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The steeplechase first became an Olympic event in 1920. Scratch that, it became half of an Olympic event in 1920. The steeplechase didn’t become a full event until 2008, when women competed for the first time.
During the 2021 women’s race, the broadcast listed the past champions…. there were only three, compared to 23 for men. Looking at that short list of women, we saw a solid metaphor for underrepresentation in athletics more generally. Nowhere is that more evident than in the field of exercise science.
A 2016 review of 30 performance studies published in two major exercise science journals in 2015 showed that 3% of all participants were women (in non-performance studies, around 40% of participants were women).
While there are some valid exercise science challenges in accounting for how variations caused by the menstrual cycle could impact performance study design (see this 2019 article in Sports Medicine), the discrepancy really makes us think of that short list of women’s steeple winners. It’s hard to make up for lost time when the base of research is formed from a partial picture of human physiology.
Things are changing.
Dr. Stacy Sims, Dr. Kate Ackerman, Dr. Emily Kraus, and many others are leading the way in understanding how female athlete physiology may differ in response to training interventions and other stressors. As Dr. Sims says, “women are not small men.” Unique aspects of female athlete physiology could change optimal training approaches, with variation in responses to fasted training, within-day energy deficits, and hormonal fluctuation at different times during the menstrual cycle (or with variation in birth control methods).
In July 2021, Dr. Sims, Laura Ware, and Emily Capodilupo published an expansive study in BMJ Open Sport & Exercise Medicine that examines how recovery metrics respond at different phases of the menstrual cycle for athletes with and without the use of hormonal contraceptives. Using data from WHOOP, the study compiled user-input and consented data from 4594 participants, with 3870 having a natural menstrual cycle, 455 on combined birth control (estrogen and progestin), and 269 on progestin-only birth control. Exclusion criteria included menstrual cycles that didn’t last between 25 and 35 days, irregular menstrual cycles of 2 or more missed periods for athletes not taking hormonal contraceptives, and/or stopping hormonal contraceptives in the last 9 months.
Researchers analyzed resting heart rate (RHR), heart rate variability (HRV), and respiratory rate. Generally, RHR and respiratory rate go up when stress goes up. HRV behaves inversely. A 2018 review article in the journal Psychiatry Investigations found that lower HRV correlates with stress, validating its use as one data point to inform stress management practices. HRV and RHR were measured in the last slow wave sleep cycle and respiratory rate was measured as a median over the entire sleep cycle. WHOOP’s proprietary recovery metric incorporated the variables into an output recovery score between 0% and 100%.
There were 4 big takeaways that could influence training approach and inform future research.
One: For participants with a natural menstrual cycle, HRV and recovery decreased across the menstrual cycle from a peak in the early follicular phase (days 1-5 after menstruation) to a low point in the late luteal phase (just before menstruation).
Respiratory rate hit a low point in the late follicular phase, before rising in the late luteal phase (likely validating the shortness-of-breath feeling that some women describe during PMS). RHR went up slightly throughout the menstrual cycle.
Two: Participants taking combined birth control had a more complex pattern, with HRV and recovery starting higher at the beginning of the inactive pill week, reaching the lowest point a few days later during the withdrawal bleed, then rising through the active pill weeks.
Whereas participants with natural menstrual cycles had lower recovery scores in the luteal phase, participants on combined birth control had recovery scores rise during the corresponding active pill weeks.
Three: Participants taking progestin-only birth control had similar patterns as participants with natural menstrual cycles.
Relative to participants with natural menstrual cycles, participants on progestin-only birth control had slightly lower HRV and recovery scores in the early follicular phase and slightly higher HRV and recovery scores in the late luteal phase, but the differences were minimal.
Four: Participants taking hormonal contraceptives had slightly greater decreases in recovery score after cardiovascular strain.
In addition, for many participants, high levels of strain led to slightly greater decreases in recovery score in the late luteal phase.
What a cool study!
The dataset is so large that it likely smooths over individual variation and gives us an exciting window into how physiology responds based on hormonal context. Sex hormones affect the nervous system, which feeds directly into HRV and recovery metrics, along with impacting the metabolic, musculoskeletal, and even biomechanical systems.
In the early follicular phase, progesterone and estrogen are low. Estrogen has two peaks, one peak as the follicular phase progresses, and a lower peak in the luteal phase. Progesterone rises to one peak in the luteal phase. The authors highlight that estrogens “act centrally to modulate the autonomic nervous system, increasing vagal and decreasing sympathetic activity, whereas progesterone appears to have an opposing effect, elevating central norepinephrine release inducing a sympathetic drive.”
What does that mean? The sympathetic nervous system is colloquially known as the “fight or flight” system, so a simple way to think about it is that progesterone may decrease HRV and recovery as the menstrual cycle progresses. While the explanations for the birth control groups are more complex, they similarly rely on introduction of hormone pulses at varying times.
The authors also theorize that athletes taking combined birth control may have slightly reduced next-day recovery due to increased inflammatory and stress responses (see this 2020 study in Frontiers of Physiology). Oral contraceptives could increase levels of the stress hormone cortisol in some athletes, though that is not certain.
