Purpose: This systematic review and meta-analysis examined the influence of menstrual cycle (MC) phases on physical fitness components in eumenorrhoeic women to inform training practices. Methods: Studies including eumenorrhoeic women aged = 18 years comparing different MC phases, with the early follicular phase as reference, were included. Data were extracted for maximal oxygen uptake (VO2max), anaerobic power, knee extensor and flexor strength, sprint, and jumping performance. Risk of bias was assessed using an adapted RoB 2.0 tool. Standardised mean differences were meta-analysed using a random-effects model; heterogeneity and publication bias were evaluated. Meta-regression assessed MC phase as a moderator. Results: From 15,423 records screened, 91 studies met the inclusion criteria, and 24 were included in the meta-analysis. VO2max was significantly lower during the early follicular phase compared to other phases [SMD = -0.28, 95% CI (-0.40, -0.15)]. Sprint time was higher (worse) in the early follicular phase versus the ovulatory and late follicular phases [SMD = 0.25, 95% CI (0.02, 0.48)]. No consistent significant differences were found for anaerobic power, knee strength, or jumping performance. Heterogeneity varied across outcomes; some strength analyses required the exclusion of influential studies. MC phase was not a significant moderator for any outcome. The certainty of evidence was low for all outcomes. Conclusions: These findings suggest VO2max and sprint performance may decrease during the early follicular phase, but conclusions are limited due to low evidence certainty. Other fitness components showed no consistent influence from MC phases.
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