Do you have any more info on the muscle scan? I'm curious since I never heard of it and it sounds interesting. I'm just wondering what it's scanning for. Torn, or inflamed muscles?
Anyway, I also had lower back problems with those 1 legged back extensions, and also a little bit with 2 legged back extensions. The weird thing for me was that once I did rom. deads, or regular deads right after the extensions, the tight, burning sensation went away.
I have no idea why, but every week, my lower back would be tight and burning (more so with higher reps) and every week, the heavier deads made it go away, and it went away faster than the times when back ext. were the last exercise I did.