After an ACL repair, my physical therapist is helping me get back my "proprioception." Just exactly what is this?

Proprioception is joint's sense of position and joint motion. For example, imagine you are blindfolded. The therapist moves your left big toe upwards 10 degrees. You could match that position with your right big toe without looking. You do this using proprioception.

The sense of joint position is important to protect the joint. Once the joint senses motion, the muscles contract to hold it steady. Scientists think that once the ACL is torn or damaged, the joint loses this protective sense.

Researchers at the University of Chicago found that proprioception improves gradually after ACL repair. This change can be measured around six months after the operation.