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Introduction to VMI

Posted: 2021-06-23 08:36:38 (ET)    [ 513 views ]

If you are relatively new to BaseballVMI, the following may move you along a little faster in your studies:

Let's talk about Air Density as related to baseball hitting and pitching.

What causes air density to change?  Air becomes thinner as the elevation increases.  It also becomes thinner as the temperature goes up.  As the humidity percentage increases the air density actually gets thinner (or lighter) as well. Why? Because there is more hydrogen in humid air and less nitrogen.  Yes, high humidity feels sticky, but it is not heavier for a baseball to push through.

So, in thin air in Denver in the 90 degree temperature ranges the air is very light, so a baseball can push through the air more easily and therefore curveballs get lesser bite against the air.  This fact is the same with fastballs and every other pitch.  However, at sea level and closer to sea level, the air is heavier because of the altitude, cooler temperatures, but rarely lesser humidity.  Humidity does not have much effect on how a baseball flies at either end of the altitude spectrum for MLB pitchers.

For more information about the air density and the Air Density Index (ADI) see the Science tab on this website.

Which Pitches favor the pitcher in Heavy Air and which in Light Air?

The objective of this section is to help you gain insight into "when" teams and individual players will be more effective.   If you know the VMI, then you know the hitter's set-up tendency for today, and you will have a clearer picture of the hitter's capability to adjust to a particular pitch.  For example; if the "tail-off" on the 95 mph Four-Seamer is lifting 6 inches today at a 65 ADI as shown on the charts under "Pitch-Mix" on this site, but the hitter is used to more than that amount of movement and sports a +4.00 VMI today, then the following should be a tip for you, or the pitcher, or catcher:

This hitter may be more susceptible to being induced into a groundball with the two-seamer or cutter.  This is because his set-up (that is; his swing vs. what he sees) will be above the four-seamer's tail-off and fully prepared to meet that lifting pitch upon its arrival at the zone.  Two-seamers and cutters however, lift less than the hitter is expecting from the four-seamer for which he is most often prepared, so he will need to bring the bat further downward to hit it and most likely will result in a groundball.

Everyone in baseball knows why and how to use two-seamers and cutters, they just don't always know "when," as there has never been an analysis of how players react to the physical properties of the air which causes late movement and allowing further analysis of hitters' preparation.

Now, imagine the opposite scenario, whereby the hitter is not used to that amount of movement, but has been playing in about 59 ADI.  His VMI will be as great as -4.00 or greater negative, therefore his setup to swing will naturally be below the center of the four-seamer until he adjusts.  

This team's hitters would tend to be more dangerous against the two-seamer/cutter, because the set-up is closer to where the hand-eye coordination needs to be to hit it on a line.

Under "Pitch-Mix" on this site, there is a more detailed description of each VMI range and each type of pitch.  Also, there are many previous blog articles you can access by scrolling down on this blog page. 

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