Lying face down on top of a stability ball, walk the arms out slowly until the lower legs are resting on top of the ball. Bring one knee off the ball and bend it under the chest. Keep the neck straight and spine in a neutral position. With the leg that is on the ball, pull the knee in towards the chest, bending the knee while kicking the opposite leg straight out over the ball. Continue alternating these leg positions for the desired number of repetitions before switching legs.