I often get this sensation when seeing a photograph or hearing a description of a pretty bad injury, usually a deep cut. Sometimes it’s in multiple joints (knees, elbows, wrists).
This sensation usually leads to people going, “For the love of God, stop talking,” and then walking off while attempting to shake off the sensation.
So…any ideas? My WAG is it has to do with reflexes. Maybe upon seeing the injury, you imagine it happening to yourself and your body reacts in the way it would if the injury was, in fact happening to you. I imagine it’s mostly psychological, but I’m willing to bet a good deal of it is physiological.