Lie face-up with your arms pressed into the floor by your sides, knees bent, heels placed on top of the ball. Press your heels into the ball, brace your abs in tight, and lift your hips up off the floor. Keeping your hips high, slowly extend your legs and roll the ball away from your body (be careful not to lock out your knees). Bend your knees and curl the ball back into your body, returning to the start position. Repeat for all reps.