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These are looking good! You definitely have a good handle on the ball and stick method of skeletal construction! My favorite is the upper right hand one-you can feel the movement and life in the pose!

If you want to reduce stiffness and improve flow, maybe try looking for the line of action, the flow line that defines the pose, before you start drawing the body structures themselves. This is difficult to explain in words but I made a little drawing to try and explain what I mean https://www.dropbox.com/s/l1nd8oclmy5thix/30secgesture.jpg?dl=0

I hope that helps! Definitely keep at it-your stuff is very good!

I'm working on the same area as well. I think an action line will help your work. This will allow your drawings to a rhythm to them. I would also recommend that you pause the video for some poses, study the pose for a minute and then do your quick sketch. I also notice that breaking down the figure by parts is very helpful. I receive most of tips from artists from a live figure drawing class. Hope this is helpful, because I need to follow this tips now because I'm super rusty now lol. Best of luck.