How to: Using a stability ball (55cm for most), lay on your back placing both heels on the center of the stability ball. Elevate your hips off the floor driving through your glutes (butt muscles), creating a straight line through your ankles, knees, hips and shoulders. You should feel most of the posterior chain working together and engaged throughout. Slowly roll the ball towards your butt, driving through your hamstrings, while keeping the glutes, etc. firing. Then slowly roll the ball back to the starting position, drop the hips back to the floor and repeat.