These looks really nice, you definitely put a lot effort in your works. The proportion and perspective looks solid, great job. It is not easy for anyone to create such work, however i do see some specific things you might want to improve.
1.It is rather tedious to memorize all the muscles of our body, but doing so may help a lot when you try to push your work to a more "finished" level. I would highly recommend you to study a little bit about how these muscles are placed and how they move. The shoulder part of all your works seems less convincing, it might be a good idea to start from there.
2.The second picture is roughly grey, that might not be good. Although grey is usually the largest tune in such drawings, we do need darker and brighter colors in order to create a sense of light. If your drawing is not about smoke and fog, I suggest add some black/white to the dark/bright side. (B<15/B>85 if you are using HSB)
3.Outline is great, but try not to start from it. Create a good looking art piece is not the purpose of these sketches, understand the structure of the body is. Of course you may use any method you see fit, but start from outline might become extremely hard and disappointing for those who are less experienced. It might worth a try to draw main body parts(for example, head and rib cage) as small boxes when you start the sketch. You can render them later, or if time is short, just round them a little bit. Also, for the third picture, solid black outline may gives a wrong message that there is a shadow, which should not be there.
That’s pretty much all feedback I can give for now, hope it will help and wish you success on your artist path/career. I would be more than happy to provide further explanations or share more detail and tips if the need rises.