Originally Posted by kaldaniels
Not being a MLB player I've always just assumed it was a tired arm...although on a hot day it certainly could be the entire body.
I think fatigue is a word meaning a lot of different things. Because if you're pitching correctly, you're using your whole body, the whole body will eventually get tired.
But even getting tired isn't really the issue. The issue is that when your body gets tired, you stop doing the repeatable mechanics you've been coached to do. All of a sudden your stride may be shorter, or you can't turn as far back on a wind up, or 100 other things that since I'm not a pitcher I don't even know.
Then, you either a) start missing location because all of these 100 things throw off your release point by 1/2 a cm, or b)you start having to concentrate on one or two or ten different mechanical things so you can't actually focus on where you want to put the ball.
This happens in golf all the time, by the 18th hole, you feel tired, so you don't bend your knees as far on a chip shot, or if you do remember to bend your knees further, you're focusing on that, and not where you want the ball to go so you flub a chip.