Injured Knees are More Common in Girl.  

The reason why is because when girls grow there ligaments get loose as there bones grow.  This is a proven fact by scientist that girls tear there ACL more than guys do.  Your ACL is not a bone it is a ligament. If you stretch or tear your ACL you won't be doing anything for a while.  Girls are six times the worry to tear there ACL than a boy because they either stretch there ligament to far and it can't handle the pressure  or they turn there leg the wrong way and it just snaps basically.  Here's a picture of a torn ACL.