The anterior cruciate ligament (ACL) is one of four knee ligaments
that connect the upper leg bone (femur) with the lower leg bone (tibia) by
running crosswise inside the center of the knee joint. The ACL stabilizes knee
movement in a forward and backward direction.
Specifically, the ACL prevents the tibia from sliding or twisting abnormally on the femur when the leg is straight or slightly bent. And the ACL prevents the knee from being
stretched or straightened beyond its normal limits (hyperextended). It also
supports other knee ligaments that keep the knee from bending sideways.