Part of the typical bodybuilder’s V shaped upper body is the small waist and together with it the, washboard-like abdominals. Nobody can deny that, as the abdominals dominate the central part of the body and as such, they’re the element that visually links all other parts of the body together. Because of this fact, a small waist and well developed and defined abdominals can have a startling effect on other body-parts. They can compensate for lack of mass in the thighs and the chest as well as the shoulders. Besides the visual effect the abdominals have a very important role in the body. Not only do they pull the ribcage closer to the pelvis making it possible for us to sit up, but they also provide vital support for the backbone when lifting heavy weights. Nowadays no serious bodybuilder can exist without a well developed set of abdominals, though in the old days, many bodybuilders came onto stage with their abs not at 100%. Just because they wanted to look bigger in other areas, they didn’t burn off the fat as well as they would’ve otherwise.
The training of the abs is an altogether different issue from the training of any other muscle group. The keyword here is “definition”. Mass is not really a factor. Many bodybuilders believe that by thoroughly isolating the muscle group, hitting it with sets made up of a high number of reps, will make it show better but they’re only partly right.
Hitting it with a bunch of high-rep sets is indeed the way to achieve good definition, but the problem is that someone may possess a terrific set of abdominals only to have it obscured by the layer of fat it’s hiding under. These exercises - though very effective in developing the muscle - will do nothing against the fat. Doing a lot of high-intensity, high-rep exercises for a certain muscle group, generally speaking won’t burn the fat off it. Apparently everybody’s organism is programmed in a certain way, when it comes to burning fat, and this program tells it the sequence it follows, when it taps into the body’s reserves, stored in the form of adipose cells.
Melting the fat away from the already nicely developed abdominals becomes the task of diet and aerobics.
Taking a closer look at the muscles that make up the abdomen we will find the following:
The obliquus externus, a long and rather wide muscle, that makes the link between the pelvis and the lower ribs on the two sides of the body. These make it possible for us to lean sideways and to rotate our torso.
The rectus abdominis - this is the muscle that everybody thinks of, when they talk about abdominals. Giving the front part of the waist the “washboard” look, this muscle makes the connection between the ribcage and the pelvis upfront, making it possible for a person to sit up.
Last but obviously not least, there are the intercoastals. These are small ripples of muscle on the two sides of the rectus abdominis above the obliques that link the abdomen to the serratus anterior. Filling the spaces between ribs, they’re used to pull the ribs together.
As said before, the training of these muscles is something quite different from other muscle-groups. The high-rep approach is a good one, but for really efficient training one has to keep in sight what the function of these muscles really is. Straight leg, straight back leg-raises or sit ups are definitely not the way to go about them. During these exercises the abdominals act as mere stabilizers and are not effectively worked. The function of the abdominal muscles is to pull the ribcage (torso) closer to the pelvis. This is why crunches are probably the most efficient way to work this muscle-group.
Famous bodybuilders to exemplify great abdominal development would be Serge Nubret (absolutely impressively small waist) and Flex Wheeler.
The exercises done for the abdominals are as follows:
First off the much-hyped crunches:

1) Simple crunches
2) Crunches done twisting the torso
3) Reverse Crunches done while hanging
4) Bent knee reverse crunches
5) Crunches done on a vertical bench
6) Kneeling crunches done with cables
7) Leg tucks on a horizontal bench

Oblique exercises:

1) Twists while seated
2) Twists while bent over
3) Straight leg leg-raises done on a horizontal bench
4) Leg-raises done on a horizontal bench with bent knees
5) Leg-raises done on a vertical bench with bent knees
6) Bent-knee leg-raises twisting and hanging off a bar

Additional abdominal exercises:

1) Situps on the roman chair
2) Leg-raises done sideways
3) Kicking back on a horizontal bench
4) Drawing the belly in, for a vacuum