Well, it is going to be hard for me to crique anything here, because you seem to be a fair bit above my level. The first picture has weird proportions, but I guess this was one of your earlier works? Anyway, the closer we get to picture eight, the better your drawings become which is good. Keep going like this!
From what I can tell, some proportions are still ever so slightly off and you seem to be drawing only from what you can see without fully considering the underlying distribution of muscles fat and bones. Maybe getting an overview over the human anaotmy might help you to get even better. (Where the muscles are placed, how they're connected to the skeleton and what this means for the shape of the human body in certain poses)
The best book on the market for studying anatomy is Gottfried Bammes "Die Gestalt des Menschen" (At least in my opinion it is) It is a fantastic book that goes into great detail and explains everything you need to know in text and picture. Sadly the book has only been published in German as far as I am aware and itt is also quite expensive (80€ or 90$). Nonetheless I have heard people who do not speak German say that the book helped them nonetheless because the drawings in there are so fantastic (and they are. Here are some samples: http://www.conceptart.org/forums/showthread.php/119441-A-wonderful-book-Die-Gestalt-des-Menschen-by-Gottfried-Bammes ).
Other than that I'd suggest to continue on to the next step which is adding more shading/lighting to your figures, color and so on.
Somebody else might be able to give you better feedback since I am nor quite on the same level as you and probably miss a lot of things a better artist might see. Maybe wait for some more feedback from others.