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 both of your arms and legs stretched out. Step 2 Pull one of your legs up and crunch up towards it with both of your arms. Step 3 Lie back down to the starting position and repeat.