Your sketches are getting better, keep up the good work.
I think your work is a little too concerned with the two dimensional right now. Right now I think you should do a few sketches that specify on NOTHING but three dimensional forms. I'd say still do figures (as that seems to be the element I'm trying to discuss), but simplify everything down to bare cylinders and spheres.
Study the rib cage more, learn it in and out. Do the same thing with every other part of the body, think about form and learn to see these things as 3D forms so you can start making the Z axis work more in your pictures.
Keep going, you're working hard!