How the menstrual cycle impacts athletic efficiency

The current state of research on gender differences in sports science recalls a famous quote from a friend of Mark Twain's Charles Dudley Warner: "Everyone talks about the weather, but nobody does anything about it." The problems of doing decades of predominantly male research and simply assuming that the conclusions can be applied to women are clear, and people are sure to talk about them. However, translating this new awareness into action and identifying specific methods by which women should train and compete differently than men remains a challenge.

All the more welcome is a new open-access study in Sports Medicine, published by a group of researchers in the UK led by Kelly McNulty from Northumbria University and Kirsty Elliott-Sale from Nottingham Trent University. The research team performed a meta-analysis of all the studies they could find on the effects of the menstrual cycle phase on exercise performance. As it turns out, the results are as interesting for what they didn't find as for what they did.

At the beginning some brief background information. The two main reproductive hormones in women are estrogen and progesterone. They rise and fall in a predictable pattern during the nominal 28-day menstrual cycle. (In practice, cycles are not always 28 days. The inclusion criteria for the subjects in this analysis were regular cycles of 21 to 35 days.) Due to its effects on the muscles, estrogen is considered to be potentially performance-enhancing -building, carbohydrate metabolism and neuromuscular Signal transmission. In contrast, progesterone inhibits the effects of estrogen.

Here is a chart from the paper showing the rise and fall of the two hormones (with estrogen adding an additional “o” in British spelling):

(Figure: Sports medicine)

There are three key phases in which the hormonal milieu has the sharpest contrasts. In the early follicular phase, both estrogen and progesterone are the lowest. Both are elevated in the middle luteal phase. This is the comparison that many studies make, assuming you see the greatest performance differences between low and high hormone phases. But the time around ovulation when estrogen is at its highest without progesterone disruption could be even better for performance – at least in theory.

The researchers located 78 relevant studies with a total of 1,193 participants, then evaluated their quality, extracted the data and carried out a series of analyzes. The clearest pattern emerged when they compared performance during the early follicular phase – the "bad" time – with all other phases. Performance measurements included a variety of results, both in terms of strength and endurance, including race times, VO2 max and power output.

This is what this data looked like in the form of a forest plot. Each point below represents a single study. If it is to the right of the dashed vertical line, it means that the subjects in the early follicular phase performed better than at other times. If it's left, they did worse. The horizontal lines attached to each point show the uncertainty associated with each estimate. For example, a small study with a few subjects would have a very broad line. And the dot at the bottom shows the average of all individual studies.

sports medicine(Figure: Sports medicine)

Take a good look. Are there more points to the right or left of the line? Below are some studies that are far to the left, but otherwise it's a fairly even split. The average result indicates a slightly negative effect size, which means that the overall performance was worse in the early follicular phase, but the uncertainty interval overlaps zero. The researchers write that the size of the effect is "trivial". Furthermore, the large variation between studies – some positive, some negative – makes it almost impossible to draw general conclusions from this data.

There are a number of caveats that are worth acknowledging. The quality of many studies was judged to be poor, often because the methods used to assess the menstrual cycle phase were not reliable. The wide range of outcome measures could also be a problem: for example, certain cycle phases can increase your endurance but decrease your strength, which can contribute to the mixed results. Similarly, the subjects in the various studies ranged from sedentary to elite athletes who may have had different reactions. However, the zero result did not change if it only contained high quality studies (indicated by asterisks in the forest property above).

Finally, as expected, researchers are calling for more, higher quality research in this area to get better answers. At the moment, "the impact of these results is likely to be so small that they are meaningless to most of the population," they write. Athletes should consider their menstrual cycles and be aware of possible changes in performance, but they should not assume that the average results will apply to them. This message of customization was highlighted by the Canadian Olympic team's sports physiologist, Trent Stellingwerff, on Twitter: "I don't think there is nearly enough published evidence to suggest changes in diet and / or exercise recommendations during menstrual cycles," he wrote. "It is just as effective if athletes track period cycles with symptoms and performance metrics with pen and paper."

That seems to be an unsatisfactory conclusion. ("(W) We're not that special that there are 4 billion responses to our periods," one critic replied on Twitter. "That is absurd.") But, as Stellingwerff countered, people are incredibly variable and don't always fall into it properly Patterns with actionable insights. It should be remembered that the Warner quote about the weather doesn't really suggest that we should build a massive weather changing device. It is, in fact, a Warner profile from 1901 in Harper's Magazine highlighting that recognizes the "subtle irony of human senselessness". We still can't change the weather, but we've learned a lot about how to predict it since Warner's time. Again, this is probably the best approach, both for our collective understanding of performance fluctuations during the menstrual cycle and for individual athletes planning their training and competition plans: gather more data and look for patterns.

If you want to learn more about Sweat Science, visit me on Twitter and Facebook, subscribe to the email newsletter and read my book Endure: Mind, Body and the strange elastic limits of human performance.

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