Lie on your back with your arms straight at your sides, palms flat on the floor, and heels on top of a stability ball.
Press your heels into the ball, then tighten your core and butt muscles. Slowly raise your hips, and draw your heels – and the ball – toward you as far as you can. Pause, then roll the ball back by straightening your legs; your hips will naturally lower back down. Repeat.