Women may be more vulnerable to injuring the knee’s anterior cruciate ligament (ACL) during the first half of their menstrual cycle, new study findings suggest.

It’s known that female athletes are up to eight times more likely to suffer an ACL tear than men are, and researchers have offered a number of possible explanations - including anatomical, muscular and training differences between males and females.

Some evidence also suggests that women’s menstrual cycles may be a factor.

The new study, published in the American Journal of Sports Medicine, found that female skiers were more than three times more likely to tear the ACL in the first couple weeks of the menstrual cycle than in the latter weeks.

The first half of the menstrual cycle, or preovulatory phase, begins on the first day of menstruation and continues for about two weeks. During this time, estrogen concentrations rise, while levels of another reproductive hormone, progesterone, fall.

However, the study findings do not mean that hormones are responsible for the ACL risk, stressed lead researcher Dr. Bruce D. Beynnon, an associate professor of orthopedics and rehabilitation at the University of Vermont in Burlington.

“We don’t know the mechanisms that are producing this,” he told Reuters Health. One question, for example, is whether neuromuscular function somehow fluctuates during the menstrual cycle, possibly increasing the risk of ACL injury at certain times.

Beynnon said his research so far indicates that certain factors, such as muscle strength, do not vary with the menstrual cycle.

But even if the mechanisms are not yet understood, it might be possible for women to cut their risk of ACL tears by focusing on factors that they can change, Beynnon noted.

Female skiers, for example, could learn how to fall in a way that best protects their knees.

And some studies have shown that when female athletes are specifically trained to run, jump and pivot more “like boys” - moving lower to the ground, and bending the knees properly when landing from a jump, for example - their risk of ACL injury drops.

One area where Beynnon sees room for improvement is in shoes and sports equipment, which he described as being “designed for males.” It may be possible to design shoes and equipment that specifically reduce the risk of ACL tears, which often happen in sports that involve sudden stops and pivots, like basketball, soccer and volleyball.

The study included 46 women who suffered an ACL rupture while skiing and 45 uninjured skiers who served as a control group. The injured women were all seen immediately at a ski clinic and had blood samples drawn to determine the phase of their menstrual cycle.

When skiing skills and experience were taken into account, women were still more than three times as likely to sustain an ACL tear during the preovulatory phase of the menstrual cycle.

However, Beynnon pointed out, the menstrual cycle is only one potential variable in women’s risk of the injury. Figuring out how to best deal with the multiple risk factors will be important, he said, not only in preventing the short-term consequences of ACL tears, but also the long-term risk of arthritis in the injured knee.

SOURCE: American Journal of Sports Medicine, May 2006.