While the menstrual cycle is far more complex, to look at it simply would be to divide it into two halves: the follicular phase, which begins on the first day of menstruation; and the luteal phase, which begins after 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 against to a gentler peak during the luteal phase.
Estrogen is important and is a key factor in the studies. Aside from other things, it also decreases the stiffness of tendons and ligaments. It’s good news for those who are pregnant, as elevated levels of estrogen make childbirth possible. But conversely, it also leaves knees and other joints less stable.
Consequently, injuries tend to occur most often during the late follicular phase when estrogen is highest and ligaments are loosest. Studies found that knee joints get one to five millimetres looser during this phase, making the joint less stable. In the football study, only players with regular menstrual cycles were studied (hormonal contraceptives would eliminate the large variations in estrogen levels), and it was found that muscle and tendon injuries were roughly twice as likely during the late follicular phase.
Researchers were quick to note that while it’s clear that hormonal fluctuations do have an impact on muscles and body functioning, these depictions of how estrogen might affect injury risk don’t yet capture the full complexity of life, certainly not in a sense that any guidelines on the menstrual cycle and injury risk mitigation could be ascertained. They simply suggest that female athletes should track their periods, so they’re at least aware of what’s happening with their bodies. And above all, we must listen to our bodies on the day. If the feelings of fatigue are present, listen to them - it doesn’t make you weak, it simply makes you a smart athlete.