AS THE COVID-19 pandemic abates and athletes around the world prepare to return to the sports field, new light is being shed on one of the gravest risks of injury they face. The anterior cruciate ligament (ACL) is one of four that hold the knee together. Tearing it, as at least 2m people do every year, is one of the most immobilising injuries a person can sustain. ACL tears are usually a consequence of an awkward movement during a sport such as football. The surgery and rehabilitation needed cost billions of dollars a year. It is becoming increasingly clear just how devastating—and possibly preventable—this injury really is.

Doctors have always recognised ACL tears as serious, but used to think that returning to competition was possible six months after surgery. Elizabeth Gardner, head orthopaedic surgeon at Yale University Athletics, reckons, however, that nine months to a year is more realistic. That is a significant chunk of an athlete’s career. Even then, surgery alone is no guarantee of recovery. Re-tear rates are as high as 20%, “a lot higher than you would expect for a surgery that we think we do really well and is so ridiculously well studied”, Dr Gardner says. On top of that, recent investigations suggest as many as three-quarters of those who suffer an ACL tear go on to develop arthritis of the knee 15-20 years later.

ACL tears are, moreover, increasing in frequency. In 2018 researchers at Boston Children’s Hospital found that, over the course of a decade, the number of them relative to other orthopaedic problems had tripled amongst Americans aged under 18. This may be a consequence of moves towards single-sport specialisation in American schools. The lack of variety of body-movements thus engendered tends to focus strain repeatedly on particular parts of the body. Recent studies in countries around the world, including Australia, Finland and Norway, support the idea that more young people than ever before are sustaining ACL injuries. Australian sports authorities in particular are trying to discourage such specialisation.

One of the most curious features of ACL injuries, though, is that they afflict women far more often than men—as much as eight times more, some investigations suggest. Why this might be is the subject of intensive research. But a clue lies in an apparent connection with the menstrual cycle.

A study published in 2013, of a group of women skiers in the Alps, for example, found that those in the pre-ovulatory stage of the cycle were more than twice as likely to suffer an ACL tear than were those in the post-ovulatory stage. A four-year survey of 113 female England footballers, published in March, also found a clear correlation. Muscle and tendon injuries were far more common in the late follicular phase of the cycle, just prior to ovulation, than in the other phases.

The reason for this menstrual-cycle link is unclear. The ACL has oestrogen receptors, which might help to explain what is happening. But it is not unique among ligaments in this, and the receptors’ job is, in any case, obscure. Levels of oestrogen in the body do spike just before ovulation—the point when tear-frequency rises—but uncertainty remains about the exact link.

Other contributory factors to women’s higher ACL tear rate may be female body shapes and movement patterns. Compared with men, women have wider hips, more inverted knees and “over-dominant“ quad muscles (meaning that the quadriceps femoris muscle group in front of the thigh bone is relatively stronger than the hamstring group behind it). All these factors put pressure on the elaborate workings of the knee joint. Women also tend to land in a more flat-footed manner than men do, and to pivot more awkwardly.

Anatomy is what it is, so not much can be done about sex differences in hip shape and knee orientation. All athletes, though, can be trained how to move more safely, and this is particularly relevant for women. Straightforward exercise classes in balance and agility have been found to reduce ACL tears by 50%. Strengthening the muscles around the knee—especially the hamstring—by focused exercises, is another way to reduce the chances of a tear.

Dealing with the menstrual cycle has, in the past, been trickier, since it requires that individual athletes keep detailed track of their cycles. This, though, has been made easier by modern gadgetry. For example, the American women’s football team (real football, not the gridiron sort), who are the current world champions, use FitrWoman, an app that monitors a user’s cycle and tells her on which days it may be risky to train intensely. This is something that the team’s former high-performance coach, Dawn Scott, reckons contributed to their retention of the World Cup in 2019. And back in football’s home country, England, FitrWoman is also making a mark. The women’s team at Chelsea football club, who won this year’s Super League, have adopted the app as well.

This is the sort of thing that could be encouraged more widely, by introducing it into school and college sports. That does, though, require awareness of the problem—and this is still lacking. In 2019, for example, St Mary’s University, Twickenham, in England, conducted a survey of more than 14,000 female athletes. It revealed that 81% of those with coaches never discussed the impact of their menstrual cycles on training with them. Nearly three-quarters said they had received no education regarding the cycle’s impact on their exercise regime and fitness, and vice versa.

The topic of menses can be taboo even at the highest level of sport. In 2015 Heather Watson, a British tennis player, withdrew from the Australian Open citing “girl things”. She was applauded for publicly linking her performance to her period, even in the vaguest of terms, which is still a rarity among top athletes. From the top to the bottom of women’s sport, then, a bit more openness and physiological realism might work wonders for knee preservation and the avoidance of problems in later life. Considering the rigour of modern, professional athletes’ training regimes, it is surprising this is not happening already.