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 by lying on your back with your arms and legs raised up in the air with the stability ball resting between them. Step 2 Using your opposite arm and leg, lower them to the ground while keeping the stability ball in place with your other arm and leg. Step 3 Return to the starting position, alternate sides and repeat the action.