Really good stick figure. I would recommend using this method to do short action sketches. You can use this as a start on a longer sketch. If you want to know more about anatomy then study muscle and bone structures. There is much info on the internet. Draw many pictures to that you, in the end, know what is where. Also, practice on models provided by Line of Action and On Air Video.
In the end, remember: Practice makes better.
So, practice, practice, practice, practice some more.
In the end, remember: Practice makes better.
So, practice, practice, practice, practice some more.