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. Exercise Instructions Step 1 Start with the small of your back placed on top of the Bosu. Have your hands at the side of your head and one leg extended out in front of you. Step 2 Pull your knee in towards your chest and crunch up at the same time. Fully contract your abs at the top of the motion. Step 3 Extend your leg back out in front of you and open your body back up over the Bosu.