ANTERIOR CRUCIATE LIGAMENT INJURY ACL is the most commonly injured ligament of the knee. The injury is common during sports.Twisting of the knee is a common cause of over-stretching or tearing the ACL. When the ACL is injured one may hear a popping sound and the leg may suddenly give out. Besides swelling and pain, walking may be painful and the knee will feel unstable. Minor tears of the anterior cruciate ligament may heal over time, but a torn ACL requires surgery.After surgery, recovery is prolonged and low impact exercises are recommended to strengthen the joint.