The ACL acts within the knee alongside the posterior cruciate ligament (PCL). Together they form a cross-shaped structure which is vital for holding the knee joint in place. Without these two ligaments in perfect health, your knee stands a high chance of giving way beneath you at zero notice.
The ACL is most commonly injured by the forces generating during twisting motions. It is one of the most often seen sporting injuries as players try to change direction quickly while their feet are planted firmly on the ground. This scenario creates a shearing effect that tears the ACL apart.