Actually, as long as they have something resembling arm or leg muscles, they can do isometric exercises. A simple one is to lift a small weight, no more than 3 to 5 pounds, with an outstretched arm parallel to the body as much as possible. Keep the arm straight and lower the weight without putting the arm down completely. Then raise the weight again slowly to the parallel position. You'd be surprised at what a good workout that is for the upper arm muscles. For legs and knees. Stand with the legs positioned slightly apart, with the back as straight as possible. Bending only the knees, keeping the back straight, slowly squat down, but only a short distance, and then slowly straighten the legs again. It doesn't seem like much, but again, a hell of a leg workout. Very slow and low impact. And easily modified for people who have mobility limits. I know dozens of "at your desk" exercise routines. And ones for people who are bedridden. It helped me heal, and lose weight, while I was bedridden for 2 months after an accident.