The abs you think of first, the muscle that becomes the six-pack, or washboard is called the rectus abdominus.

Rectus Abdominus It originates at the bottom of the pelvis–the crest of the pubis–and inserts onto the rib cage at the bottom of the breastbone, on the cartilage of the 5th, 6th, and 7th ribs, and the xiphoid process.

Training Your Rectus Abdominus

One of the best ways to train this muscle is by crunching, or simply flexing your spine forward.