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Earlier this week, MLB published it’s first round of bat tracking metrics. This release included bat speed and swing length metrics at the pitch level. This is massive for advancing public evaluation of hitters, as it’s the first real advanced tracking data released specific to hitters. For years, we have had access to troves of pitcher-centric metrics like velocity, movement, spin, and more, and although we can use this data in our evaluation of hitters as well, bat speed and swing length (at least to an extent) are directly under hitter control. These metrics still don’t tell the full story, but there is still a ton of insight to be gleaned from what we do have. Today, let’s take a dive into bat speed and swing length and the context needed to understand them.

Sitting atop the bat speed leaderboards are largely guys you’d expect, hulking sluggers like Giancarlo Stanton, Oneil Cruz, and Kyle Schwarber. However, there are a few somewhat unexpected names, like Christopher Morel and Jo Adell. Morel is much smaller than the rest of the players on this list, and although Adell has struggled following his prospect hype, this is representative of the tantalizing raw tools he possesses.

A partial explanation for why Adell has struggled over his career despite his elite bat speed is his inability to purely contact the ball. By employing Dr. Alan Nathan’s batted ball collision formula, we can estimate the maximum attainable exit velocity of a swing. Hitters who can get the most out of their maximum attainable exit velocity are those who most frequently make solid contact that efficiently converts bat speed into exit velocity. MLB has a statistic for this, which they call Squared-Up%, and it is the rate at which hitters make contact that achieves at least 80% a batted ball’s maximum attainable EV. The stat is dominated by contact savants like Luis Arráez and Nolan Schanuel, as well as some of MLB’s elite hitters like Juan Soto and Mookie Betts. Per Kyle Bland’s fantastic Swing Speed App, Adell has done the worst job in MLB (through May 13th) of converting his bat speed into exit velocity.

There is a negative relationship between bat speed and squared up rate, which is part of what makes guys like Soto who combine strong marks in both metrics so special. In the limited data we have so far, it appears that hitters make the purest contact when they hit line drives, in the neighborhood of 10 degrees.

Swing length is defined as the “total (sum) distance (in feet) traveled of the head of the bat in X/Y/Z space, from the start of bat tracking data (generally around 150 ms), up until impact point.” This metric quantifies the idea of being short to the ball, a common refrain among coaches. Being short to the ball, all else held equal, is intuitively preferable. However, swing length is often not just a measure of a hitter’s preferred or natural swing path. Swing length is related to many other factors, like batted ball direction approach, pitch location, bat speed and more. Because swing length is calculated until point of contact, pulled batted balls have longer swing lengths.

This isn’t anything groundbreaking, obviously. It does, however, demonstrate the nature of this metric. Hitters who are seeking to meet the ball out in front will naturally show up with longer swings, even if through a central point (for this example, let’s say the front of home plate) they have the exact same swing length as someone who makes contact with the pitch deeper.

Also unsurprisingly, pitch location is also very influential on swing length. In fact, smoothed pitch location (standardized for handedness) can explain nearly one third of the variance in swing length. Swing length generally increases on lower pitches, as the barrel has to travel further from a traditional loaded position to meet that location.

Faster swings generally coincide with longer swings, as they give the hitter more time to get the barrel up to speed. The coefficient of correlation between swing length is 0.54, which isn’t incredibly high but definitely meaningful. This isn’t the case for every hitter, though. Certain hitters, like the following, exhibit almost no relationship between the speed and length of their swing:

Other hitters, though, consistently swing harder when they swing longer:

Hitters like these are good candidates to pull the ball more frequently, because as mentioned pulled batted balls require longer swings and this would allow them get to their top end bat speed more consistently.

A good way of contextualizing bat speed for the length of a swing is by looking at bat acceleration (credit again to Kyle Bland). This more accurately credits a hitter for getting the barrel moving quickly in a short period of time, which is helpful for both making contact as well as creating real damage on contact. Although many have found the players with higher bat speeds generally have lower contact rates, there doesn’t seem to be much of a difference in the distribution of acceleration among swings that resulted in whiffs versus contact. In fact, swings that resulted in whiffs have a distribution skewed more towards the low end of acceleration values than swings that resulted in contact

I also personally look at this as a measure of athleticism or explosiveness, as it is related to the rotational strength of an athlete. The following are the leaders in a super simple Dynamism metric, which is just average acceleration over player weight. We know larger guys can get the barrel moving fast, but I’m interested in the guys with leaner builds who can still get real speed in their swing in a short window. Some of the metrics are definitely skewed by incorrect weights (there is no chance that Christopher Morel is 145 pounds), but it still provides a decent estimation of the dynamic athlete type we’re looking for.

This list is full of exciting young guys, and I think evaluation in this vein can serve as a sort of proxy for raw athleticism, especially paired with things like sprint speed.

Bat speed and swing length don’t tell the whole story and hopefully future release include things like miss distance or horizontal/vertical bat angle, but it’s very encouraging to see MLB making an effort to distribute valuable data like this. The public has had access to this data for just a short amount of time and tons of interesting research has already been done, and I’m sure with more time we will better understand how to use and interpret this data.