Ligamentous laxity has been linked with athletic injury. It's commonly believed that knee hyperextension puts people at risk for anterior cruciate ligament (ACL) injury. There isn't much to back that idea up. Only one study has been done in this area.
Researchers at the SUNY Upstate Medical University in New York reported on the effect of knee hyperextension and ACL injury. They looked at joint mobility among college athletes.
White men and women were included. They found both knee and elbow hyperextension were common among athletes who had noncontact ACL tears.
Women have more ACL injuries than men--even when both have generalized ligamentous laxity and hyperextended joints. So female gender is a risk factor, too.