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 with the small of your back on the stability ball, your knees bent and your feet shoulder width. Step 2 Crunch up and exhale as you fully contract your abs. Make sure that the stability ball does not move underneath you. Step 3 Inhale as you open your abs back up over the stability ball and make sure that your tailbone stays in contact with the ball throughout the entire range of motion.