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. Exercise Instructions Step 1 Start on your back with your feet together and your heels tucked in close to your body. Step 2 Sit all the way up and touch in front of your feet to make sure that your shoulders pass your hip. Step 3 Lie back down and fully extend your abs.