Spock use to do that! Sit with one leg up, still does it but a lot of my drilling 'get your feet off the table' into her and stopped it a fair bit.
I struggle to sit on chairs properly, if I'm sitting on a chair I need to cross my legs or put my legs up, it's just so uncomfortable for my hips otherwise...no pain...hard to explain.
That and the elbow thing does sound like 'double joints'