MUSCLES ENGAGED Abdominals The abdominal muscles (abs) provide movement and support to your core area. The deeper and closer to the spine the particular abdominal muscle is, the more effect over body posture it will have, and this often contributes significantly to a healthy back. Equipment Used Stability Ball A stability or exercise ball is a ball constructed of soft elastic and filled with air until firm.Most stability balls range from a diameter of approximately 35 to 85 centimeters (14 to 34 inches). Exercise Instructions Step 1 Start in a push-up position with your legs up on the stability ball. Make sure that your back is straight, your hips are tucked and your core is engaged. Step 2 Lift your hips upwards by contracting your upper abs. Raise your body up to a V or until your feet are in the middle of the stability ball. Step 3 Slowly lower your body back to the starting position and repeat.