good questions.... good questions. i was a little bit addicted to situps i my early 20's. they just felt so RIGHT, and they didn't give me any trouble at the time.
“We stopped teaching people to do crunches a long, long time ago,” says Dr. Richard Guyer, president of the Texas Back Institute. That’s because the “full flex” movement—the actual “crunch” part of crunches – puts an unhealthy strain on your back at its weakest point. The section with the most nerves (and most potential for nerve damage) is in the back of the spine, which is the very part that bends and strains during a sit-up.
“There are only so many bends or a ‘fatigue life’,” in your spinal disks,” says Stuart M. McGill, a professor of spine biomechanics at the University of Waterloo. Inside each disk is a mucus-like nucleus, he says, and “if you keep flexing your spine and bending the disk over and over again, that nucleus slowly breaches the layers and causes a disk bulge, or a disk herniation.” A herniated disk won’t show through your swimsuit, but it’s no fun, and can cause persistent back and leg pain, weakness, and tingling.
anyway, there's a crapload of articles like that published all over the place. for what that's all worth! i would just recommend caution and good sense, is all. i'm not a health professional.