These are very clean, good work. But I feel that your drawings are a bit stiff. Try using more curves.

So, the trick is to try to ignore them. Look for landmarks in the skeleton. For example, figure three, you clearly see the clavicles, the rib cage so you're seeing it from above, not from below as you drew it. By changing the perspective that crease that confused you makes sense; the torso is rotating and so are the muscles and skin, following the ribcage. 4 is right, those parts are in perspective, as is 5 where you see the top of the ribcage.