The bones are growing and the muscles gets stretched and the tendon pulls on the bone. Ages 13 to 18, players must condition the body to make it through this growth period. Proper warmup and stretching. This is accomplished by the hitter looking at the release point of the pitcher. This is where the ball leaves the pitchers hand.
Timing is everything.
When the pitcher starts his motion, focus your eyes on his hat, and when he separates his hands, shift your eyes to the release point. You will then see his hand positioning on the ball. If his hand is behind the ball, it's a fastball, if his hand is on the side, it's a curveball. The change-up is a tough pitch to pick up. You can also tell what pitch is coming, by the way the ball is rotating.