The abdominal region is made up of several different muscles, but the actual 6-pack that shows through is the rectus abdominis.
It’s a single muscle that’s long and flat like a small surfboard, and it runs vertically from your hips to your rib cage.
The 6-pack is formed by the fibrous tendons reinforcing the fascia that covers the muscle.
You could possess abdominal muscles that resemble the underside of an ice cube tray, but if your body fat level isn’t low enough, no one will ever see them.
The rectus abdominis has a couple functions.
The primary role is “anti-extension” of the spine; the secondary role is to pull the hips toward the chest or the chest toward the hips.
Think of it this way: If your spine was a bow, your abs would be the string.
This muscle is made up of mostly slow-twitch fibers, designed for endurance; but it’s basically a muscle like any other, and it should be worked with the same principles of muscle building that you would apply to all your other muscle groups.