"Torso muscle reference. :3 So backs aren't really my strong suit and I think the relaxed one is slightly out of proportion.. but abs are something I can do.
I think I made the lines pretty clear for the muscle layout.
Also threw some tips in there for proportioning of the waist to shoulder and waist to neck width.
Belly button placement, ab placement, serratus placement.. etc.
Enjoy~! Hope this helps, I think I got it mostly correct."