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 Hold the stability ball between your legs and slowly lower your legs to the ground while exhaling and contracting your lower abs. Step 2 Bring the ball back up and the crunch up towards the ball by contracting your upper abs. Step 3 Lower your body back down to the mat then start lowering the stability ball back towards the floor and repeat action.