The arms I like a lot, but the back seems off it me. It looks like he has two spines, and I can't really tell where his neck connects to his spine (I'm looking for a continuous crevice but I can't find one). If I shrink down the image a lot the thumbnail looks right except that there seems to be a blob where the spine crevice and other back muscles should be.

But, I'm not experienced in anatomy, so my thoughts may just be because of an untrained eye. Still, I don't think you need me to tell you this but everything else looks very nice. The hair seems a bit bland, but that's because everything else is so colorful.

youre not wrong, at first glance, the drawing is pretty much perfect.
but upon clsoer inspection, the top of his traps are comlpetely screwed and the left side of his back is either very flat or very wrong, i cant really tell.
his right trap is just horribly deformed, and his left arm appears to be sprouting out the back of his shoulder, its only slight mistake that i cant really figure out, but its bothering me now.
the right side however, besides the traps, is perfect, and as an anatomy study, is spot on.

also, a side note, this one isnt that important, but the side is a little undefined, the back lats and rib area all interact a little, and i cant really tell the difference, especially with the lats and ribs. its the only area that you just sorta drew, rather than studied

Kind of looks like his muscles are made out of metal, really hard lighting there. Wrists seems somewhat deformed, might just be the perspective though.

Pretty badass, like the ambient purple.

Well, sketch was ok. Looking back at it I guess I overdone it with 'volumes', trying to make him look as mountain of muscles...