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. Obliques Many people put a lot of focus on working their abs but forget their surrounding neighbors; the oblique muscles. Obliques serve as stabilizers, and are used in almost every physical activity. Exercising your obliques should be a very important part of your workout program. 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 Balance your upper body in a plank position on the stability ball using your forearms. Keep your back straight and your abs tight. Step 2 Lift your foot and bring your knee to the center of the stability ball. Be sure to keep your body rigid throughout the motion. Step 3 Return your leg back to starting position. Perform the same motion with your other leg.