My feet! and good walking shoes. The feet were inexpensive and the shoes last for years! All jokes aside, I just walk, walk, walk. With a bad back, it is the best exercise for me and pretty much injury proof. I bought an bike, but I find it hard to get interested in doing that type of exercise. When the weather is inclimate, I bought a Zumba tape to try since I like to dance. When winter sets in, I'll have to update you and let you know how the Zumba is going. Good luck.