Work the middle abs and obliques. For these, do bicycle crunches. Begin on your back with your feet in the air, shins parallel to the ground and knees bent to 90 degrees. Interlock your hands behind your head, keeping your elbows out wide. Straighten your right leg, right foot in the air, and lift your chest and right shoulder to your left knee. Come back to the center and straighten the left leg, bringing your left shoulder toward your right knee. Keep going from side to side, keeping your chest lifted and elbows wide next to your ears.