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). Bosu The BOSU balance trainer stands for "Both Sides Utilized" and is most often used for balance training, various athletic drills and aerobic activities. Exercise Instructions Step 1 Start in a plank position with your feet on top of the Bosu. Make sure your back is flat and your core is engaged. Step 2 Push out the stability ball a bit before pulling it back in and meeting it with your knee. Step 3 Alternate legs and make sure to push out between every repetition.