I use a machine to do 8-12 crunches to failure. I also like to perform sit-ups to failure (and I feel it in the front of my hips, not my abs). Does it matter in which order I do these two exercises?
Edit: I was wondering whether wearing-out the abdominals would make the sit-ups less effective. I've read you're supposed to do certain exercises before others, but the reason is not always clear to me.