The ACL tear is a common injury and, while it happens most often in sports, it is the result of a sudden stop and twisting motion, or if the front or side of the knee receives a sudden blow. In sports, it’s often the result of an athlete running and coming to a quick stop and then changing direction at the same time. Pivoting on one leg, overextending the knee joint, or a hard landing from a jump can also cause the damage.