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 Place both feet on top of the stability ball. Hook your heels into the stability ball and use it to anchor your lower back to the floor during the crunches. Step 2 Keep the stability ball tucked close to your body with your legs. Pull your knees in towards your chest while crunching up towards the ball. Step 3 Lower yourself back down until your shoulder blades touch the floor and repeat.