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 in a push-up position on top of the stability ball. Make sure that your hands are underneath your shoulders and your back flat. Step 2 Drive your knees into the stability ball staying light on your toes. Keep your core engaged. Step 3 Continue for a recommended number of repetitions or length of time.