Your scetches seem well thought-trough and I think you grasp the proportions of human body quite well, too! The differences in flown between 30, 60, and 90 second-longs suggest that with more time you don't actually study the posture more rather than focusing on your lines and overall shapes. If I could recommend an excercise to loosen up a bit, I would say exaggerate the poses a little until they feel too much. You're doing great, keep it up!
