Lie on your back with your knees bent and your arms on the floor alongside your body. Place the sole of one foot on top of the medicine ball. Raise your butt off the floor making a straight line from your knee to your shoulder. Raise your hips even higher and drive your opposite knee toward your chest. Lower the leg, placing this foot on the ball and driving your other knee toward your chest. Don't let those hips sag at any time.