Artloader - Sketchbook

[walent]

mnope, the top muscle you labeled is not the erector spinae, that's actually where the fibers of the lats end and the fascia begins, so that's why you see the plane change. At least that's what I manage to see through that watermark. I suggest trying to find reference that shows muscles a bit better, flexed poses and better lighting. Otherwise it won't help, because you'll have to invent the muscles.

Also, now that I see the reference, I can see you pushed the scapulas too much towards the lateral side. In this pose, the opposite happens, the arms go back so the scapulas go closer to the spine and the rhomboids and trapezius bulge.
I know back muscles are freaking hard, I consider them second hardest, after the forearm extensors. So look out for the scapula movements, it's pretty important.

Here's how the erectors create surface (on the right image, the clear highlight on the right side of the spine)


On the left image, you can clearly see the medial end of the latissimus fibers. The erectors create just a slight bulge close to the spine (that's their function, help hold the spine)
Try studying these.

Oh, I also noticed you ignored the serratus, you should look out for that one too, because it creates some surface as well and it got me pretty confused at first. You can basically see it in the right image, under the right scapula and above the fiber ending of the lats, going down from the medial border of the scapula. So there are also a lot of rhythms there, those will help a lot with positioning.
Anyway, hope this helps and hope it's not too much info.