An ACL injury refers to damage or tearing of the anterior cruciate ligament (ACL), which is one of the major ligaments in the knee. The ACL plays a crucial role in stabilizing the knee joint and controlling its movements, particularly during activities that involve pivoting, jumping, and sudden changes in direction. Symptoms: A person with an ACL injury may experience: A popping sensation or sound at the scene of the injury. Immediate knee pain and edoema. Having trouble putting weight on the hurt leg. Instability or a sensation that your knee is "Giving way."