Dale Zanine-USA TODAY Sports
Statcast’s new public repository of bat tracking data has been out for a few weeks now. Like every number manipulator with a sense of curiosity and middling technical skills, I’ve been messing around with the data in my spare time, and also in my working time, because messing around with data is both my job and hobby. Mostly, I’ve been reaching some conclusions that mirror what others have already shown, only with less technical sophistication on my part. This article by Sky Kalkman does a great job summing up the biggest conclusion: Pitch location and spray angle (pull/oppo) influence swing length so much that you probably shouldn’t quote raw swing length.
But I thought I’d look for something slightly different, and I think I found something. Here’s the high level conclusion of my search: When pitchers throw harder fastballs, hitters slow down their swings to compensate. It sounds counterintuitive. Shouldn’t hitters speed up their bats to try to get to the faster pitch? But I had a hunch that this wasn’t the case. If you listen to hitters describe their approach against flamethrowers, they focus on shortening up and putting the ball in play. “Shortening up” might sound like it describes swing length, but it also surely describes swing speed. A hitter who is just punching at the ball likely won’t swing as hard as one trying to launch one. If you’re prioritizing having your bat on plane with the ball as long as possible, you probably aren’t focusing as much on raw speed.
Here’s a further conclusion: Pitch velocity impacts hitters’ ability to make optimal contact with the ball. You can measure this effect, and it’s greater in magnitude than the variation in swing speed. Hitters are shortening up to make more consistent contact, but it’s just downright harder to square up a faster pitch. The result is weaker contact on average. Combine the slightly slower swing speed and the worse contact quality, and you get a delightful result: The combination of slower swings and worse contact offsets the gain in exit velocity attendant in faster pitch speeds almost exactly.
That’s the conclusion, but I’ll be honest with you, I’m eager to share my methodology because I want to know if I did this right. The internet is full of people who are good at statistics, and I’m certainly not the best at it. I’ll walk through how I did it, explain the magnitude of the effect, and then try to make some conclusions based on that effect. I’d love for someone to point out where I’ve messed up my methods or should make an adjustment, because that’s a great way to learn.
Let’s start with the dataset. I took every single four-seam fastball and sinker for which Statcast recorded a swing speed. That gave me 44,084 observations. Clearly, the first thing we need to do is control for hitter identity. Hitters have far more to say about swing speed than pitchers. In other words, it matters much more which hitters face 99 mph fastballs than how those hitters change their approach. My first step, then, was to find each hitter’s average swing speed against each type of fastball. I calculated this for every single hitter who has recorded a swing against a fastball in 2024. As an example, Shohei Ohtani’s average swing speed against four-seamers this year is 73.9 mph; against sinkers, he’s averaged 74.2 mph. I noted each batter’s average against each pitch type.
Next, we have to control for count. As demonstrated in this excellent article by Steve Brown and Patrick Dubuque, count is vitally important in determining swing speed, so I controlled for that as well. For each combination of pitch type and count (0-0 sinkers, 1-2 four-seamers, etc.), I calculated the average swing speed you’d expect given the population of hitters using the calculated average swing speeds from up above. This gave me an expected swing speed in the absence of count effects. I compared the realized swing speeds to those expectations to determine the effect of count. My findings largely mirrored those of Brown and Dubuque.
This says roughly what you’d expect. Hitters let it eat when they’re ahead in the count and swing more slowly to prioritize contact when they’re behind. If you don’t account for this effect, your answer won’t make sense.
Consider an obvious cross-correlation: Pitchers throw harder when they reach two-strike counts. Hitters naturally swing more defensively in two-strike counts. If you don’t control for count, you’ll find that hitters swing slower at faster pitches, and it’ll be an artifact of count.
Using these two controls, I produced an estimated “control” swing speed for each swing against a fastball this year: hitter average swing speed against a given type of fastball plus the count adjustment. This metric isn’t perfect, because different hitters adjust their swings differently with two strikes, but to be honest with you, it sounded daunting to chop up my data further and I was skeptical that it would improve things very much, so I stuck with this approach.
In any case, I now had a number to work off of, namely how fast I expected each batter to swing given who they were and what count they were facing. The difference between that expectation and their actual swing speed could be down to any number of factors, including game state, how they were feeling that day, what pitch they were looking for, whether the umpire sneezed during their backswing and distracted them slightly, random noise – you name it. But pitch speed is also something that can affect swing speed, and in aggregate, the hope is that those other random factors even out.
How does the data work out in practice? Here’s a table of the difference between my expected swing speed and the actual swing speeds recorded, grouped by pitch velocity:
Four-Seam Velo and Swing Speed
| Velo | Swing Speed | Pred Swing Speed | Difference | Observations |
|---|---|---|---|---|
| 89 | 69.70 | 69.62 | 0.08 | 807 |
| 90 | 69.90 | 69.38 | 0.52 | 922 |
Sinker Velo and Swing Speed
| Velo | Swing Speed | Pred Swing Speed | Difference | Observations |
|---|---|---|---|---|
| 89 | 69.52 | 69.83 | -0.31 | 1140 |
| 90 | 70.33 | 69.77 | 0.55 | 1052 |
As you can see, the bulk of the observations occur between 92 and 97 mph, which makes good sense: that’s the cruising speed of most pitchers. Relative to what you’d expect based on hitter identity and count adjustments, hitters seem to adjust their swing speeds almost instinctively based on pitch velocity. The data shows that when faced with faster pitches, hitters do slow down their swings to compensate, ultimately leading to a balance between pitch speed, swing speed, and contact quality.
