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Back in August, we wrote about a fascinating study in BMJ Open Sport & Exercise Medicine that tracked heart-rate variability, resting heart rate, and respiratory rate from WHOOP devices to draw conclusions about how the menstrual cycle influences recovery. It found that hormone fluctuations generally led to increased recovery in the early follicular phase and decreased recovery in the late luteal phase. In summarizing the implications of the study, we wrote:
“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.”
More studies are addressing that uncertainty all the time, reducing some of the error bars on our knowledge about how the menstrual cycle impacts training and performance. A December 2021 study in Frontiers in Physiology by an amazing research team led by Ritva S. Taipale-Mikkonen looked at that question directly: do changes during the menstrual cycle impact running performance?
The answer in this particular study is no, at least not in a systematic way, underscoring the difficulty of drawing cross-population conclusions for physiological processes with high individual variability. Athletes and coaches can tune into that individual variability by monitoring fatigue, perceived exertion, performance, HRV, RHR, and/or respiratory rate, but there appears to be no one-size-fits-all answer about performance throughout the menstrual cycle. Let’s dive into the study design!
The study included 28 women: 16 with a consistent menstrual cycle and no hormonal contraceptive use for the previous 12 months; and 12 using combined hormonal contraceptives (including estrogen and progestin). Each eumenorrheic participant underwent four test sessions across the menstrual cycle:
- Test 1: Bleeding (days 2-4 of cycle as measured from the start of menstruation)
- Test 2: Mid-Follicular (days 7-11)
- Test 3: Ovulation (determined from urine tests)
- Test 4: Mid-Luteal Phase (7 days after ovulation)
The participants taking hormonal contraceptives had their test sessions during bleeding, the first active pill week, the second active pill week, and the inactive pill week.
The test initiation was randomized, so athletes did their first test session at different times in the menstrual cycle.
The sessions were delightfully difficult–the dreaded “time to exhaustion” treadmill test. Athletes started at 6 km/h at 0.5% grade, increasing by 1 km/h every 3 minutes until volitional exhaustion. The authors also recorded heart rate, blood lactate, VO2 max, aerobic threshold, and anaerobic threshold. There were three main findings.
One: There were no significant changes in performance metrics or physiological responses across different phases of the menstrual cycle.
That’s the headline finding of the study, which may contrast with what might be expected based on the study on recovery metrics–that hormone fluctuations (particularly the rise in progesterone) may cause performance decreases for some athletes in the luteal phase. It’s important to note that the authors are not saying that this finding applies in all contexts. For example, a 2021 study in Medicine & Science in Sport & Exercise found that recreationally active women performed worse on a ~20 minute cycling time trial during the luteal phase, along with having higher ratings of fatigue.
Similarly, studies have different findings about heart rate responses during the menstrual cycle. The new study found no significant heart rate changes. Another 2021 study found elevated heart rate during the luteal phase during an 8 x 3 minute workout. A 2019 study also found heart rate differences during a 40 minute run at 75% of maximal aerobic speed, but that did not correspond to performance differences. When doing heart rate training, it’s key to monitor individual variation over time and in different conditions.
Two: Heart rate was higher at aerobic and anaerobic threshold for athletes using hormonal contraceptives
The authors caution that their purpose was not to compare across groups, but they did find a medium to large effect size of heart rate response at aerobic and anaerobic threshold (but not at max heart rate) for athletes using hormonal contraceptives compared to those that were not. It’s uncertain how that would impact performance and adaptation, though a fantastic 2020 meta-analysis in Sports Medicine found that oral contraceptives “might result in slightly inferior exercise performance on average when compared to naturally menstruating women, although any group-level effect is most likely to be trivial.” That finding underscores the importance of treating each athlete individually, since performance impacts are uncertain. And when using heart-rate based training, it’s not a simple equation to derive submaximal thresholds from max heart rate (or age).
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Three: While there was no systematic response across the study, individual responses varied substantially
This study includes what may be one of the more beautiful figures ever made to illustrate the complexity of exercise physiology. Figure 2 charts the change in performance for each athlete (EUM = menstrual cycle, CHC = hormonal contraception), standardizing baseline as the test result during bleeding. Check it out!
At least one athlete has their peak performance and one athlete has their worst performance at each phase of the menstrual cycle. And the hormonal contraceptive chart shows a similar scatter. The individual results could apply to each athlete over time, allowing for training adjustments. Or perhaps the results would vary even for the individuals with repeat testing.
Read together with other studies, it’s clear that the menstrual cycle matters for training and performance. Unfortunately, there is no easy answer on how exactly it matters and how (or if) it should alter approaches for individual athletes. An amazing 2020 review in Sports Medicine examined data from 51 studies to find a possible trivial reduction in performance during the early follicular phase, but the effect size was low, with no uniform guidelines possible. The authors “recommended that a personalised approach should be taken based on each individual’s response to exercise performance.”
It’s also an open question how these processes might play out over time. Right now, there is no set understanding of how/if the menstrual cycle impacts performance as a snapshot of a given moment. The study we reviewed today showed no systematic changes at all. Could there be any changes to adaptation from doing hard training or racing in different phases of the menstrual cycle? How predictable are individual variations over time?
This emerging field of study will play out over the coming years and decades, and we are so excited to see where it goes from here. For now, pay attention to how you feel and respond, adjusting as needed (see our general recommendations here).
The menstrual cycle works like a lot of physiology–general principles and physical responses that get stirred into a chaotic stew of thousands of variables that impact performance. While we might never have a certain answer that applies to everyone, it is possible to develop some uncertain answers that apply to individuals. And that’s a great place to start.
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.