Wednesday, July 8, 2020

Statcast Aging Curves: Looking at How Hitter Exit Velocity Behavior Changes

Recently I looked at aging curves in the NHL. In my last couple of posts, I used the delta method to determine how forward skills age and how overall player value changes. Now, I am moving to baseball and, more specifically, exit velocity aging curves from the Statcast data. Since 2015, MLBAM has made some of the data recorded by TrackMan (and in the future Hawk-Eye) available to the public through its website Baseball Savant.

I wanted to see how hitter batted ball profiles aged, so I pulled a few batted ball metrics from the leader-boards and developed an aging pattern. In this study, I weighted each delta by the average number of batted balls between two player seasons.

First, the most commonly cited metric from Statcast, average exit velocity. While I have some qualms with how average exit velocity is presented versus what it actually means in practice (average exit velocity gives no context to the spread of a hitter's exit velocities, so Christian Yelich and Franmil Reyes profile similarly). Nevertheless, here is how average exit velocity ages during a hitter's career:
Age, to reiterate the point I made in my first aging curve post, is the second age in a given bucket. So age 24 on the chart corresponds to the delta between age 23 and age 24. The peak is a bit later than I expected, given our understanding that generally peak performance generally occurs somewhere between age 24 and 27. Maybe average exit velocity over a season is not a good indicator of whether or not that season was successful, relative to that player's ability. Let us now look at how maximum exit velocity ages: 
This is more in line with the traditional aging curve for player performance. For the sake of easy comparison, I put the maximum and average exit velocity curves on the same chart, along with average line-drive/fly-ball exit velocity.
Average exit velocities in balls put into the air age similarly to total average exit velocity (i.e. a less robust curve and later peak). This makes me think that maximum exit velocity is a distillation of where a player is in his career than either total average exit velocity or air-ball exit velocity. This finding is similar to the conclusion Rob Arthur came too in a 2018 study at The Athletic (subscription required). This is despite the fact that the maximum exit velocity for a player in a given season consists of a sample size of one (because it is the maximum of a distribution) while the other two measures consider all balls put in play. Subjectively, I would think that maximum exit velocity is a better indicator of physicality than average exit velocity, the latter of which can remain relatively unchanged even if the maximum declines through the development of some of the "soft skills" of being a major league hitter (waiting for good pitches to drive, avoiding swinging at pitches outside the strike-zone, etc).  

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