I just read an article in Men’s journal that indicated that ab exercises might worsen back pain. Don’t have the article in front of me, but it was in the October 2009 issue.
I agree with those who say to have one foot higher. A physical therapist recommended it for me when I do dishes, and it helps. A short stool would work, as would yellow pages, etc. Alternate which foot is elevated.
I hurt my lower back doing that machine and my doctor and 2 physical therapists begged me not to do it anymore. I was 23 at the time and fairly fit so I’m reasonably sure they were opposed to the exercise itself and not something specific to me doing it. On the other hand in therapy we did a lot of exercises that were very similar in motion. As we built up to it they recommended doing a similar exercise on a swiss ball with a reduced range of motion and less stress on your back.