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 Medicine Ball A medicine ball (also known as an exercise ball, med ball, or fitness ball) is a weighted ball roughly the diameter of the shoulders (approx. 14 inches). Medicine balls come in a variety of weights from 2lbs. to 30lbs. and can be made of hard plastic, dense rubber or synthetic leather material. Exercise Instructions Step 1 Start in a plank position with both hands on the medicine ball, your hands under your shoulders your lower back flat and one leg raised in the air. Step 2 Pull your leg up into your chest and fully contract your abs. Step 3 Place push your leg back out to the starting position and repeat.