This is what I learned too. No longer have my reference book, it was like 20-years ago, but situps tug at you lower vertebrae and increase the risk of lower-back injuries. Crunches exercise your abdomen without that undesirable side-effect. Since it's harder to isolate your lower abs, it's best to target those first (leg-raises with bent-knees and pelvis, or easier knee-ups) and then hit the upper abs which by now have already done some work and will really burn.