Parenting Tips: The Dangers of Overzealous Swinging

Jay Biggerstaff-USA TODAY Sports

I can’t possibly begin to cover all the excellent work concerning Statcast’s new bat tracking data. Now as much as ever, it’s important to support your local Baseball Prospectuses, PitcherLists, Baseball Americas, FanGraphses and freelance Substack writers. We move quickly in these parts. There’s so much analysis to consume, all of it superb. When confronted with this new data, one of my first instincts was to see which metrics from other areas of sabermetric analysis could be replicated within the bat tracking framework. Enter 90th-percentile exit velocity (90EV); it’s a powerful shorthand metric that distills a lot of information about the top end of a hitter’s exit velocity distribution into a single number. It’s not perfect, and other metrics outperform it, but it’s easy to see how it has become popular in contemporary analysis, especially in prospecting and scouting circles. So I looked to 90EV for inspiration with respect to bat speed. I assumed bat speed, being a primary input to exit velocity, would exhibit similar strengths in characterizing a hitter’s distribution of bat speeds by consolidating the most crucial speeds into a single value. Indeed, parsing bat speeds into deciles appears to work really well:

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Average exit velocity increases linearly with bat speed. So does contact quality (xwOBA on contact, or xwOBAcon) and barrel rate (Barrel%). All of these trends meet expectations. What defies expectations, however, is that contact rate (Contact%) spikes for a hitter’s top 10% of bat speeds. Whereas bat speed correlates positively with whiffs (that is, more bat speed typically means more whiffs), the average hitter’s fastest swings evidently produce more contact, not less, and by a substantial margin to boot. That’s an amazing collective feat. It’s like the numeric equivalent of “seeing beach balls.” There’s more to this than meets the eye. Why is this possible? Let’s view the predicament through the lens of physics. Do you remember physics? One thing I remember — in addition to spending an inordinate amount of time across several days with my friend Kevin trying to bounce an empty plastic apple juice bottle on its side to get it to land upside-down on its cap — is that force equals mass times acceleration. Not velocity, but acceleration. The typical swing starts at an initial velocity of zero and makes contact at a final instantaneous velocity of X (in this case, “Bat Speed”). To reach X mph from 0 mph, the bat must accelerate. How much the bat accelerates determines how much force the hitter imparts upon the ball. And what is a hitter’s primary job if not to impart force upon an unsuspecting, unassuming baseball?

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In moving from the penultimate decile to the top decile, the league-average exit velocity plateaus at 86.4 mph (rather than continuing to increase linearly). Contact rates peak closer to somewhere between the 60th and 70th percentiles, as does Statcast’s new square-up percentage (Square-Up%). Barrels peak below the 90th percentile, as does Statcast’s new Blast rate metric (Blast%), as well as our tried-and-true xwOBAcon. The texture of assessing swing efficacy (through the viewpoint of a single bat tracking measurement) has changed entirely. All of which is to say that daddy hacks — true swing-out-of-your-shoes daddy hacks — perhaps aren’t the best way for a hitter to optimize his offensive production. Which isn’t to say that daddy hacks aren’t good, productive swings — they are still among the most productive swings a hitter can make, and a hitter ought to err on the side of too much acceleration rather than not enough — but this is a question of optimization, not magnitude.

What, then, is the best way to optimize? What’s the ideal combination of bat speed and bat acceleration, using this unsophisticated percentile framework? It’s somewhere in the red-highlighted area below, which is typically not where a hitter maximizes his acceleration:

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It is where he maximizes bat speed, but through a longer swing that allows more time to accelerate (at a slightly lower rate of acceleration) to its maximum speed. That’s not something you can glean from bat speed alone. As such, top-10% bat speed as a 90EV-equivalent metric simply does not capture the information we want it to capture, at least not in its entirety. The solution to the problem of optimization vis-à-vis contact quality for any hitter, regardless of size and strength, is to minimize swing length for any given bat speed (red = high average exit velocity, blue = low). To be clear, long swings aren’t bad; in fact, a longer swing (in distance, not time) tends to denote more distance across which to accelerate, which naturally creates more bat speed. You can actually see that longer swings are associated with higher bat speeds and higher exit velocities. But if a…