Why your hormones are not a side issue in endurance training

You know that route well. Same route, same effort, same conditions, but everything about it feels completely different. Heart rate is higher than the training load justifies, legs are heavy, and the watch says recovery looks fine.

This is not a bad day or a fitness question. It is oestrogen and progesterone operating on an internal schedule that most training plans have never accounted for. Here is what is actually happening.

One note before proceeding: the typical 28-day cycle is a statistical average, and cycle length ranges from 21 to 35 days in women with regular periods. What follows describes the hormonal pattern, rather than a fixed day schedule.

What oestrogen and progesterone are actually doing

The menstrual cycle is governed mainly by two hormones: oestrogen and progesterone. Neither stays steady. Both rise and fall in a predictable wave pattern throughout the cycle, affecting energy metabolism, thermoregulation, muscle repair, connective tissue, and the autonomic nervous system. These interactions affect many of the signals that wearables use to estimate recovery, readiness, and strain.

The follicular phase: rising oestrogen and fuel availability

The follicular phase starts on day one of menstruation and ends at ovulation. Oestrogen rises steadily while progesterone stays low throughout.

Some research suggests oestrogen may promote fat oxidation during endurance exercise. However, the effect is more consistently observed when comparing the luteal phase to the early follicular phase, and findings across studies are not uniform. Since carbohydrate remains the primary fuel for higher-intensity work such as intervals, tempo, and threshold efforts, some runners report feeling stronger and recovering better during the follicular phase, though individual responses vary considerably.

Some studies have reported higher HRV and lower resting heart rate during parts of the follicular phase compared with the luteal phase, which may show up as improved recovery scores on devices from Garmin or WHOOP.

One caveat worth noting: some research has linked oestrogen to changes in connective tissue properties in certain athletes. The clearest evidence for increased laxity appears around ovulation rather than in the follicular phase, as discussed below.

Ovulation: the hormonal peak

A surge in luteinising hormone (LH) triggers ovulation. Peak oestrogen is observed at this point, while progesterone remains low and begins its substantial rise only after ovulation.

Some women report feeling stronger or finding hard workouts more manageable around ovulation, although responses vary considerably. HRV may be higher and resting heart rate lower than during the luteal phase, though the effect is individual rather than universal. Body temperature rises after ovulation as progesterone increases, a shift detectable on a basal body temperature chart.

It is also worth flagging the connective tissue question here. A 2017 meta-analysis by Herzberg et al. found significantly increased ACL laxity during the ovulatory phase compared with the follicular phase, coinciding with peak oestrogen. The overall strength of that evidence is graded low, and whether it meaningfully increases injury risk in practice remains debated. Wearable data cannot capture this.

The luteal phase: progesterone changes the equation

After ovulation, progesterone rises sharply and becomes the dominant hormone. This is the phase most runners notice most clearly, and the one most visible in wearable data.

Thermoregulation and cardiovascular load

Baker et al. (2020) showed that progesterone elevates core body temperature by approximately 0.3-0.7°C from baseline. That shift means the body has to work harder to stay cool during training. Heart rate at the same pace will be slightly higher, particularly toward the end of the luteal phase and more pronounced in warmer conditions. It is not a sign of declining fitness.

Fuel use

Some studies suggest the body may rely slightly more on fat oxidation and slightly less on carbohydrate metabolism during the luteal phase. Since high-intensity efforts depend heavily on carbohydrate, intervals and tempo runs may feel harder. Some researchers have suggested protein requirements may be slightly higher during this phase in well-trained athletes, although evidence remains limited.

HRV, recovery, and sleep

The luteal phase is where hormonal effects on wearable data are most apparent. Many women see reduced HRV, elevated resting heart rate, increased skin or body temperature, and lower recovery scores, all without any change in training load. Sleep may also suffer, with more nighttime awakenings or reduced sleep quality.

These changes collectively explain why training can feel harder in the second half of the cycle, even when fitness and workload are identical. For more on how heart rate and HRV interact with recovery metrics, see the heart rate guide.

Menstruation: reset, not shutdown

From a hormonal perspective, menstruation is the beginning of the cycle, not the end. Oestrogen and progesterone are low, and the conditions of the follicular phase begin to return.

