Caffeine is one of the most widely used and extensively studied ergogenic aids in sport, yet whether its well-established acute benefits extend to the chronic remodelling of skeletal muscle remains unresolved. In this narrative review, we distinguish the acute ergogenic effects of caffeine from its potential influence on chronic skeletal muscle remodelling, because the mechanisms that improve acute performance need not be those that govern repeated tissue adaptation. The evidence for acute ergogenicity rests on a large human literature, in which adenosine receptor antagonism is the probable dominant mediator, with additional contributions from potassium handling, ryanodine receptor 1 (RyR1) sensitisation, altered contractile behaviour, and reduced perceived effort. Evidence that repeated caffeine exposure around exercise modifies chronic skeletal muscle adaptation in humans remains limited; available training trials are short, narrow in modality, and lack muscle biopsy endpoints, and several acute endurance studies under substrate restriction are better interpreted as training quality mediator evidence. Preclinical work is more mechanistic but directionally mixed, supporting Ca² ⁺ linked CaMKKβ (Ca² ⁺ /calmodulin-dependent protein kinase kinase β) and AMP-activated protein kinase (AMPK) signalling, autophagy, peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α) related transcription, and mitochondrial quality control on one side, and raising an attenuation hypothesis through protein synthesis, recovery, and tissue-specific remodelling on the other. We therefore evaluate three working models in which caffeine acts as an amplifier, a partial mimic, or an attenuator of exercise-related signalling. Current human evidence is most compatible with partial mimicry and training quality mediation rather than direct amplification or impairment of long-term tissue adaptation. Resolving this question will require human training studies that combine muscle biopsy endpoints, caffeine-abstinent post-training testing, objective sleep monitoring, and explicit control of external training load. On balance, the current evidence supports interpreting caffeine as a reliable acute ergogenic aid and a plausible mediator of training quality, rather than as a proven direct modifier of chronic skeletal muscle adaptation.

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.

Background and Objectives: Evidence on the acute ergogenic effects of caffeine in adolescent athletes remains limited. This systematic review quantified acute performance-related effects and summarised adverse-event reporting in this population. Methods: PubMed and Web of Science were searched from inception to 13 May 2026 and SPORTDiscus was additionally searched from inception to 15 July 2026 for placebo-controlled studies of acute, dose-defined caffeine ingestion in adolescent athletes aged 10–19 years. Paired Hedges’ g values were synthesised separately across six outcome domains using three-level random-effects models with CR2 cluster-robust inference. Risk of bias and certainty of evidence were assessed using RoB 2 and GRADE. Safety findings were summarised narratively. Results: Thirty-five studies were included, of which 32, representing a total of 726 participants, contributed 363 effect sizes. In the primary analysis, which excluded a priori a small number of extreme combat-sport count-test effects arising from implausibly small reported standard deviations (Hedges’ g > 3.0), caffeine produced a small favourable average effect on physical performance (g = 0.33, 95% CI 0.17 to 0.49). When all effect sizes were included in a sensitivity analysis, the estimate was larger but inflated by these extreme values (g = 0.43, 95% CI 0.20 to 0.67) and was accompanied by substantial heterogeneity (I² = 84%), a prediction interval crossing the null (−0.71 to 1.58), and significant small-study asymmetry. Pooled effects were not statistically significant for sport-specific performance, perceptual response, or physiology. Cognitive findings were based on only two study clusters and were highly uncertain, while psychological outcomes were summarised narratively because the evidence was too sparse and heterogeneous for a meaningful pooled estimate. Exploratory moderator analyses did not provide reliable evidence that effects differed by caffeine dose or ingestion timing. No serious adverse events were reported, but adverse-event assessment was inconsistent. Conclusions: Acute caffeine ingestion may produce a small and uncertain benefit for selected physical-performance tasks in adolescent athletes. Adverse-event reporting was inconsistent, and current evidence is insufficient to support routine caffeine use in youth sport. These findings are specific to acute caffeine exposure under controlled experimental conditions in adolescent athletes and should not be extrapolated to children, non-athlete adolescents, repeated or chronic caffeine use, or energy-drink consumption.

Background and Objectives: Evidence on the acute effects of caffeine in female team-sport athletes is limited. This study aimed to quantify the acute effects of caffeine on female team-sport athletes across physical performance, sport-specific performance, physiological responses, perceptual responses, and cognitive performance and determine moderators. Methods: PubMed and Web of Science were systematically searched for randomized, placebo-controlled crossover trials of acute, dose-defined caffeine in female team-sport athletes. Effect sizes were expressed as Hedges’ g and synthesized within each domain using a three-level CHE model with CR2 cluster-robust variance estimation and Satterthwaite degrees of freedom; 95% confidence intervals and prediction intervals were reported. Risk of bias (RoB 2), methodological quality (PEDro), and certainty of evidence (GRADE) were assessed. Results: Twenty-six studies were included, comprising 26 randomized, blind, placebo-controlled crossover trials. As a descriptive cross-domain summary, acute caffeine intake showed a small overall effect when all available domains were pooled (Hedges’ g = 0.24, 95% CI 0.13 to 0.35); however, because these domains represent different constructs, domain-specific estimates were considered the primary interpretable findings. For physical performance, caffeine showed a small favorable effect (g = 0.32, 95% CI 0.22 to 0.42), with statistical evidence for repeated sprint ability (g = 0.43), agility or change in direction (g = 0.42), sprint or speed performance (g = 0.39), anaerobic power (g = 0.30), and jumping performance (g = 0.29). For sport-specific performance, the pooled estimate indicated a small favorable effect but did not reach conventional statistical significance (g = 0.36, 95% CI: −0.01 to 0.73). Exploratory sub-indicator analyses, each based on only a few studies, suggested possible favorable directions for sport-specific locomotion or running performance (g = 0.46) and throwing or ball-speed outcomes (g = 0.19), whereas evidence was insufficient for shooting or scoring accuracy. Physiological outcomes were direction-aligned to the value conventionally regarded as favourable for each indicator (i.e., lower heart rate, lactate, and glucose), so that positive values denote the conventionally favourable direction; because the domain aggregates indicators of differing clinical desirability, its pooled value is a descriptive summary only and showed no clear domain-level direction (g = −0.05, 95% CI −0.31 to 0.21). The heart rate submetric showed a statistically detectable difference (g = −0.30), indicating that caffeine increased heart rate, the expected pharmacological response to a stimulant rather than an adverse effect; there were no clear effects on blood lactate or blood glucose. Perceptual responses improved modestly (g = 0.42), largely through reduced rating of perceived exertion (g = 0.35). Cognitive performance, based on only four studies with low degrees of freedom and wide uncertainty, showed a nonsignificant direction (g = 0.37) with no clear evidence for reaction time or cognitive accuracy, and these data should not be interpreted as showing that caffeine improves cognitive performance in female team-sport athletes. Exploratory moderator analyses did not provide statistically reliable evidence of moderation by caffeine dose, timing of intake, formulation or source, sport type, competitive level, habitual caffeine intake, or blinding status. Conclusions: Available evidence most consistently supports small, outcome-specific benefits of acute caffeine for physical performance and reduced perceived exertion. Evidence for sport-specific skills and cognitive outcomes remained uncertain, based on few studies per outcome and not supporting a general benefit. Current data do not support dose-, timing-, formulation-, habitual-intake-, sport-, or population-stratified recommendations. Future trials should be adequately powered, prospectively report menstrual-cycle phase, hormonal-contraceptive use, habitual caffeine intake, and adverse symptoms, and verify the integrity of blinding.