Abdominal crunch legs on stability ball supine

Lie on your back with your knees bent and your lower legs resting on a stability ball.
Place your hands across your chest.
Push your lower back down into the floor.
Tighten your stomach and pelvic floor muscles.
Gently lift your head and shoulders a little way off the floor.
Slowly lower back down.
Try to keep the ball still during the movement.