Studies have shown that women are two to eight times more likely than men to suffer ACL tears. Such injuries often require surgery and up to a year of rehabilitation.
Theories abound over why women suffer more knee injuries, but there has been little objective proof.
The U-M research team, led by orthopaedic surgeon Edward M. Wojtys, M.D., evaluated 40 young females with acute ACL injuries and discovered a disproportionately high number of the injuries occurred during the ovulatory phase of the athlete's menstrual cycle. The ovulatory phase typically occurs during days 10-14 of the cycle and is marked by a significant rise in estrogen levels as well as high levels of a hormone called relaxin. Wojtys says these hormone fluctuations may be a significant factor in the high number of ACL tears suffered by women, although it has not been ascertained precisely how estrogen contributes to knee injuries. The researchers suspect estrogen and relaxin have profound effects on women's neuro-muscular systems and the mechanics of soft tissue such as ligaments and tendons. The link between ACL injuries and the hormonal cycle has big implications for female athletes and raises the question of whether that cycle can be manipulated.