Influence of Caffeine Consumption on Maximal Strength Performance in Resistance-Trained Females During Various Menstrual Cycle Phases: A Double-Blind Placebo-Controlled Crossover Study

Caffeine is widely used to enhance neuromuscular performance; however, whether its ergogenic effects vary across menstrual cycle phases remains unclear. This study investigated the acute effects of caffeine on isokinetic and isometric strength performance during the follicular and luteal phases in resistance-trained females. Sixteen resistance-trained females (26.4 ± 3.9 years) completed a randomized, double-blind, placebo-controlled crossover trial. Participants ingested caffeine (6 mg·kg−1) or placebo during both menstrual cycle phases under standardized experimental conditions. Outcomes included isokinetic peak torque (PT) at 60°·s−1 and 180°·s−1, average rate of force development, time to peak torque, and maximal voluntary isometric contraction. Data were analyzed using linear mixed-effects models. Caffeine significantly increased knee extensor absolute and relative PT at 60°·s−1 (p = .009, ηp2 = 0.146; p = .005, ηp2 = 0.166) and 180°·s−1 (both p < .001, ηp2 = 0.293 and 0.310, respectively). Significant Phase × Condition interactions were observed for knee extensor absolute and relative PT at 60°·s−1 (p = .016, ηp2 = 0.125 and 0.126, respectively) and 180°·s−1 (p = .025, ηp2 = 0.109; p = .022, ηp2 = 0.114). For knee flexors, the interaction was not significant for absolute PT at 180°·s−1 (p = .069, ηp2 = 0.073) but was significant for relative PT (p = .050, ηp2 = 0.085). No significant interactions were observed for other neuromuscular outcomes (all p > .05). These findings suggest that menstrual cycle phase may modify the ergogenic effects of caffeine, particularly for knee extensor PT, with greater benefits during the follicular phase.