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 you back with you legs straight and up at a 90 degree angle. Step 2 Slowly lower both legs down, while contracting your abdominals and keeping your lower back pressed against the floor. Step 3 Lift legs back up to starting position, keep your legs straight and repeat the action.