When I train my abs I break the abdominals down into four main muscle groups:
1. The Rectus Abdominis (composed of upper and lower abdominals)
2. The Oblique Muscles
3. The Intercostal Muscles
4. The Serratus Anterior
Oblique Muscles Function & Exercises
The external obliques are the muscles at the sides of the waist. The external oblique complex actually consists of three layers of muscles: the internal obliques, the transverse obliques and external obliques. Together, these muscles contract to tilt the torso, as well as twist it, from side to side.
While a bodybuilder would not want massive obliques as this would take away from symmetry and give the illusion of a thick waist, these muscle do need to be trained in order to maintain ideal postural alignment. A great exercise for these muscles is the side bends performed on a swiss ball. Another exercise that also needs to be performed for these muscles in order to exercise its rotating capabilities are the Russian Twists.
The Intercostal Muscles
The intercostals are the muscles of breathing that lie between the ribs and show as bands of muscle angling downward in the sides of the rib cage and the upper abdomen. The intercostals come into play by flexing the torso and causing it to twist, so doing any type of twisting crunch on a swiss ball will stimulate this group maximally.
The Serratus Muscles
The serratus anterior muscles are the finger-like strands of muscle on the rib cage between the front abs and the lats. Their job is to depress the rib cage and also assist in bringing the upper arms from a position pointing directly up from the shoulders to one pointing directly below the shoulders. A good exercise that will stimulate these muscles is the one-arm cable crunches (using an overhead pulley).
How Much, How Often?
Since we have now identified which exercises should be present in our specialized abdominal training routine, let's now figure out how much will we do of each and how often.
Believe it or not, the abdominal muscles are composed mainly of fast twitch fibers. These fibers (as opposed to the slow twitch, endurance type ones), are composed of the strongest types of muscle fibers and are thus designed for short bouts of explosive hard work. Because of this, fast-twitch fibers respond best to heavy weight/low repetition work. Therefore, performing more than 15 repetitions per set on your abdominal exercises will be largely a waste of time!
So for abs, lets keep the repetitions from as low as 5 to a maximum of 15. As far as sets, if you perform days of lower repetitions, you can do as much as 5 sets per exercise, while on higher repetition days you can get away with 3 sets. For abs, you want to mainly concentrate on the intensity of the contraction and you really want to feel the movement. However, ensure that you choose a tempo that allows you to finish all of your repetitions within a time span of 40 seconds.
Typically, as you lower the repetitions and increase the resistance, the lower the tempo should be and the higher the repetitions, the faster the exercise should be performed. Also, as you get stronger, you may want to start adding resistance to your exercises except for the ones that target the obliques. Oblique exercises should be executed with no weights even as the repetitions get lower. On lower repetition days, just concentrate on holding the contraction longer at the peak of the movement.
As far as how often, beginners need to keep in mind that fast twitch fibers take long to recover. Therefore, we need to train these muscles like we train any other muscles and give them the rest they need. Experienced trainers can vary because they understand their bodies...