The back of my knees are generally stiff and tender, but get unbearably painful if I stand
for about an hour (say, if cooking), or if I walk
for about 30 minutes. Similarly, my knees ache (along with the lower parts of my quads) after a flight or two of stairs.
The most plausible cause of this is some sort of inflammatory auto immune condition; my rheumatoligist thinks Reactive Arthritis.