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 the stability ball between your ankles. Step 2 Keeping your lower back pressed against the floor, squeeze the ball and lift it up, extending your hands out to grab it. Step 3 Holding the ball in your hands, lower the ball behind your head. Perform this action in reverse, and repeat.