One thing I can't help but notice is that your lines look like you draw then in very short strokes. One thing that has hugely helped me get more comfortable with figure drawing is by using quick, long strokes.
The reason this is helpful is because when I narrow it down to a single line to capture the shape and motion of the model, it's not the way the line looks that matters - it's what the line represents. By drawing fast and with large motions, you're able to capture the figure much more accurately.
Right now, your gesture drawing do feel a bit stiff, especially the ones from the first link. Try drawing them again, but this time draw each line in one quick swoop instead of small short sketchlines. The point is not to make it look as accurate as possible, so don't worry if they don't look exact. See how this feels and how it looks. Chances are the ones with quicker strokes look more natural and dynamic.
Once you have this down, you can build on these stick figures to study the models further.
Let me know if this is helpful to you! :) And good luck.