It could be mild chondromalacia (runners knee) where the patella doesn't quite track right. I had a case of it and noticed it more when I sat than when I ran. My doctor called it "theater knee" because the pressure builds up when you rest it.
I also have night legs. Is the "emptyness" associated with any need to twitch your legs?
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I teach the LSAT Test. The LSAT Test is not as hard as people think, just like hanging leg raises.
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