Biomechanics of the anterior cruciate ligament

The principal role of the anterior cruciate ligament (ACL) is to resist anterior displacement of the tibia on the femur. The ACL also has a major role in resisting internal rotation. The posterior fibers are longest, or most tense, in extension, and are therefore the major resistance to hyperextension and are more susceptible to injury in extension. The anterior fibers are longest, or most tense, in flexion and therefore are not susceptible to injuries with the knee flexed. The ACL is strong and stiff, yet because of its unique structure, it is precariously susceptible to injury.