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 on your back with your legs on top of the stability ball. Step 2 Tuck the stability ball in tightly with your legs to keep your back pressed down firmly to the ground. Step 3 Crunch up towards your knees and fully contract your abs.