The stability (or balance ) ball is wonderful for limited mobility. You can sit on it (which takes practice) and just bounce and it's aerobic. Add arm motions while you bounce if you like. When you just plain old sit on them they build your core muscles. You have to hold your abs tight to do that. If you lift them over your head while holding them tight they build other neck, arm and shoulder muscles. There are tapes and videos that come with them too. I worked for 6 months with my ball and the trainer and it's amazing how toned I got using that as the main exercise equipt.