I feel your pain, so sore…so stiff…
I had this problem a while back but i noticed that on top of it i also had problems on proportion. And in the end this is what the core of the problem was.
Because i had not yet internalised my awareness of proportions to the point that it was 2nd nature - as it should be - i would spend a large time of drawing poses towards getting proportions right, in doing so the pose was no longer dynamic and fluid. It was the complete opposite - stiff and rigid.
So basically what im saying is until you have that awareness of proportions to the point that it is second nature, your poses will continue to look stiff.
Dont stress though because with spirit, discipline and some quality guidance from the usual cats (loomis, hamm, hogarth) you will gain that awareness sooner then you think.
And trust me the effort is worth it, the ability to draw figures straight from the dome is an awesome thing - and it scores major cool points with chicks…ahh…honest.