Bad knee is a little vague. I have all kinds of knee injuries. Some kept me from walking, but I was fine on the elliptical. I have yet to see the knee injury that prevents you getting on a bike, though, so this must be a new one.
I agree with the pp that you should see your doctor and a physical therapist. The exercise I do most when I have knee injuries, though, is yoga. It's typically gentle on the knees and if you do it right it can get the heart rate up.
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