By some estimates, 200,000 people tear their anterior cruciate ligaments (ACLs) in the United States each year, the vast majority of them women. And the timing of these injuries isn’t coincidental. For more than two decades, researchers have known that ACL tears are more likely during certain phases of the menstrual cycle, presumably because changing hormone levels affect ligament properties.
It’s not just ACLs: a new study in Frontiers in Physiology, from researchers at the University of Lincoln, Nottingham Trent University, and The Football Association, followed women’s national-team soccer players from England over a four-year period, and found evidence that muscle, tendon, joint, and ligament injuries are unevenly distributed across the menstrual cycle. Hormones clearly affect injury risk—but the tricky part is figuring out what the mechanisms are and what to do about it.
The key factor appears to be estrogen, which has broad effects on the body including decreasing the stiffness of tendons and ligaments—a handy trick that helps make childbirth possible but leaves knees and other joints less stable when levels are elevated. In the simplest terms, you can divide the menstrual cycle into two halves: the follicular phase, which begins on the first day of menstruation; and the luteal phase, which begins with ovulation. Estrogen is lowest at the beginning of the follicular phase, then rises to its highest peak shortly before ovulation. Then it drops sharply, and rises again to a gentler peak during the luteal phase.
With this in mind, you’d expect ACL injuries to occur most often during the late follicular phase (sometimes called the ovulatory phase), when estrogen is highest and ligaments are loosest. Studies have found that knee joints get one to five millimeters looser during this phase, making the joint less stable. And that pattern of injuries is, indeed, what’s generally observed for ACL tears. But it’s not obvious that the same pattern should hold for other injuries. A 2019 article by Nkechinyere Chidi-Ogbolu and Keith Baar of the University of California Davis argued that laxer tendons might actually decrease the risk of muscle injuries, because they would be better able to absorb some of the impact of jolts that might otherwise strain or tear a muscle.
The new soccer study, led by Nottingham Trent’s Ian Varley, followed players ranging from England’s under-15 national team to its senior squad, tracking injuries during all training camps and competitions over four years. Only players with regular menstrual cycles who did not use hormonal contraceptives were included in the analysis, since contraceptives eliminate the large variations in estrogen levels. A total of 156 eligible injuries from 113 players were observed.
Surprisingly, only one player suffered an ACL rupture during the study period, and she was taking oral contraceptives so was not included in the analysis. Muscle and tendon injuries were roughly twice as likely during the late follicular phase (with estrogen leading to lax tendons and ligaments) compared to the other phases. Conversely, joint and ligament injuries were significantly less likely during the late follicular phase, though the smaller number of these injuries (24 in total) makes that observation weaker. I didn’t get that backwards: this is exactly the opposite of the hypothesis (lax ligaments bad, lax tendons good) I outlined two paragraphs ago.
There was one other surprising detail: 20 percent of the injuries occurred when a player’s menstrual cycle was “overdue,” based on when they expected their next period to start. That’s particularly surprising because the overall proportion of time spent while overdue is relatively small. Irregularity from cycle to cycle is common even among women (like those included in the study) who report regular menstrual cycles, but it may be that some were on the border of menstrual dysfunction associated with Relative Energy Deficiency in Sport. That condition, which is an updated definition of what used to be known as the “female athlete triad,” involves persistent calorie deficits and leads to problems including missed or irregular periods and lower bone density—and heightened overall injury risk, which might be what happened here.
What do we do with this somewhat unexpected information? “As this research is in its infancy,” the researchers caution, “we do not recommend that this data is used to inform exercise practice or participation as further work is needed before clear guidelines on the menstrual cycle phase and injury risk mitigation can be generated.” Fair enough. It’s clear that the hormonal fluctuations matter, but it’s also clear that the simplest models of how estrogen might affect injury risk don’t fully capture the complexity of real life.
It may be tempting to see hormonal contraceptives as a ligament protector, since they suppress the highest peaks in estrogen. There’s some evidence that this is indeed the case: for example, a 2014 study found that oral contraceptive users were about 20 percent less likely to need an ACL operation than non-users. But, as Chidi-Ogbolu and Baar point out, there are trade-offs: high estrogen levels also promote muscle-building and the repair of muscle and tendon in response to training. You can start to formulate schemes for avoiding contraceptives during training then using them during the competitive season, but the evidence is awfully thin for that kind of decision-making.
For now, Varley and his colleagues start with one straightforward practical suggestion: female athletes should track their periods, so that they’re at least aware of what factors may be at work on any given day. That’s pretty much the same conclusion drawn by researchers who recently investigated the effects of menstrual cycle on athletic performance. Knowledge is power. It’s not as much power as we might like, and more research is urgently needed, but it’s a start.
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