I had a freak bicycle accident about ten weeks ago. For about four weeks, walking was quite painful and running impossible. I spent four weeks as a couch potato. I began walking after that, but then took a three week cruise. By the end of the cruise, I was able to walk fairly well, and had begun doing some upper-body weight lifting.
I began jogging about two weeks ago, and my legs were quite weak and my knees were stiff and sore. I had expected to be weak, but was surprised by the pain and stiffness in my knees. I had assumed they would have benefited from the rest, but was completely wrong. It is as if some lubricant had dried up as a result of inactivity. There seems to be some feedback mechanism -- perhaps hormonal -- that "decided" I was old and would not be using my knees any more.
After about two weeks of jogging, shooting baskets and doing leg presses, my knees feel about the same as they did before the accident. (I am still weaker than before, but improving).
My knees came back faster than my leg strength, but this seems to be a clear case of use it or lose it.