Some people believe that concentric ab work is unnecessary. Abs, by Mark Rippetoe outlines the common reasoning and alternatives to concentic ab work. Here's some summary and excerpts.
- The primary role of the abs is isometric stabilization.
- "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."
- "As the loads [on a squat or deadlift] 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."
- Chin-ups, while not supporting the spine also require isometric ab work to hold the body in position. During later reps, there even is a little bit of eccentric/concentric contraction as your abs become tired and relax/tighten every rep.
- "But for most lifters – and I mean the vast majority who will never squat 600, or even 500 – the stresses normally encountered under the bar provide all the work the abs need."
- "For those of you with recurring low back problems, see what six situp-free months does to your back problems."
I find this to be true. When my deadlift started to get heavy, I noticed my abs got sore! This happens now with my heavier squat sets. Chin-ups and pull-ups always leave my abs sore. I haven't done a sit-up in three months, but my plank has increased from a 1m30s to 3m (and I don't train the plank; I just test it occasionally).