Good start Matthew! The first thing I advice you is to not be concerned with shading for these kind of drawings. Keep your focus on capturing the action.
The second is that you should develop your skill in drawing form. It seems like you're focusing on copying the outlines, and you won't learn much with this approach. Not if your goal is to understand the human body in action well enough to be able to draw it from imagination. For this you'll need to understand the structure of the human body and how it functions to perform various actions.
I hope this helps. If you have any questions feel free to ask :).