ROSEMONT, IL - Women who participate in jumping and pivoting sports, such as basketball, volleyball, and soccer, are up to eight times more likely to rupture the anterior cruciate ligament of the knee than are same-size men participating in these same sports, according to a study in the May 2003 issue of The Journal of Bone and Joint Surgery.

Edward Michael Wojtys, M.D. Professor of Surgery, Sports Medicine

Ligaments are tough, complex structures that hold bones together. The cruciate ligaments crisscross in the knee joints and provide stability to the knees. The cruciate ligament located toward the front of the knee is the anterior cruciate ligament (ACL), and the one located toward the rear of the knee is called the posterior cruciate ligament (PCL). Athletes can injure or rupture the cruciate ligaments by pivoting or changing direction rapidly, landing from a jump, or slowing down from running. Voluntarily contracting the muscles supporting the knees stiffens the knees and may protect the ligaments. The lower the stiffness of the muscles bracing the knee joint, the greater the load that the ligaments must bear. Thus, insufficient muscular stiffness increases the risk for damage to the knee's ligaments.

Because both the cross-sectional area and the strength of the leg muscles are known to be greater in men than in women and because muscle stiffness increases during muscle contraction, researchers at the University of Michigan tested the hypothesis that healthy young women are not as able to increase the stiffness of the knee voluntarily by activating the knee muscles as are same-size men participating in the same sport. Edward M. Wojtys, MD, Laura J. Huston, MS, Harold J. Schock, BS, James P. Boylan, BS, and James A. Ashton-Miller, PhD theorized that men might be able to use their knee muscles to protect the cruciate ligaments more effectively than women. The authors are part of the Medsport program in the Orthopaedic Surgery Department in the U-M Health System

The investigators compared 24 athletes (12 women and 12 men) who were competing in sports associated with risk of ligament injury to 28 endurance athletes (14 men and 14 women) active in sports with a low risk of such injuries (bicycling, crew, and running). They matched pairs of men and women for age, height, weight, body mass index, shoe size, and activity level. Pairing by these criteria downplayed size differences and allowed a clearer evaluation of gender variations in knee joint stiffness.

Devising several tests to measure knee stiffness, the researchers found that voluntary increase in knee stiffness after contracting knee muscles during pivoting, jumping, and turning sports was greater in men than in women participating in the same sports. Interestingly, women competing in non-pivoting sports exhibited a higher increase in knee stiffness on muscle contraction than either men in non-pivoting sports or women in pivoting sports.

"Historically, training has been the same for both men and women," states Wojtys. "This may not be the correct approach." He concluded that there are physical differences in knee function between men and women and that researchers need to ask, "What is it about the female physiology that is different from that of same-sized men?"

The scope of this study speculated about, but did not explore, the reasons for the gender differences other than to theorize that men and women may activate the knee muscles differently during athletic activities. According to Wojtys, researchers are conducting additional studies to determine gender dissimilarities and responses to training.

The investigators concluded, however, that, because women athletes exhibited less muscular protection of the knee ligaments during pivoting and jumping sports than men did, improving active muscle protection of the knee during training may help decrease rates of knee injury among women athletes. Further studies will try to pinpoint specific protective measures. In addition, Wojtys emphasized that coaches and trainers should rethink training programs for women athletes based on an awareness of the female knee physiology.

The Journal of Bone and Joint Surgery (JBJS) is a publication of the 26,000-member American Academy of Orthopaedic Surgeons, a not-for-profit organization that provides education programs for orthopaedic surgeons, allied health professionals, and the public. The peer-reviewed JBJS, located in Needham, Massachusetts, is published monthly. Abstracts are available online at http://www.jbjs.org.

An orthopaedic surgeon is a physician with extensive training in the diagnosis and nonsurgical as well as surgical treatment of the musculoskeletal system including bones, joints, ligaments, tendons, and nerves.

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