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 Lie on your back with your legs up and knees bent at 90 degrees. Place a stability ball on top of your legs and have it rest on your shins. Step 2 Try to keep your lower body still and you cruch up towards your knees by contacting your upper abs. Step 3 Lower your body back onto the floor and repeat action.