Some nice work Rainier(BTW you spelled your own name wrong on the thread title
).
You have created some nice moods with these works, Keep it up.
As for critiques...
As you already mentioned, contrast. It is one thing I see that needs more attention in a lot of in my fellow classmates work as well. Many have problems going dark enough. You seem to have that issue to a degree and the mid tones seem a bit off which makes for a blurred/less crisp design. I think working on getting the right mid tones to go along with the extremes will help give the work more intensity. Try and get some areas where there is stark contrasts to bring more excitement to the piece. Contrast can also come into play with color choices if more drama is needed.
You also mentioned anatomy, I noticed that as well. The forearm in the one pic seems a bit shorter than it should be and the muscle/bone structure could also use some refinement.
Overall they work fine for what they are but there is always room for improvement. Keep up the good work and I hope these tips have been helpful.