After trauma to the body we compensate movement to avoid pain. This leads to avoidance of using certain muscles. If we avoid using a muscle for an extended period our brains will decide that muscle is no longer needed and stop sending electrical signals to that muscle and it will waste away. Thus the importance of quality physio therapy. This is very important following total knee replacement. I learned this when my physio therapist starting me working with various mechanical contraptions that require me to lift, push and pull 70 lbs with my legs.