I did a High School summer league tournament last month, in which I had 4 games on one day...and three on the plate. (Don't ask!).
By game four (my last plate game), I was zonked. So I worked the knee. I'm still young enough (
) that I was able to get up and out from behind the plate without a problem (for instance with the base umpire going out with no runners on base).
As for the strike zone: I have worked the scissors in every other game I've umpired for the last 5 years. To me, personally, the pitches did look quite a bit different than in the scissors. I found myself calling more high strikes.
It was a very, very long day for the teams (they were ready to go home, too), plus I have a very good reputation with both coaches, so my new higher strike zone caused no problems. They wanted strikes! In fact one of the coaches teased me the next week accusing me of thinking I was so good that I could now work the knee like a MLBer. I laughed and said that I was just too tired to keep standing for another 7 innings.