I know it varies per individual but if I were to say what muscle group is the most underdeveloped on naturals more often then not, HANDS DOWN it's the lower back,. no natural has a thick lower back, in fact even the ones that juice at the lower doses develope no muscle in that area.
In my opinion, the reason the low back isnt developed on most of them are for these reasons:
1) they dont want to do things like deadlifts because the exercise is 'worthless' / builds a 'thicker waist'
2) If not complete deads, they will rely on 'rack deads' because they read about DoggCrap and he recommends is
3) stack the shit out of some poorly designed low back extension machine rather than doing them right
4) Refuse to do hyperextensions because its a 'pussy exercise'
5) Partial squats...the ol' 3" depth is good enough to keep adding weight to the bar....