The physical therapist did a test on my knees before I had a total joint replacement. While blind folded, I pushed a button anytime I felt the knee joint move. What does this information tell him?

It sounds like the therapist was measuring joint position sense. This is called proprioception. The joint is bent or straightened by the therapist and the patient tells when movement is first felt.

Right now researchers are trying to find out more about proprioception. What's normal? Does it change as we get older? Is it the same for men and women? What happens when the joint becomes arthritic? Scientists hope to find an exercise program to restore proprioception--maybe even reverse the effects of arthritis.