Starting Position: Lie supine (on your back) on a mat, placing the backs of your lower legs and heels on the top of a stability ball. Position your feet hip-width apart on the ball with your ankles slightly dorsiflexed (toes pointing towards the ceiling). Gently contract your abdominal/core muscles to flatten your low back into the floor. Attempt to maintain this gentle muscle contraction throughout the exercise. Extend your arms out to your sides with palms turned to the floor to help stabilize your body during the exercise.
Gently exhale while holding your abdominal contraction and press your hips upwards off the floor into extension by contracting your glutes (butt muscles). At the same time press the backs of your lower legs and heels into the ball for additional stability. Avoid pushing your hips too high as this generally increases the amount of hyperextension (arching) in your low back. Maintaining your abdominal contraction helps avoid excessive arching in your low back. Continue to press upwards until your legs and hips are straight with your torso and legs in alignment.
Upward Phase: Exhale and slowly contract your hamstrings to move your heels towards your hips while plantar flexing (toes pointed away from your shines) your ankles to rest the soles of your feet on top of the ball. Continue to pull your heels towards hips, raising your hips further off the floor. Maintain a stable torso, keeping it parallel with your upper thighs.
Lowering Phase: Inhale and slowly lower yourself back towards your starting position.
Exercise Variation (1): To increase the balance challenge, move your feet closer together on the stability ball.
Exercise Variation (2): To increase the balance challenge, move your arms towards your side or place them on your chest to reduce your stabilizing points of contact with the floor.
Exercise Variation (3): To increase the balance challenge, raise one leg off the ball in the starting position and curl with one leg.