One of the ongoing arguments among baseball coaches and instructors is whether or not to use weighted baseballs for training purposes. And while I'll give you my opinion, as always, it is up to you to make your own decision.
The exercise scientist in me say that it is a bad idea for a two main reasons. The smaller shoulder muscles that you rely on to throw aren't used to handling a six or seven or ten ounce ball (a regulation ball is five ounces). By putting more stress on your shoulder, you increase your risk of injury.
The other aspect you need to look at is the difference between fast-twitch and slow-twitch muscles. In order to throw faster, your arm has to move faster. When you throw a heavier ball, your shoulder moves through the pitching motion slower. And when you consider muscle memory (which is your body adapting to performing a motion over and over), it isn't out of the question that if you throw a heavy ball too much, you will actually decrease your velocity.
So when you throw a heavier ball, your pitching arm is slower and you are putting a heavier load on the smaller, stabilizing muscles of the shoulder.
You are better off to work on proper mechanical development and correctly using your body. By using your entire body, you can generate power you weren't aware of and throw harder. It isn't the quick fix, but it is a better solution.
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