Hey, good job on these. You're putting in the effort, and that's the most important thing to getting better. What I'd say is to start "drawing through" more. What that means is to try to find ways to connect pieces of the body together with gesture. @Stan Prokopenko's video Q&A – Gesture vs Contour and Scribbly Lines goes through what I'm talking about. When doing quick sketch (especially 2 min) your goal is to get the gesture down and that's it. In these drawings you're over complicating your drawings with contour lines. A contour line is any line that describes the exterior shapes of the body, and if you only do that you're giving up how the parts of the body relate to each other and as a consequence they feel stiff. If your goal is to make your drawings feel less stiff, really start focusing on simplifying your gesture. What I mean by that is try and find a single line to describe what the body is doing. I've included some draw overs I've done in the past, and some drawings from @Erik Gist that I think communicate the concept well. I don't have my drawing tablet with me right this second but if you'd like I can do a draw over later, just let me know.

I like the third one best and I agree with your self critique. Try shorter timed poses to force you to make longer/bolder marks. Use a few key lines to capture the flow of gravity and the angles of the shoulders and hips. Consider the inner structure before working on the contour which might be making it feel stiff. Also, practice ‘the bean’ which conveys volume. Hope this helps.