My lower back never hurts while I do any of my ab workouts. I mostly do the machines or I use a cable and do the crunches on my knees. During or after each set, my lower back never hurts. However, when I try to tight my abs in order to see if I'm gaining any progress, then my lower back hurts. I can't tighten my abs without my back hurting, it seems really weird. Maybe my lower back is tight and I need to stretch it and my hamstrings more.
What do you guys think?