Looks like a pretty happy monkey despite the huge dip going down his spine and up to his head.
Hell I didn't know how else to call it, first thing that popped in my mind when I saw it was disproportional lats lol.
The main problem I see with the muscle right now is that they don't really have origins, the creases that are created seem rather random. I find these references pretty good for the arm and chest, but obviously feel free to search on your own and do w/e you want; it's your model.
lots of pictures ahead:
Also, I couldn't help but notice the delts on the back look like they have just been carved into the shoulders, so you might rethink creating the creases created by muscles with the crease tool (ironically), and rather go for adding layers on top of each other. Although, I won't lie, sometimes the crease tool is very useless, but generally only for small details that do look like they're carved in, like the serratus for an instance, and sometimes when used in moderation looks pretty decent too.
Oh also, completely forgot to mention this but still wanted to put notice to this; I don't know how familiar you are to studying anatomy, but at first it seems really overwhelming and complex, but as you go along and memorise the general looks of where the muscles originate from and attach to it gets really really easy.
Some quick links for myself (and you all too).