Your threatening monster painting is really coming along, I look forward to seeing the final.
I've been studying the muscles of the back lately so I hope you don't mind a little critique of your sketch. Your proportions are mostly good, the only thing that could be changed there is in the muscles around the shoulder blade. The Infraspinatus ( the top shoulder blade muscle ) needs to be the biggest muscle of the three, and the Teres Major (the bottom muscle) needs to attach under the armpit.
The Lats overlap the Teres Major and the Traps but the Lats are so thin on the back that it can be hard to tell.
You've got the proportions and connections down for all the really important stuff. Hope that was more helpful than annoying