This is an exercise you can do with or without a stability ball. In fact, many of the stability ball exercises can be performed without a ball! But using a ball creates an unbalanced environment, forcing you to use your stomach muscles to keep your body in line. No matter what you’re doing with the ball, you constantly have to activate your core to stay stable. I love doing push ups with the ball. A regular push up requires you to use your core muscular to keep your back flat (something a lot of us forget to do!), but with a ball- you have not choice but to use your core!