Your small sketches are really good. I would like to see larger sketches. You will need to work on human anatomy -- muscle & bone structure. Larger drawings will mean you need a good knowledge to produce really good drawings. The internet can provide you with the information you need. Be sure to make many drawings of the information.
In any case, practice makes better. Consequently, practice, practice, then practice some more.
Your gestures look awesome. In your case, I would experiment with pushing the actual poses a little bit more with sharp angles even though they can't be really observed on the subject. For example, if the figure is bend put there a sharp angle to show the tension in the pose. Otherwise, keep the good work. Also when shading, try to experiment with different pressure and linework. Unless you trying to go for a comics-style shading the actual shades have different values.