sit in a chair that is high enough from the floor that you can do leg lifts. do as many as you can without causing pain, then take a break and do the other leg. or you can do both at a time and that will help strengthen your abdominal muscles as well as the "butt". any type of exercise that you can do over normal daily activity will help boost your metabolism. you could also stand about 3 feet from a wall and place your hands on the wall about chest height and lean into the wall and push off the wall, like a push up. these would be knee friendly too. hope this helps.