[greymule]

My friend said something like "all that up and down," but he might have used the phrase broadly to refer to all the various movements required of a plate umpire. I agree that just going up and down isn't likely to destroy knees unless you're constantly going way down, as in deep knee bends. It may be that after his knees were damaged by other movements or blows, he noticed pain when moving up and down and attributed the damage to that type of motion.

Most of the athletes I know whose knees are gone either suffered blows to the knee (mostly from football) or constant pounding within the knee (basketball on hard surfaces).

For some reason, my knees have held up well, and I've never had any problems with them. A few years ago, however, after a couple of slow-pitch marathons, it was my feet that were killing me, mainly from the incorrect way I was standing.