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 legs straight and your hands over your head. Step 2 Pull your legs up to 90 degrees and crunch up to your shins at the same time. Step 3 Lower your arms and legs back down. Make sure to keep your back pressed firmly to the ground.