Have a look at this video and concentrate on the guitarist:

http://www.youtube.com/watch?v=vgj9Q4-YEBc&feature=related

As a person who's had tennis elbow, I have the following thoughts:

His right arm is doing the same thing too hard, too fast and too many times without a rest.

His hand is tense from holding the pick. This is not good.

The sharp edge of the guitar is digging into his elbow while the whatchacallit nerve is having to move, back and forth, back and forth through the elbow joint.

I have seen other guitarists who move that arm further, harder and faster than the man in the video.   
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It would be interesting to calculate how many times a right arm does this during rehearsals followed by a tour. Hundreds, thousands?

In my opinion, someone who plays like that very long is headed for joint trouble and real pain.

I've known two musicians (one keyboard, one fiddle) who've gone to doctors with tendonitis and been told to stop playing for A Year.