I also find it to be a tricky one - however, it makes sense in my opinion if we consider the central lines (front and back). For the back, we just see half of it, the part until the spine/central line (that we cannot see). Therefore, on your rectangle for the back, you should be able to see half of it. On the front, you can clearly see the central line at the pelvic region, but it then disappears because it is masked by the shoulders. Therefore, the top of your rectangle should cross above the front plane. So I thought it was exaggeration at first, but this is actually accurate. Hope that helps!

Whoops... there may be a few forshortening and twist beans on the first page too

I see a major improvement in line quality as you go. The final drawings look very confident and seem to use bolder + fewer lines. I'm assuming these show up in order form oldest to newest and I feel your 3rd page strikes a nice balance of gesture, detail, and confidence. Your final 2 pages have excellent line quality but may follow the contours at times or have details in the edges of the elbow / bumps that ultimately distract from the gesture. In fact I can often see the main line of action really faintly and then the bolder strokes are mostly straight lines as opposed to C, or S curves. Awesome drawings!