Keeping a Stiff Upper Knee: Why Men Are Less Prone to ACL TearsIt's a well-known fact that women athletes injure the anterior cruciate ligament (ACL) of their knees more often than men. Many explanations have been suggested. Perhaps the training of men and women is different enough to increase risk for ACL injury in women. There are gender differences in hormone levels, knee structure, hip width, and the angles at which muscles attach around the knee. However, no single cause or combination of factors has been proven.
One of the functions of the ACL is to keep the larger bone in the lower leg (tibia) from sliding forward of the upper leg bone (femur). This movement of the tibia at the knee is called tibial translation. The ACL keeps forward tibial translation in check. Contracting the muscles around the knee increases the stiffness of the knee. This also reduces forward tibial motion and offers additional protection to the ACL.
Since scientists haven't been able to pin gender differences on anything within the joint, they decided to test the muscles around the joint. Researchers measured how much tibial translation occurs in men and women with and without the muscles around the knee contracted. In this way, researchers could tell how much of the knee stiffness was from muscles contracting and how much came from other structures in the knee. They found that men were able to produce more stiffness in the knee than women. Women could double the stiffness of the knee by contracting the muscles, but men could more than triple the knee stiffness.
There were no significant differences in actual muscle strength between men and women. There were no differences in knee stiffness based on height or muscle strength. The only difference was by gender. One more interesting observation was noted. Women used the quadriceps muscle (front of the thigh) more than men did. Meanwhile, men activated the hamstring muscle (back of the thigh) more than women did.
Men and women achieve different amounts of knee stiffness by contracting the muscles around the knee. Men and women also activate different muscles to stiffen the knee--hamstrings for men versus quadriceps for women. Can these patterns be changed by muscle training or conditioning exercises? That's the next question for researchers to answer.
Edward M. Wojtys, MD, et al. A Gender-Related Difference in the Contribution of the Knee Musculature to Sagittal-Plane Shear Stiffness in Subjects With Similar Knee Laxity. In The Journal of Bone and Joint Surgery. January 2002. Vol. 84A. No. 1. Pp. 10-16.
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