"The abs are a strange breed. They can stand an amazing amount of punishment during one workout and then come back for more after a short periods of rest. That's because the abs are a rapidly reoxygenating muscle group. They recover quickly after a set and are soon ready for more work. Most muscles take much longer to recover. Lactic acid builds up, and you're wrecked for at least two to three days."
"But the abs are different. After being subjected to a gut-burning giant set, they can recover within 60 seconds and be ready to go again because they have the capacity to rapidly clear away the lactic acid and pump fresh oxygen into the area. This ability may be due to their unique structure or their location close to the heart. Whatever the reason, they respond to high-intensity training."
Concerning SETS and REPS
SETS--
"In general you can forget what you've heard about 'less is more' because in developing the abs, 'more is more!' Increasing the total number of sets in a workout can produce great results....The key to using sets as a basis for progress is to create as much overall stress to the muscle as possible between brief rest periods. Understand that more sets can only help your cause, as the ab muscles are able to withstand exertion over a long period of time. Work supersets and giant sets into your routine. This practice is beneficial to the abs because it places stress on them almost nonstop. Since the abs reoxygenate rapidly, continuous action can be just what's needed to take them to the next level. Rest between sets only long enough to move from one station or position to the next."
REPS--
"Varying reps can be one of the most effective methods of effecting change in the abdominal region because the rep scheme is indicative of overall workload. More reps equal intensity. Many people believe the abs should be trained like any other muscle group and that muscles respond only to heavy weight. The fact of the matter is, without a high rep scheme the intensity in your abdominal routine would be in adequate.
"That's not to say everyone should be doing 1000 crunches every day, but working your way up to 1000 crunches isn't beyond the realm of possibility. You just have to be properly conditioned. High-rep sets take a lot of energy and readily drain the average person. Getting accustomed to this regime is difficult. Most people are, after all, used to a slower pace that includes less intense work. High rep schemes challenge this indulgence by forcing an excess of lactic acid deep into the muscles."