What is the primary role of the ACL in the knee?

The primary role of the anterior cruciate ligament (ACL) in the knee is to prevent the forward movement of the tibia relative to the femur. This ligament is crucial for maintaining stability in the knee joint, especially during activities that involve sudden changes in direction, jumping, and pivoting. When the ACL is intact, it helps control the motion of the knee and prevents injuries during dynamic movements.

While stability during lateral movements is important and the ACL contributes to that stability, its main function is specifically to limit the anterior translation of the tibia. Flexibility of the knee joint is not a direct role of the ACL, as ligaments primarily act to stabilize rather than provide flexibility. Support during running involves multiple structures of the knee, including muscles and other ligaments, so it cannot be solely attributed to the ACL. Thus, the correct response highlights the specific and critical function of the ACL in preventing the forward movement of the tibia, ensuring the knee remains stable during high-impact and dynamic activities.