The ACL helps the knee joint to know what position it is in, a function called proprioception. This sense of joint position is very important for movement, joint stability, and function.
For example, when stepping down off a curb, the knee will bend and straighten just the right amount and at just the right speed to keep you from falling. Proprioception helps the knee joint do this. Joints, ligaments, and muscles have tiny sensors signaling the body about the joint's position. These sensors also start the reflex that tells the muscles to contract. Without this function of the ACL, a person may feel as if the knee is unsteady or unable to keep the leg from collapsing.