If no one else is around yet, I'd be a bit more attentive. People respond to pain differently; and it's not necessarily a guy thing, that's a bit stereotypical/condescending right there :-) He's an individual you hurt; see him through the hurt. Knees are important; I don't know what the "strange spot" is but he could have all sorts of trouble with that knee/be crippled in 20-30 years because of this hit (I have trouble with one of mine now at 57 because I fell on mine 2-3 times in my "youth" :-)

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Osteoarthritis of the knee usually occurs in knees that have experienced trauma, infection, or injury.

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From: http://orthoinfo.aaos.org/topic.cfm?topic=A00228