I have a 74 year old male client who is awaiting a knee replacement (the other knee has already been replaced). He has had numerous surgeries (quadruple bypass, brain stem, removal of fluid from his knee, etc.) and is a real trooper. He cannot stand for very long so I have been doing most of my workouts with him in a chair. He would like to build more leg strength so he can stop using his walker and use a cane but it is very difficult to work his lower body with strength building exercises. Any suggestions on what I should be doing to (1) build his leg strength up and (2) keep him from getting frustrated until he gets his knee surgery?