I'm not the greatest at pointing out advice, and I don't admit to being super good, but here's what I can say about these.
First off, I'd like to say that you're off to a great start, especially with the fine details and such, much more than I could do myself.
Secondly, there seems to be slight proportional errors with the head and hips. The head appears to be a bit too small for the body, as well as the hips leaning too far forward. Also, even though seeing the character being a muscular type, it would help to make the shoulders slightly less bulky.
Lastly, I would say that it would be helpful to position the arms better, otherwise, everything seems fine. You could also apply shadows and such to take the sketch further if you wanted to.
To help with this, you could look at proportion and positioning tutorials on deviantART, which I find very helpful if a good one is found.