Often when I sit for an extended period of time, one (I don't necessarily think that it's always the same one) of the vertebrae near the bottom of my spine comes out. When I stand it can feel like my hip is coming off. Put another way, when I lift my foot off the ground my leg gives a sense of dangling there. I have to (multiple times per day) arch my back and push my hips forward to (sometimes smoothly, often not), pop it back into place.