"Hello James, I have a question for you: I am a large guy (about 320), and I started working out. When I do situps, my lower back really hurts. It doesn't matter if I do crunches or full situps. Is this normal?"
Thanks!!! -Matthew V.
My Answer: If you're using every other muscle BUT your abs, then yes, it would be normal for you to have lower back pain. But at 320, ditch the ab work. You won't get a six-pack or reduce the size of your waist by doing sit-ups or crunches. Work on losing fat throughout your body overall by working out your whole body, not just your abs. This means predominantly whole body movements. A good primer on using whole body movements to generate whole body fat loss is my article Strength Training for Fat Loss.
Once you generate total body fat loss, THEN you can work on your abs.