The menstrual cycle and the bar: real hormones, a small average effect

A menstrual cycle is an ovarian clock with two halves. In the follicular phase a follicle ripens under FSH, oestradiol rises, and a mid-cycle LH surge releases the egg. In the luteal phase the leftover follicle, the corpus luteum, makes progesterone, and oestradiol stays meaningfully high as well. Progesterone lifts basal temperature by a few tenths of a degree, drives ventilation up, and shifts which fuel the muscle reaches for. Oestradiol is anabolic-leaning in tendon and in muscle in the cell papers, and it is also the hormone that, later, restrains kisspeptin if the energy is there to run a cycle at all. Strength lives inside that weather. It is not scheduled by it as neatly as a programme card wants.

In short. The first half of the cycle is dominated by rising oestradiol. After ovulation, progesterone joins it, temperature ticks up, and breathing changes. Strength sits in that weather. It is not on a rigid timetable.

McNulty and colleagues, Sports Medicine, 2020, pooled the studies that actually compared strength and power across phases. The average effect was trivial. Late follicular, when oestradiol is high and progesterone has not arrived, is the phase people feel strongest, and the pooled numbers do not turn that feeling into a large, reliable bonus. Individual women still differ by more than the average phase effect. A lifter who has tracked her own bar speed for six months knows more about her bar than the meta-analysis knows about her. The meta-analysis knows that writing a universal 'go heavy in week two, deload in week three' rule oversells a small mean. Both facts can stand. The hormone changes are real. The translation into kilos, across all women, is modest.

In short. A big review found the average strength difference between phases is tiny. Some women still notice a real difference. A rule that says everyone is strongest in week two overclaims the average.

Progesterone is thermogenic and it is a respiratory stimulant. Luteal-phase sessions in the heat feel harder at the same pace because you are already a bit warmer and already breathing more. Substrate use leans away from carbohydrate. None of that is weakness as a moral category. It is a different physiological set-point, and RPE, the effort number in your head, moves even when the muscle's maximum force hardly does. Force and effort coming apart is why a session can feel awful and the bar can still move. It is also why a session can feel easy in the late follicular phase without the muscle having become a different tissue overnight. Tendons express oestrogen receptors. Ligament laxity across a normal cycle is much smaller than the pregnancy story people import. Pregnancy relaxin is not a week of the luteal phase.

In short. After ovulation you run slightly hotter and breathe more, so the same pace feels harder. The muscle's actual maximum often barely changes. Feeling and force are not the same number. Ordinary cycles are not pregnancy-level joint laxity.

The result that matters more than phase timing is whether the cycle is happening. Low energy availability suppresses kisspeptin, GnRH goes quiet, LH falls, and bleeding stops. That is functional hypothalamic amenorrhoea. Bone loses the oestradiol pulses it was using, and the training adaptations you wanted are being attempted in a body that has shut an entire axis down to save fuel. RED-S, the relative energy deficiency syndrome, is this physiology plus the immune, mood and iron problems that travel with it. No clever split across a vanished cycle will replace the energy signal those neurons require. The kisspeptin page is the cellular version. This page is the barbell version of the same silence.

In short. A missing period is a bigger story than which week you should squat. Under-eating switches the cycle off via the brain, oestradiol pulses stop, and bone and recovery pay. Timing workouts cannot replace the fuel.

A combined pill replaces the ovarian pulse with a steady ethinyloestradiol, which occupies the oestrogen receptor, and a progestin, which occupies the progesterone receptor. The hypothalamus stops the GnRH pulse, so there is no follicular rise and no luteal progesterone peak, because the LH surge that would have released the egg never comes. Strength studies on the pill are mixed and mostly small. The climate is flat on purpose, so a programme written around a natural oestradiol peak is describing a cycle the pill has switched off. A progestin-only method is another climate again: many of them leave ovulation in place. Tracking a bleed on the combined pill and calling it ovulation is the common mistake. The bleed is a withdrawal from the tablet, not an LH surge. Train the muscle. Note the climate you actually have.

In short. The combined pill stops ovulation, so there is no natural oestradiol peak to time. A bleed on the pill is withdrawal from the tablet, not a real period. Write the programme for the hormone climate you are actually in.

A period is a blood loss. For some women it is a small one. For some it is enough, month after month, to pull ferritin down while the haemoglobin still looks acceptable. Low ferritin shows up as heavy legs, a session that dies early, and a doctor who says the blood count is fine. Strength work on an empty iron store is a different limitation from progesterone making a session feel hot. It is worth knowing which one you have before you rewrite the programme around the calendar. Fuel use shifts in the luteal phase toward fat at a given intensity, and carbohydrate becomes slightly less willing. A long session that was fed in the follicular phase can feel underfed in the luteal phase at the same breakfast. That is substrate, not character. Eat for the work. The phase does not suspend the need for carbohydrate when the session is hard.

In short. Heavy periods can empty iron stores while the basic blood count still looks normal. That will flatten a session more honestly than the luteal phase will. Late in the cycle, fat is used a bit more and hard sessions still want carbohydrate.

Not all progestins in a pill are the same climate. Levonorgestrel binds the androgen receptor more readily than drospirenone, which is anti-androgenic and also blocks the mineralocorticoid receptor, so water and skin can move when the ligand changes. A woman who changed pills and changed water retention, skin, or mood changed the ligand, not her discipline. Strength data are still too thin to rank pills by kilos on the bar. They are not too thin to stop pretending every 'pill' is one hormone. A hormonal IUD releases a local progestin at the progesterone receptor in the uterus and usually leaves ovulation happening, so a phase pattern can persist. A combined pill does not. Perimenopause is a third pattern: irregular surges, high and then absent oestradiol, sleep that breaks, and a strength change that will not sit on a 28-day wheel. The ovary is failing at its clock. The programme is not.

In short. Levonorgestrel and drospirenone are not the same progestin. A hormonal coil often leaves ovulation going, so a cycle can still be felt. The combined pill stops it. The years before menopause are irregular surges, not a tidy four-week plan.