[USMCRET6391]

My suggestion is to do what I call 30 sec speed drills. Do as many sit up/crunches as you can in 30 secs then rest for 60 secs and do another set. Do 4 to 5 sets.....you should be doing significantly fewer reps on the 4th and 5th set. Abs are the easiest muscles to build and once you have driven them to failure (which you should be at by set 4 or 5 above) you arent doing much good, so 400 sit ups a day sounds really excessive to me, they are only going to get so big and/or strong.