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No, I'm not referring to a standing up quicky in some dark doorway.

Why do some youngish guys indulge in knee trembling? Whats it for?
I mean they are sitting down and rapidly lifting/vibrating one heel while keeping the sole of the foot on the ground. And not always listening to music.

Mart

For those who are listening to music, it is trying to keep their "Foot Vibrations" in time with the beat of the Music, which is rare.

For those who aren't listening to Music, it could be the common phenomenon i have seen of late, a "Nervous Reaction" to whatever the given situation is.

or excessive masturbation

was that what tom robinson meant in his song up against the wall?