Single and combined effect of beetroot juice and caffeine intake on muscular strength, power and endurance performance in resistance-trained males

To examine the single and combined effect of acute beetroot juice and caffeine supplementation in muscular strength, power, and endurance performance. Thirteen resistance-trained males participated in a triple-blind, cross-over, randomized controlled-trial with four conditions: (a) caffeine (CAF); (b) beetroot juice (BJ); (c) caffeine and beetroot juice (CAF + BJ); (d) placebo (PLA). Participants ingested 70 mL of beetroot juice, concentrated NO₃^--rich beverage (BJ, 6.4 mmol NO₃^-) or PLA (~ 0.04 mmol NO₃^-) 180 min and caffeine or placebo (3 mg/kg) 60 min before the trial. Muscular strength/power was evaluated at 25%, 50%, 75%, 90% and 100%1RM and muscular endurance at 65%1RM, in bench press (BP) and back squat (BS). In all tests, mean (V_mean and W_mean) and peak (V_peak and W_peak) velocity and power output were measured. In BS, muscular strength/power showed a supplement-by-load effect in V_mean and W_mean (P < 0.05, η_p² = 0.167-0.173), with caffeine increased compared to placebo at 75%, 90% and 100%1RM (9-25%, P < 0.005, g = 0.51-1.47); while in muscular endurance, significant differences were found in number of repetitions, V_mean and W_mean (P < 0.05, η_p² > 0.277), in all experimental groups (CAF, BJ and CAF + BJ) compared to placebo (6-17%, P < 0.05, g = 0.46-94). No differences in muscular strength/power or endurance were found in BP. Single and combined acute beetroot juice and caffeine intake increased muscular endurance performance at 65%1RM in back squat but not in bench press exercise.