If you've ever spent any time around a pitching coach, you've probably heard the phrase "good extension" or "great extension." This is a reference to how a pitcher uses his arm. Unfortunately, there are a lot of people -- pitching coaches included -- who simply do not know good or great extension when they see it because they don't really know what it means.
The other day I saw a very good pitching coach complement his pitcher's extension on a particular pitch. The player kind of mimed extension by reaching toward home plate. The pitching coach stopped him and asked, "What does 'extension' mean?"
I'm sure that some of you are confused. You probably think of extension the same way this pitcher did -- drive off the rubber, extend toward the target. Unfortunately, that's not the type of extension that should be extolled.
In any athletic action, several extensions take place in several different places throughout the body. In the act of pitching, the term "extension" should refer to the position of the arm when the baseball leaves the pitcher's hand.
In pitching, "extension" is a generalized term that refers to elbow extension, but good extension isn't just about releasing the baseball with an extended elbow. Finding good extension is about releasing the baseball in a mechanically efficient position.
The most efficient release will occur when the hand reaches its maximum velocity in the direction of home plate.
The physics of rotational acceleration tells us that this happens when the forearm (the acting lever for the hand and ball) is perpendicular to the target because at this point, the hand is moving directly toward the target. 100% of the hand's -- and therefore the ball's -- velocity is directed toward home plate.
When this physics concept is applied to elbow extension in the throwing motion, "good extension" is seen in a full extension release point that is perpendicular to home plate rather than one that is reaching forward toward the plate.
Because the hand is connected to the elbow, the faster the elbow moves, the faster the hand will move. The elbow is connected to the shoulder, so the faster the shoulder moves, the faster the elbow will move.
Put together, these ideas build a concept of the release point in which the pitching shoulder, elbow, hand, and the baseball itself are moving straight toward home plate with near-peak velocity. The problem with that concept is that the human body is not made up of perfect levers like the ones that introductory physics classes love to pretend exist.
The result is that good extension can take many forms -- varying widely from pitcher to pitcher -- but true extension looks the same from pitcher to pitcher no matter how different their deliveries are. In most pitchers, good extension will occur slightly in front of perpendicular.
Now that you have an idea what good extension looks like, how important is it? That's a question that's not easy to answer.
As with most pitching concepts, there are always exceptions to "rules" like this. UCLA's Trevor Bauer is very good at what he does, has been clocked in the mid- to upper-90s, and is a great example of someone who does not have "complete" extension.
Inefficient extension -- such as short-arming the ball (a lack of extension) or "reaching through the target" (the wrong kind of extension) -- will likely result in lower velocities, but that doesn't mean that someone can't throw hard without efficient extension. On the other hand, overly aggressive extension can lead to cartilage irritation, joint swelling, and even olecranon fractures (Jay Powell, Joel Zumaya).
Proper understanding of concepts like this are essential for coaches that work with youth pitchers. Once improper techniques are assimilated, especially in kids with less natural athleticism, they can be extremely difficult to overcome.