Female athletes appear to be at greater risk for tears to their anterior cruciate ligament during the preovulation phase of the menstrual cycle, according to the National Athletic Trainers Association.

The ACL is a rubber band-like fiber that is attached to the femur in the upper leg and the tibia in the lower and stabilizes the knee. The Post-Gazette reported May 28 that girls are five to eight times more likely to suffer ACL tears while playing the same sports (chiefly soccer) that boys play.

Doctors aren't certain why. One hypothesis is that girls bend and flex their knees less while running and jumping than boys do.

"There are differences in muscular proprioception, our body's ability to sense our body's joint position in space without our thinking about it," said Dr. Patrick DeMeo, director of the division of sports medicine at Allegheny General Hospital and medical director of the Pittsburgh Pirates. "Because of that, girls may put their knees more at 'at-risk' positions. Their legs are more straight than they are bent."

Female hormones also contribute, said Dr. Moira Davenport, a sports medicine and emergency medicine physician at AGH.

"Levels of estrogen may contribute to laxity of the (anterior cruciate) ligament," she said.

Medical researchers who've studied ACL injuries haven't taken sex-based differences sufficiently into account, said Sandra Shultz, the lead author of a consensus statement published last month in the Journal of Athletic Training, the scientific publication of the National Athletic Trainers' Association.

Research conducted to date suggests the mechanical and molecular properties of the ACL likely are influenced not only by estrogen, but also by the interaction of several sex hormones, processes within the ligament cells and mechanical stresses, the consensus statement said.

"When examining hormonal influences on knee joint function, women using oral contraceptives, and those with irregular menstrual cycles should also be investigated by researchers," said Ms. Shultz, a certified athletic trainer and associate professor of exercise and sport science at the University of North Carolina-Greensboro.

Balance and plyometric training can reduce the likelihood of ACL injuries, but the benefits appear to be transient, the statement said.

"While we work to identify the risk factors for ACL injury, we should also continue to conduct controlled studies to evaluate the ability of prevention strategies to alter risk factors and prevent ACL injuries," Ms. Shultz said.