Every time a coach gets a new team or pitching staff, I think it's important to establish every pitcher's natural arm angle during the first few weeks of practice.
This is the arm slot from which the player most instinctively throws the baseball on every throw. Everyone's different. So it's important to find out what each pitcher does individually. Arm angles are something that you generally should not change with a kid. (Now I'm not talking arm path here -- or the direction of the hands when they separate from the glove. These can be changed and corrected, if a player is performing improper mechanics. Arm angle specifically is the angle of the hand and arm at the moment a throw is made.)
Perhaps the best way to determine a kid's natural arm angle is to have the pitcher go to the shortstop position and field grounders to the right and left of where he's standing. Have him come up and make a throw to first base.
After 5 or 10 throws, have the player go to the outfield and catch a series of fly balls, and make a series of 5 or 10 throws to home plate. (Try to get your pitcher to keep his throws on a line, with little arc. It's OK to one-hop the throw, if your player can't reach home plate in the air.)
Finally, have the pitcher play catch at about 80 percent effort at 60 yards.
After watching the pitcher throw from these three positions, you should be able to determine the precise throwing slot for the pitcher. This is where he should be throwing from every time he makes a pitch.
I feel strongly that a coach should not change a pitcher's natural arm arm as it can cause significant injury. Everybody's made to throw a certain way, so diversity among arm angles on a pitch staff is perfectly normal.
The coach should also refrain from the pitcher throwing at different arm angles -- for different "looks," to try and fool the batter. This never works. Pitching is difficult enough from one arm angle to experiment with various arm positions. And at the higher levels of the game, good hitters usually can pick up on the arm angle variations, so it's not particularly effective.
Each season, remember to determine a player's natural arm angle during early practices. Then have that pitcher throw only from that arm angle throughout the season.
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