http://startingstrength.com/articles/abs_rippetoe.pdf
Read that. Here's an excerpt.
Conversely, the abs’ job is primarily isometric, since spinal stabilization is their principal task. If the
skeletal relationships they maintain are motionless, then their primary function is to exert force while
allowing no position change, and to do this they must remain the same length under whatever load
the spine must be stable against. Thus isometric contraction is their principle mode of action. They
can be pressed into service to do a situp, acting concentrically/eccentrically to flex the spine while you
are lying down, but it’s not their “normal” function, the one they have developed over millions of years
to accomplish. We haven’t been doing situps that long – only since they were invented by Joe Weider
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Abs
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back in 1980 – hardly long enough to have changed our inherited muscle physiology to accommodate
him. Abs are supposed to keep the spine rigid, and this has some rather important implications for the
way we have been thinking about training them to do this function.
Since the basic nature of correct ab function is isometric, the exercises in which the abs perform this
function will provide exercises for the abs as well. This may seem childishly apparent, yet virtually every
strength coach adds extra concentric/eccentric ab work to the program anyway. The thinking must be
that just squatting, deadlifting, pressing, cleaning, snatching, chins, and barbell curls – all of which
involve trunk stabilization as a critical performance component – do not provide sufficient ab work
by themselves. I disagree. First, these lifts are not done by themselves. They are performed together in
workouts composed of several of them the same day. I don’t think a novice needs to do situps as a part
of novice-level programming; the program relies so heavily on the good form provided by a rigid spine
during all the barbell exercises that the abs are receiving as much work as they can possibly do. This is
especially true of heavy work sets in the squat and deadlift which require a high degree of focus on a
flat back for completion of the set as the lifter gets stronger. It’s not a factor at first because the weights
at first are light, and this is why it is safe. As the loads pass 200 and then 300+ pounds, it becomes
enough of a challenge for the now more experienced lifter that a helluva lot of ab work is required to
squat and pull properly. At weights above this range, most lifters find that a belt helps them produce
harder ab contractions and therefore maintain better spinal stability, and this is why it is common that
a lifter’s first squat workout with a belt produces new levels of ab fatigue.