The eternal cat and mouse game between pitchers and hitters took another turn this past month, as Major League Baseball made the decision to start enforcing its long-dormant prohibitions against use of foreign substances by pitchers.
First announced to owners and teams at the start of the month, and then officially revealed to the public on June 15th, umpires now perform a series of mandatory spot-checks over the course of each game to ensure that pitchers are not using illegal foreign substances.
“Why now?”, some might ask - this seems like kind of a random rule to “pick back up”… and that isn’t necessarily wrong. MLB has received a lot of criticism for how they have chosen to communicate and message the inspections to coaches and players. However, I do think that intervention of some sort has definitely been needed - let’s take a peek at some numbers to understand why.
When pitchers throw the ball, they use their fingers to impart spin - giving the ball “break” of all different kinds. For a curveball, this is often a “bite” that pulls the ball down and away from the hitter, and for fastballs, this is often backspin which helps keep the ball tracking higher and harder than the hitter might expect. Note that this spin is measured in revolutions per minute (RPMs) of the baseball when released, so it can be easily tracked over time and by pitch type.
For a long time, spin rate was viewed purely as a byproduct of throwing good pitches - we can visually see a hard-breaking curveball, so why would we need numbers to tell us that?
But over the last ~5 seasons, pitching analytics began to reveal that spin rate was an under-optimized tool - pitchers could reap significant benefits in terms of pitch velocity and movement by focusing on increasing the RPMs of their pitches, and making spin rate improvements a primary goal of their pitching training.
Over time, spin rate became an ordinary part of the pitching discourse - with the MLB website prominently featuring discussions of Spin Rate and it’s impact on hitters - even using their Statcast platform to publish a live leaderboard of pitchers achieving the highest spin rates for different pitches.
Unfortunately, it had become something of an open secret across baseball of late that pitchers have been using different substances and aides to help increase their spin rates - everything from hyper-grippy Spider Tack to the relatively “innocent” swipe of sunscreen with rosin - to the detriment of hitters everywhere, who have been tasked with hitting these higher RPM curveballs and fastballs, and who are struggling (to put it mildly).
With this context in mind, it’s incredibly interesting to see what a big bite the new foreign substance rules seem to have taken out of spin rates across the league. The data is still young, but seems to strongly suggest that pitchers had relied heavily on various grip aides (which, as mentioned, exist very much on a spectrum) to up their spin rates, and things have regressed to a more natural level in the past few weeks.
Spin Rates Across the League