Lying face up, keep both legs straight and rest the heels on top of a stability ball. Press through the heels and raise the hips off the ground. Lift one leg straight up into the air directly up from the hips, and drive the other heel into the ball. With the leg on the ball, bend at the knee and drive the hips into the air while pulling that heel in towards the body. Slowly straighten that leg until the body is in a straight line, but keep the other leg straight up in the air.