Acute caffeine supplementation and resistance exercise performance in female participants: a systematic review and three-level meta-analysis.

Background and objectives: Caffeine is a widely studied ergogenic aid, but evidence for its acute effects on resistance exercise performance is primarily derived from male or mixed-sex samples, with scarce data specific to women. Previous women-specific syntheses did not model the multiple dependent outcomes extracted from the same crossover trial, which may have produced overly narrow uncertainty estimates. Methods: PubMed, Web of Science Core Collection, Scopus and Cochrane Library were systematically searched through 26 April 2026 to include randomized, placebo-controlled trials testing acute, dose-defined caffeine before resistance exercise in female participants or independently extractable female subgroups. Effect sizes were paired Hedges' g and were synthesized separately for six outcome domains using three-level random-effects models with cluster-robust variance estimation: maximal strength, muscular endurance, movement speed, functional strength, functional endurance, and total load. 95% confidence intervals and prediction intervals were reported, and RoB 2 and GRADE were used to assess risk of bias and certainty of evidence. Results: A total of 24 studies were included (23 of which were included in the quantitative synthesis, with 107 effect sizes). Maximal strength showed a small mean effect (g = 0.17, 95% CI 0.06 to 0.29; low GRADE); muscular endurance showed a larger mean effect (g = 0.54, 95% CI 0.06 to 1.02; low GRADE), but its wide prediction interval and influence diagnostics indicated low reproducibility across protocols; movement speed showed a small, model-based estimate, but cluster-robust inference was uncertain because only three study clusters were available (g = 0.13; CR2 p = 0.072; very low GRADE). The study clusters for functional strength, functional endurance, and total load were too small and were not pooled for inference, so these domains are summarized descriptively. Meta-regressions of caffeine dosage, mean age of participants, and resistance load did not identify reliable moderation. Conclusion: Acute caffeine may slightly improve maximal strength and may benefit muscular endurance in some protocols, but the certainty of evidence is low or very low for most outcomes and prediction intervals indicate limited reproducibility. Current evidence therefore does not support strong recommendations by dosage, age, resistance load, menstrual cycle phase, hormonal status, or habitual caffeine intake. Future trials should report menstrual cycle phase, hormonal contraceptive use, menopausal status, habitual caffeine intake, adverse symptoms, sleep disturbance, blinding integrity, washout duration, and paired/change-score data with within-participant correlations. Systematic review registration: https://osf.io/2msf6/overview, identifier 10.17605/OSF.IO/2MSF6.