Automatically linking knee problems to fixed gear is a good sign of not looking at the whole picture. Sure, it might be part of the problem for some people, but most of the time there are other, more important factors. Pushing too high a gear, for example, is going to put unneeded strain on your knees whether or not your bike coasts. A singlespeed with a 90" gear is going to be a lot worse than a fixed gear running a 65" gear. I'm not pointing fingers here, I might add - this is just a major peeve of mine. Understand and listen to your body and you're fine, fixed *or* free.