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 with your hands behind your head, one knee tucked in to your chest and the other leg stretched out. Step 2 Pull in your opposite knee as you stretch out your leg. Step 3 Alternate between legs for the recommended number of reps.