA) are widely used ergogenic supplements, but whether acute co-ingestion provides additional performance or task-specific cognitive benefits beyond either supplement alone remains uncertain. This study compared placebo, CR, LA, and LA+CR on repeated-sprint power, countermovement jump (CMJ) height, and Stroop reaction time (RT). Methods: In this randomized, double-blind, four-period crossover trial, 18 recreationally active men completed placebo (CG), CR (0.3 g·kg⁻¹), LA (6 g), and LA+CR conditions separated by at least 72 h (prospectively registered at ClinicalTrials.gov: NCT07548541). Testing began 60 min after ingestion with the Stroop test, followed by CMJ and the Running-Based Anaerobic Sprint Test (RAST). Co-primary outcomes were RAST average power (AP), peak power (PP), CMJ height, and incongruent Stroop median RT. Results: Condition effects were observed for AP, PP, minimum power, CMJ height, and Stroop RT (all p ≤ 0.011), but not fatigue index or Stroop accuracy. All four co-primary omnibus effects remained significant after Holm correction (all pHolm < 0.001). LA+CR produced higher PP than CG (MD 99.01 W; p < 0.001), LA (MD 51.29 W; p = 0.002), and CR (MD 46.71 W; p < 0.001), and greater CMJ height than all comparison conditions (all p < 0.001). LA+CR also produced lower Stroop RT than CG (MD 0.089 s; p < 0.001) and CR (p = 0.041), but not LA (p = 0.271). For AP, LA+CR exceeded CG and CR, but not LA after Bonferroni adjustment. In a secondary 2 × 2 factorial decomposition, no LA×CR interaction for a co-primary outcome survived Holm correction (all pHolm ≥ 0.104). Conclusions: Acute LA+CR produced outcome-specific improvements in peak sprint power and CMJ and selectively reduced Stroop RT, but did not establish biological synergy. Interpretation should remain exploratory given the small male-only sample, unmodelled period/sequence effects, and possible creatine-related carryover.