Home Run Rates and Fastball Movement

In the offseason, there was a lively debate about Matt Cain and his ability to keep the ball in the park. There was one hypothesis that suggested that pitchers who throw a hard, rising fastball* give up less home runs per fly ball than would be expected.

I did some initial research into this and found that there is a fairly strong negative correlation between vertical movement and home run rate. My initial research looked just at individual starting pitchers and was not able to isolate just fastballs, giving my concern that there might be a lot of noise that could have distorted the true effects. Well, now I have much better data that gives me every fastball tracked by Pitch F/X from 2008 through last night's games to study from.

This new sample is huge (54,396 pitches) and should hopefully give me some robust answers as I work with it. My first exercise was to break the pitches into buckets grouped by vertical movement and look at the corresponding home run rate.

The correlation here is pretty astounding (R² of 97). In my previous study, there was some correlation but nothing close to the magnitude here. This seems to lend credence to my theory that fastballs that drop less than expected induce more "lazy" fly balls than hard hit fly balls.

To me, this seems like common sense reasoning as well. Most would tend to agree that if a pitch has more sink, it would have more gravity that would cause batters to hit the top portion of the ball and induce ground balls.The opposite should be true as well. If a pitch fails to drop as much as gravity suggests, batters would hit the bottom portion of the ball and induce more pop ups and fly balls.

The next step will be to create a regression estimate including everything that goes into explaining a pitcher's home run per fly ball rate. At least this gives me an idea that I am on the right path.

*The fastball is not literally rising but would be in a gravity neutral environment. The vertical movement described here means that the pitch is dropping less than expected.

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