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). Bench A weight training bench is a piece of equipment that has a resemblance to a normal (e.g. park) bench, but is cushioned and designed for use in the weight room. Exercise Instructions Step 1 Lie on your back on top of the bench holding the stability ball between your hands and your knees. Step 2 While exhaling, slowly lower one of your legs and the opposite arm. Be sure to contract your abs at the full extension of the movement. Step 3 Raise your arm and leg back to the starting position, alternate to the other arm and leg and repeat action.