I've said it before and I'll say it again - sit-ups are not an athletic movement and are not something that athletes, or anyone else for that matter, should be doing. Sure, they're easy to do and you can rack some impressive numbers, but they aren't going to do your back any good in the long run... and if you really want a six-pack then quit eating Doritos and drinking Diet Coke! Now, if you really want to talk about hammering your abs, I'll be happy to sing the praises of the Ab Wheel aka "The Wheel of Pain" all day long... but I'll save that for another post.
Anyway, I wanted to expose you to some excellent information about ab strength and back strength coming from the one and only Dr. Stuart McGill. Dr. McGill is a renowned researcher in the exercise field but is known for his outstanding research on protecting and rehabing the back. Check out this two part interview with Dr. McGill on the Testosterone.com website: Part One and Part Two.
Also, check out this seemingly "easy" ab exercise that Dr. McGill highly recommends for strengthening both the abs and reinforcing the back. I first tried this at the Dragon Door Bodyweight Workshop in 2011 and thought it was a keeper!