The flow in your sketches and your ability to very quickly mimic the proportions and anatomy in the image are remarkable. They are not as stiff as you think they are, but if that's something you're really worried about i would recommend slightly exaggerating the poses and limiting your use of straight lines, only using them when absolutely necessary (which is rare, the human body being made up of organic shapes and curves). I also agree with aunt herberts suggestion in trying to not draw over the same lines multiple times, making small tweaks, and trying instead to use more clear bold single lines, even if it looks worse at first. This will help in the long run. In your shorter sketches, try not to worry about details and instead block out the figure in simple shapes and lines, you can add more details afterwards if you have extra time.
