Does the menstrual cycle really decide when you will train best?

A difference too small to schedule around. Across 78 studies, performance may be slightly lower at the start of the phase when bleeding begins. The average size of that difference was trivial, next to what moves performance anyway: sleep, food and stress. The quality of the evidence was low and the variation between studies large, so the researchers recommended fitting decisions to the individual woman.

Hormone levels change across the cycle, so it is tempting to build a training schedule by its phases. For that to work, the difference between phases has to be larger than ordinary day-to-day noise.

A review of seventy-eight studies covering nearly twelve hundred participants found that performance may be slightly lower at the start of the phase when bleeding begins, and the average size of the difference was trivial.

Trivial next to what moves performance anyway: sleep, food and stress. On top of that the quality of the evidence was rated low and the variation between studies was large, partly because many of them did not verify the cycle phase with a hormonal test.

So the recommendation is not a rigid schedule but personal tracking. When the variation between women is larger than the average difference between phases, a group average does not predict the individual.