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 Bosu The BOSU balance trainer stands for "Both Sides Utilized" and is most often used for balance training, various athletic drills and aerobic activities. 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 on the Bosu with the small of your back place right on the centre with your feet placed wider than shoulder width in front of you. Step 2 Sit-up on the Bosu without using any momentum and press the medicine ball up towards the ceiling. Step 3 Roll back down on to the Bosu and fully stretch open your abs.