For some athletes, menstrual symptoms are the primary challenge: cramping, disrupted sleep, digestive symptoms, fatigue, and heavy bleeding can all affect performance. For others, menstruation is simply another week of training. Symptoms affect performance; the hormonal environment is already shifting back toward the more positive conditions of the follicular phase.

Menstruation is also an important health marker. Changes in cycle length or loss of menstruation can be important indicators of low energy availability and possible RED-S (Relative Energy Deficiency in Sport). In that context, the cycle becomes a physiological performance measure rather than something separate from training.

Individual variation matters more than any phase chart

Some athletes notice clear changes in recovery, effort, sleep, and performance across the cycle. Others notice almost none. No chart can tell you exactly how you will perform on a given day.

Hormonal patterns provide context but do not determine outcomes. Training history, sleep, nutrition, life stress, environmental conditions, and overall health remain large factors. The most useful approach is to treat cycle tracking as another data source rather than a performance prediction. Understanding your own patterns over time is where tools such as Garmin’s menstrual cycle tracking can add practical value.

What wearables are actually measuring

No current consumer wearable measures oestrogen or progesterone directly. What devices can monitor are the downstream effects: skin temperature, HRV, resting heart rate, respiratory rate, and sleep architecture. Those signals do reflect real hormonal influences, which is why recovery scores after ovulation tend to differ from those in the follicular phase.

Several companies are exploring ways to infer hormonal changes more accurately from physiological signals, though most approaches remain in development or early validation. At present, the most practical application of wearables in this context is pattern recognition across multiple cycles. The recovery tracker guide covers which devices currently offer cycle-aware features and how to interpret them. For the underlying sports science on HRV and autonomic function, see the sports science section.

The bottom line

Hormones affect physiology more reliably than they affect performance. Wearables can now track changes in body temperature, heart rate, HRV, and recovery metrics across the cycle with increasing confidence. How those physiological changes translate to race results is more complex and more individual.

Fitness, sleep quality, nutrition, training experience, environmental conditions, and other stressors all play substantial roles. Understanding the menstrual cycle is not about finding a universal training strategy. It is about understanding why heart rate, HRV, body temperature, and perceived effort may fluctuate, and interpreting your own data in that context rather than treating every variation as a problem to fix. This article is part of the site’s female athlete tech coverage, which addresses wearables, physiology, and performance for female endurance athletes.

FAQ

Does the menstrual cycle affect heart rate during running?

Yes. Progesterone in the luteal phase raises core body temperature by approximately 0.3-0.7°C, which increases the cardiovascular demand of running at a given pace. Heart rate at the same speed may read higher than in the follicular phase, with no change in fitness. The effect is more pronounced in warmer conditions.

Why do WHOOP or Garmin recovery scores drop in the second half of the cycle?

The luteal phase typically produces lower HRV, higher resting heart rate, and elevated skin temperature. Wearables interpret these physiological signals as incomplete recovery. They are registering real hormonal effects, not training errors. Tracking multiple cycles helps distinguish a hormonal pattern from a genuine recovery problem.

Can wearables track hormone levels directly?

No current consumer device measures oestrogen or progesterone directly. Devices measure downstream effects: skin temperature, HRV, resting heart rate, and respiratory rate. Several companies are developing tools to infer hormonal shifts from these signals, but direct hormonal measurement is not yet available in a validated consumer product.

What is RED-S, and how is the menstrual cycle connected to it?

RED-S (Relative Energy Deficiency in Sport) is impaired physiological function caused by insufficient energy availability relative to exercise demands. Changes in cycle length or loss of menstruation can be important indicators of low energy availability and the development of RED-S. Any significant change in cycle regularity warrants attention, particularly in athletes who have increased training load or reduced food intake.

Does the menstrual cycle increase injury risk?

Evidence suggests that ACL laxity increases around ovulation, coinciding with peak oestrogen levels. A 2017 meta-analysis found this effect, though the overall quality of the evidence is graded as low, and the clinical significance is debated. There is no established protocol for modifying training based on the cycle phase to prevent injury. Awareness of the pattern is reasonable; restructuring training around it requires stronger evidence than currently exists.

Last Updated on 19 June 2026 by the5krunner