This study is just a start, and it isn’t without limitations.
Results could vary in different sub-populations of athletes since there is likely some selection bias in WHOOP users. Perhaps the level of chronic strain matters, or the training background of athletes. Menstrual cycle phases were not confirmed with blood tests, so the self-reporting could introduce some variance too. Finally, and perhaps most importantly, it’s hard to know how much these metrics influence performance and actual recovery for activity. It’s possible that minor changes in RHR, HRV, and respiratory rate won’t have any significant impact on output during running or other sports (or possibly it varies based on the sport), or that it’ll be so minor that it’s overwhelmed by other factors (see this 2020 review in Sports Medicine finding that due to between-study variation, “general guidelines on exercise performance across the menstrual cycle cannot be formed”).
With those limitations acknowledged, the results parallel some of the understanding of adjusting training based on individual menstrual cycle characteristics. Led by Dr. Sims, the general principle is to reduce intensity during the late luteal phase when recovery might be impacted, and modulate intensity/duration based on hormone fluctuations.
For our team, we like athletes to note menstrual cycle characteristics when possible, then monitor how athletes respond over time (we went into more detail on our podcast). When athletes note feelings of increased fatigue or decreased performance in a consistent pattern overlapping with the general trend seen in menstrual cycle research (see this 2021 study for an example), we like to emphasize aerobic volume and a focus on recovery in the late luteal phase.
Not every athlete responds the same way, or even notices much variation. For example, check out this 2020 study in the International Journal of Sports Physiology and Performance, where 47% of athletes reported their worst fitness and 30% reported their worst performance during “bleeding.” Research indicates the start of the period in the early follicular phase should be a low hormone, high recovery zone, so it underscores the importance of monitoring individual responses.
While every athlete is different, this is the general outline we use for athletes that report that their menstrual cycle significantly impacts how they feel in training. Note: the outline is for a 4-week cycle, but athletes often have substantial variation in timing, so adjust accordingly.
Week 1 (early follicular phase): Moderate/high volume and moderate/high intensity
Go for it! The recovery metrics and hormonal context are apt for heavy training. Just remember that there are tons of variables that go into performance and adaptation, so listen to the whole context for how you feel. Some athletes describe increased side effects that make the first few days tougher for heavy training.
Week 2 (follicular phase): Moderate volume and moderate/high intensity
This is a great time to keep training hard, but pay attention to how ovulation affects how you feel. We sometimes reduce volume slightly to account for harder training weeks in the early follicular phase, or increase volume for athletes that don’t respond as well to heavy training at the start of the follicular phase.
Week 3 (luteal phase): Moderate/high volume and low/moderate intensity
Here, things can really start to vary heavily. If you feel good, it’s OK to keep training as planned, but back off the intensity if recovery feels lower.
Week 4 (late luteal phase): Low/moderate/high volume and low/moderate intensity
Talk about high error bars in our suggestions! PMS can be extremely difficult for some athletes, and if that’s a consistent pattern, we suggest training a bit easier in the late luteal phase. That can still mean workouts, just not the hardest efforts of the training block. Add rest days if needed.
For others (particularly pro athletes pushing the margins of performance), it may be a great time to work in a higher volume aerobic build week. The body can still adapt to some intensity, just make sure there is plenty of space for recovery and adaptation.
The Big Question
What if a race falls in the late luteal phase, when high hormones may affect recovery? While it varies a ton, in general, try not to stress about it too much. We have seen athletes win national championships at every phase of their menstrual cycle.
It gets back to how the menstrual cycle interacts with other variables. As Dr. Sims’ study showed, the late luteal phase can introduce more stress. So can work, kids, anxiety, or even worries about where you are in the menstrual cycle as a race approaches. The menstrual cycle is one important variable that goes into performance, and there are thousands of other variables too. If one of those variables is off, whether it’s menstrual cycle timing or work schedule or something else, it’s OK–there’s a great chance you can still have your best, badass boss day.
That might not be possible for you, and that’s OK too. During the Olympic marathon, Lonah Chemtai Salpeter was in a medal position when she suddenly had to stop and walk. The announcers speculated that it was due to the heat. After the race, Salpeter said it was due to severe menstrual cramps. Her period was scheduled to arrive two days before the race, but it didn’t arrive until after. As she said on Facebook, “I think we really need to normalize the conversation around being female athletes.”
Whether you are training for the Olympics or just starting running, pay attention to how you feel. The physiology of the menstrual cycle is incredibly cool. That being said, some athletes get frustrated by the uncertainty of how it may affect training and racing. The emerging research is shifting that dialogue.
Sure, female athletes have a bit more variation to consider each month. But it can make training into an even greater process of learning and growth. And how freaking cool is that?!
Megan Roche received her medical degree from Stanford University and is currently pursuing a PhD in Epidemiology from Stanford, where she focuses on female athlete performance. David and Megan partner with runners of all abilities through their coaching service, Some Work, All Play. They host the Some Work, All Play podcast on running (and other things), and they wrote a book called The Happy Runner.