Proprioception is the ability of a joint to sense its position. When you move your knee from one position to another, sensors in the joint, ligaments, and muscles tell you how far you have moved the joint and where it is now. Sometimes this "position sense" is damaged after an ACL tear. This can lead to further injury.
How is proprioception measured? In one test, the physical therapist moves the uninjured leg to a certain position. Without looking, the patient places the injured leg in the same position. The two legs should be fairly closely matched. Another way to measure proprioception is to measure how long it takes the patient to detect motion when the therapist begins to move the leg.
There are specific exercises that can help restore joint proprioception. These are included in physical therapy whenever a joint has been injured, whether it's the knee, ankle, shoulder, or another joint.