If you can do planche or levers, those are about as hardcore ab work as you can find. Pretty much the only two that I do are dragon flags and roll-outs, which I usually do after lifting sessions. Crunches and sit-ups are pointless, especially doing 100s of them. Abs are like any other muscle... they respond to increased loading, and like any other muscle doing super high volume is silly. If you can do more than 10 reps, you're probably not creating enough stress.
6/17/12 at 11:33am