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 feet up while holding the medicine ball in both hands. Step 2 Lower both legs to the floor and pull them back up without your lower back coming off the floor. Step 3 Crunch up with the medicine ball lower back down and repeat.