Menstrual-Cycle Phase-Based Periodization of Resistance Training and Chronic Gains in Strength and Body Composition: A Discordant Evidence Base in Need of an Uncertainty-Aware Synthesis

Periodizing resistance training around the menstrual cycle, typically by concentrating load in the follicular phase, has become popular through commercial cycle-tracking applications. This opinion argues that the recommendation rests on a small, heterogeneous, and conflicting evidence base and that much of the conflict stems from conflating two questions: acute, within-cycle variation in muscular performance and chronic adaptation to phase-periodized training. Acute studies in eumenorrheic women report small or absent between-phase differences in maximal strength and power, weakening the premise of a privileged follicular anabolic window. Among the few chronic studies, follicular advantage appears mainly where phase verification was limited or when training dose was concentrated in the favored phase; it was not reproduced in the most rigorous trial to date, in which volume–load, not cycle phase, was associated with adaptation. Phase-verification rigor and volume–load equating, alongside heterogeneity in outcome measurement, are plausible, testable candidate explanations; none is yet an established cause. Existing quantitative syntheses address hormonal contraceptive use, not phase-based periodization in naturally cycling women. A registered systematic review with co-primary frequentist and Bayesian meta-analyses that model these factors as effect modifiers and test any pooled effect against a pre-specified smallest effect size of interest is warranted and introduced (INPLASY202660145).