Gelatinggem,
I think if you are struggling more with stiffness, the best thing I find is to draw one figure per page.
It is not a waste of paper. See, when you draw more than one figure on the page, you are now in the realm of composition. When we compose, we have to consider all of the other figures around it, which can hamper our feeling of dynamic movement.
If you would like to help your anatomy, I would like you to consider the movement of your lines. The line can say so much. Your fast drawing is better, I think, than your longer poses because you are not thinking about how the line forms across the body consciously. You may be better at the fast stuff because you don't have the time to think about it. -Don't sweat this; I have been there myself.
I feel you would do well to read the Practice Line chapter in The Science and Practice of Drawing.
Above is a link to the audiobook on youtube if you can not get it in your library or are disinclined to purchase the book. Yeah, it is a bit wordy, but it is also chock full of golden drawing nuggets.
All the best,
JCML Fine Art