Starting Position: Sit on a properly inflated stability ball (one that compresses approximately 6" under your body weight) with your feet flat on the floor. Slowly lean backwards while walking yourself out along the ball until your shoulders, back and tailbone make contact on the ball. Adjust your leg position to create a 90 degree bend at the knees with your thighs parallel to the floor and positioned hip-width apart, and feet facing forward with your weight distributed evenly through your feet. Your mid-back should be positioned on the top of the ball (at 12 o'clock) and your hips should be positioned at 2 o'clock.
Place your hands behind your head, squeezing your scapulae (shoulder blades) together and pulling your elbows back without arching your low back. This elbow position should be maintained throughout the exercise. Align your head with your spine, but allow it to move into slight flexion (moving the chin towards the chest) during the upward phase of the exercise.
Upward Phase: Exhale, contract your abdominal and core muscles, and flex your chin slightly towards your chest while slowly curling your torso towards your thighs. Since the abdominals attach the rib cage to the pelvis, your movement should focus on pulling these two body parts closer together (the neck stays relaxed while the chin is tucked towards the neck and focus on pulling the bottom of the chest towards the top of the pelvis). Your feet should remain firmly planted, and your tailbone and lower back should remain in contact with the ball at all times. Continue curling up until your upper back is lifted off the ball. Hold this position briefly while maintaining your balance.
Downward Phase: Gently inhale and slowly uncurl (lower) your torso back towards the ball in a controlled fashion keeping your feet firmly planted, and your tailbone and low back in contact with the ball.