Hey there, this is a cool study, thanks for posting.
I do think there’s probably not enough skin / muscle on the back, it looks a little like an ecorche / flayed model rather than a living person, but it just depends on what you intend to accomplish with your study…I think if you want to go more in the direction of ecorche, the fattiness of the pelvic area / buttocks vs. the torso seems anomalous, in other words, the pelvis isn’t as apparent as one would expect from seeing the upper part of the body. Proportionally, the head seems long in the face relative to the width of the head from the side view, maybe add more cranial mass? Again that’s subjective and only a suggestion. 
If you want I can move your thread to the Sketchbook area where you might receive more feedback and critique. 
-R