If you are developing sores form doing a sit-up and not a crunch, switch to crunches and leg lifts. Your abdomen is used to pull your shoulders to your hips (that is how I describe the contraction of the abdominal muscles). Crunches work the abs extremely well and do not "grind" the lower back. If you are using and incline sit-up board, don't let your shoulder blades or back touch the board except when you start. This will keep tension on the abs and hip flexors but it will not jeopardize your lower back
