I've had knee trouble for years and found that riding fixed caused me discomfort. I think even if you try not to resist the pedals too much you'll end up doing it anyway. If cycling with a freewheel causes you no problems then it may be that your knees aren't used to different strains and pressures being put on them by fixed cycling. Knees aren't very tolerant and doing anything your knees aren't used to can cause discomfort until they get used to it.
If you have no history of knee problems just take it easy, if you have a history of knee problems take it VERY easy and see how you go, icing your knees after riding may help, if it gives you real problems it may be worth getting looked at.
There's a knee pain thread if you search for it.
I've had knee trouble for years and found that riding fixed caused me discomfort. I think even if you try not to resist the pedals too much you'll end up doing it anyway. If cycling with a freewheel causes you no problems then it may be that your knees aren't used to different strains and pressures being put on them by fixed cycling. Knees aren't very tolerant and doing anything your knees aren't used to can cause discomfort until they get used to it.
If you have no history of knee problems just take it easy, if you have a history of knee problems take it VERY easy and see how you go, icing your knees after riding may help, if it gives you real problems it may be worth getting looked at.