The Anterior Cruciate Ligament (ACL) is the primary restraint to forward motion of the shin bone (tibia).
The anatomy of the knee joint is critical to understanding this relationship. Mostly, the femur (thigh bone) sits on top of the tibia (shin bone), and the knee joint allows movement at the junction of these bones.
Without ligaments to stabilise the knee, the joint would be unstable and prone to dislocation. The ACL prevents the tibia from sliding too far forward.
The ACL also contributes stability to other movements at the joint, including the angulation and rotation at the knee joint. The ACL performs these functions by attaching to the femur on one end to the tibia at the other.
The other major ligaments of the knee are the posterior cruciate ligament (PCL), and the medial and lateral collateral ligaments (MCL and LCL, respectively).