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 in a push up position with your hands underneath your shoulders, your butt down and your back flat. Step 2 With your feet shoulder width apart, alternate between legs and cross them under the body. Step 3 Try to make contact with your opposing forearm and fully contract your abs at the top of the knee drive.