ACL Injury – Anterior cruciate ligament (ACL) injuries are one of the most common knee injuries, and are typically seen in sports that involve change in direction or landing from a jump. The mechanism is often in a noncontact situation when the athlete feels that the give way or collapse, sometimes hearing a pop. Contact injuries usually occur when the foot is planted at an external force is suddenly applied to the knee. Swelling usually appears within a couple of hours.