An ACL injury is a tear or sprain of the anterior cruciate ligament (ACL), which is one of the strong bands of tissue that help connect the thigh bone (femur) to the shinbone (tibia) . The ACL is one of the key ligaments that help stabilize the knee joint, and its most commonly torn during sports that involve sudden stops and changes in direction, such as basketball, soccer, tennis, and volleyball. ACL injuries most commonly occur during sports that involve sudden stops or changes in direction, jumping and landing, such as soccer, basketball, football, and downhill skiing. Many people hear a pop or feel a "popping" sensation in the knee when an ACL injury occurs. The knee may swell, feel unstable, and become too painful to bear weight. Depending on the severity of the ACL injury, treatment may include rest and rehabilitation exercises to help regain strength and stability, or surgery to replace the torn ligament followed by rehabilitation.