I've blogged a few times in the past about ACL injuries, including Chipper Jones and Wes Welker. Both of these injuries were non-contact ACL tears. On Sunday, Leonard Weaver of the Eagles suffered a contact ACL injury when his knee was forced into hyperextension. You can watch the video here. I'll warn you, it is a pretty gruesome injury. His left foot is planted in the turf and the motion of his lower leg is stopped by the tackler as his upper left leg and body continue to move forward, forcing his knee into hyperextension and tearing the ACL. This is actually a good example of inertia (resistance to change in motion); his lower leg was stopped by the force applied from the defender, but his upper leg and upper body continued to fall forward due to their inertia (there was not a force applied to stop their motion), causing a bending moment at the knee, resulting in the hyperextension. Basically, the knee is not designed to bend in that direction, and when it does, an injury will occur. Many knee injuries in football are caused by blocking or tackling below the knee. There are some rules in place to prevent this from happening (chop blocks, clipping), but these injuries will always be a part of football.
