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 Start in a push up plank position with your hands on the bench, arms slightly wider than shoulder width apart and your feet on the stability ball. Step 2 Keeping your core engaged and your abs contracted, curl your knees in towards your chest without lifting your hips. Step 3 Return your feet back to the starting position maintaining a tight core and repeat action.