Teaching or allowing young baseball pitchers to throw curveballs is a decision that should be treated with care.
Reasons for not throwing a curveball are that it will hurt a young pitcher's arm (particularly his elbow) and that it hinders the development of a good fastball.
Reasons for teaching the curveball include it will not hurt the pitcher's arm if thrown correctly and that a curve ball is needed by the time they get to high school.
I think that all of these reasons are legitimate and, in general, would like to see a pitcher wait until he is at least 14 before developing this pitch. And, given different types of arms and stages of development, 14 is a fairly arbitrary age.
Youth pitchers and curveballs
As a general rule, the younger the pitcher, the fewer curveballs he should throw and the slower he should throw them.
If a younger pitcher has any soreness in his elbow (or any part of the arm) he should give up throwing curveballs for at least a couple of weeks (and shouldn't pitch for a while as well).
Curveballs should be thrown with just enough speed to get them to the plate during the first year or two of throwing this pitch. A hard curve or slider increases the risk of injury.
Reducing the risk of throwing arm injuries
If the curveball is thrown flat (lots of horizontal spin), it is not as effective as a curveball that breaks down and sideways and it increases the risk of injury. Young players often think the curveball needs a lot of sideways spin. To achieve this, they drop their elbow below their shoulder when throwing it which is not only bad mechanically but puts too much pressure on their elbow. Getting kids to stop throwing with a dropped elbow is one good reason to teach them how to throw a curve ball correctly.
Mechanically, the curveball should be thrown like the fastball with the exception that the hand pulls down in front of the body instead of extending out toward the target and the wrist turns downward instead of snapping forward. In addition (although this varies to some extent with the pitcher), the path of the ball is a little closer to the head, the stride is a little shorter and the ball may be held a little further back in the hand.
Gripping the ball also has variations. Some of the best curveballs are thrown with the seam running along the inside of the thumb and the fingers perpendicular across two seams. The most popular grip is to put the fingers fairly close together running along a single seam.
If a young pitcher has the potential to throw 90 mph in high school, or the coach cannot be trusted to keep curves to a minimum (in a game or practice), or a pitcher tends to get a sore arm (especially at the elbow), or a pitcher has not spent a good deal of time working on a change-up, or the pitcher is overused (or throws to much on his own), I would recommend that he NOT start throwing a curveball.
Think long term
In the "win now" mentality of youth baseball — even at the youth-league age — saying “no” to a pitcher who wants to learn the curveball and slider before the age of 14 and 18, respectively, is easier said than done. But it is the responsibility of the coach and parent to be thinking long-term about their youth pitcher’s careers.
By not allowing a pitcher to throw curveballs and sliders, and instead teaching them to develop the fastball and change-up, your pitcher and team may take some losses. However, the bigger loss will come five years down the road when that youth pitcher, now 17 years old, can’t pitch because his arm was damaged from improper use.
You can teach a curveball later ... but not a fastball
Pro scouts demand a pitcher have a minimum 90 mph fastball, not a great curveball. That fastball isn’t developed if a pitcher is throwing the curveball 30-, 20-, or even 10-percent of the time throughout his youth career. The fastball is developed by throwing the fastball 95- to 100-percent of the time—only going to the change-up when an off speed pitch is needed.
Professional pitching coaches can always teach a curveball or slider, you either have a 90 mph fastball or you don’t get a shot at pro ball. It's your job as a coach or parent to steer youth pitchers in the right direction. Get them throwing fastballs!
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