From Science Daily:For some reason, women tend to have knee motions that make them more susceptible to injury. Among other things, when landing from a jump their knees tend to collapse inward more than that of most men. They suffer significantly more ACL injuries during physical activity. "We're finding differences in nervous system processing that we believe are related to this," Johnson said. "The causes for those differences are unclear, but it may be due either to a biological difference, such as hormones, or a cultural difference such as different exercise and training patterns."