Sculpt a Better Butt: Stability Ball Back Extension -- If you think this move sculpts only your back, think again: Your glutes do the work to pull your body up. -- Lie facedown on a stability ball, hands behind your head, feet against a sturdy object. Squeeze your glutes and lift your torso up until your body forms a straight line. Hold for one or two seconds. Slowly return to start. That's one rep.