Singlespeed & Fixed Gear - knees and gears

Just curious, I have heard a few things recently about fixed gear riding and knees. Is it the shearing force applied to the knee, by "spinning" uphill when you have to push super hard on the single gear? how long does it take, will I notice it at all before I am 80? What's the difference between that force and one applied to the pedal on a flat when you are pushing hard?
any experience?
I have plans to take the 42-18 singlespeed up Loveland pass tomorrow before a Dr. appt. I guess I could ask him, but that will be after the ride. I've been up rabbit ears pass6 or so times single speed recently with no ill effects. only huge confidence and heart rate boosting.

I could be wrong here, but i think it is more of the sudden force of stopping your bike that effects your knees more. I read a story a few years back about messengers in San Fransisco and problems they had with their knees and they kept riding there fixed gear bikes. The sudden force of stoping your legs from spinning would seem more damaging them mashing the gears for a few miles up hill..

Loveland pass on a ss? man i wish i was in that kind of shape. If you can do it, do it. Only a little bit more time before snow and cold makes it a lot harder, but at that point, its time for the one activity i love more than cycling...

Made it! under 3 hours from silverthorne. I don't know the descent so well and there was some wind and weather to make it exciting.