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 on your back with the exercise ball tucked under your legs, your hands up by your head and your back pressed firmly against the mat. Step 2 Lift you knees up towards your chest by contracting your lower abs. At the same time crunch up towards you knees using your upper abs you lift your body up. Step 3 Lower your body and the exercise ball back down to the starting position and repeat action.