As stated on the Soloflex thread, I'm coming off of rehab for a couple of problems, one being my lower back. I have a slim build, high metabolism. However, I'm feeling very, very soft around the edges due to months of inactivity. Used to have great abs - like most skinny guys - but those days are over. My abs are now weak as hell! So I intended to do what I've done in the past - major sets of situps - but my physical therapist and others have told me situps are bad for your back and crunches are better. Any ideas out there other than crunches? I noted some ab machines being mentioned on the Soloflex thread - any of those give a good ab workout without hurting your back?