It is a well-known fact that women athletes suffer more ACL injuries than men in the same sport. The reason for this remains unknown. Although many possibilities have been suggested, no one factor or combination of factors has been identified.

One group of researchers decided to measure the effect of muscle contraction on the knees. They looked at how stiff the knee becomes when the muscles contract in men and women. Knee stiffness protects the ACL. By tightening up the knee, the bones can't slide enough to tear the ligaments inside.

Surprisingly, the researchers found that men are able to tighten the knee joint to three times its normal stiffness. Women can only double the stiffness. This difference does not appear to be caused by muscle strength, body weight, or height. It may be that men use the muscles around the knee differently than women.

The study also showed that men use the hamstring muscles on the back of the thigh more than the quadriceps muscles in front to get this much knee stiffness. It may be that women can train the hamstrings to react faster and with more force to help protect the knee from injury. Further studies are needed in this area.