Don't know the answer - but in my most recent doc visit - I mentioned pain behind the knee cap. His answer was that that's most likely deconditioning. He said to worry if the pain is on the side of the kneecaps b/c there's soft tissue that might be being damaged....
So...might be worth getting someone to look at your fit (I've done that, and I'm good to go...) to make sure that your knees line up correctly with the pedals, etc.
Good luck!
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