I asked my family doctor about this a few years ago. At the time I was complaining about how stiff I was anytime I would get up after sitting for a longer time, (this was before I had any TKR's.) He said that as you age your muscle fibers lose their ability for instant flexibility like you had when you were a child. When you sit or stay in one position, those unmoving muscles shrink and contract, and when you first get up, it takes a while to stretch them out to where you can move normally. I guess there is no cure for aging. All we can do is try to keep active so we don't completely go to pot!