Back in 2008, when he was just a young rookie and not yet the Greatest Pitcher Alive, the Los Angeles Dodgers' Clayton Kershaw threw fastballs around 70 percent of the time. That'll work, even when you have an all-time great curve you could be throwing instead. This year, however, Kershaw has gone to his fastball less than half of the time. And it's not a one-time aberration—his fastball rate last year was about 50 percent, too, and it hasn't been above 60 since 2013. What gives?
It also turns out that Kershaw is not alone. So far this season, big-league pitchers as a group are throwing fewer fastballs, as a percentage of all pitches thrown, than they have since we started collecting data on the subject a decade ago. That'snot a fluke, either, and it means that pitchers are now throwing a higher share of breaking and off-speed pitches than ever before, potentially risking injury in the process. And I think I know why it's happening.
First, the facts. Major League Baseball began keeping an accurate count of pitch types thrown back in 2008 (by coincidence, also Kershaw's rookie season), which sounds a little late in the game until you remember that half of us were lusting after Motorola RAZRs at the time and the other half were listening to Flo Rida. Let he who is blameless cast the first stone.
Anyway. Back in 2008, big-league pitchers threw fastballs 66.1 percent of the time. By the end of last year, that figure had dropped to 62.5 percent, according to Pitch Info data provided for this story by Rob McQuown of Baseball Prospectus.
As of April 16, 2017, the figure for this season is down another half-point, to 62 percent. And lest you think that fastball rate is a statistic that fluctuates wildly year-to-year, making a ten-year comparison worthless, the data shows that MLB's fastball rate has been steadily declining over that time.
Which brings us right back to the question of why: Why would pitchers, who are throwing faster than ever, move away from their fastballs? Why would Kershaw? Those questions bring me to an article the brilliant Rob Mains wrote over at Baseball Prospectus last week, in which he convincingly demonstrated that baseball's well-documented rise in strikeouts is linked to a less documented but still very scary rise in hit batsmen.
Mains showed that the change in the statistic of interest—in his case, the rate of HBPs; in ours, fastball rate—was tied not to a change in behavior but rather to a change in opportunity. In other words, pitchers weren't tryingto hit more batters, even though they were, in fact, hitting more batters. Instead, because the rise in strikeouts has brought with it a rise in situations in which pitchers are ahead of hitters in the count, pitchers are finding themselves in more situations in which they would normally throw pitches inside—the kind of pitches that hit batters.
Neat, right? I think something similar is happening with fastballs, but with a twist. In this case, I think we're seeing a change in both opportunity and behavior.
Fastballs, which are relatively easy to control, are the kind of pitch you tend to throw more often when you're behind in the count, in order to get a quick strike over. Pitchers are behind in the count less often today than they were ten years ago—about 4.4 percent less often, to be exact.
Thus, even if pitchers had changed nothing at all about how they pitch over the past ten years, you'd still expect to see a bit of a decline in fastball usage overall, given the difference in situations they're facing between 2008 and today. That, by itself, might explain part of the overall decline, but there's another thing, too: pitchers have changed how they pitch, given the count, since 2008.
Since 2008, there's been a 5.8 percent decline in the rate at which pitchers throw fastballs behind in the count. In other words, over the last decade pitchers have become a little less likely to throw a get-me-over fastball when they need a strike.
Why? Well, I don't really know. If I had to guess, I'd say it's because batters have probably become somewhat more likely to expand the zone, even when they're ahead in the count—basically every kind of swinging strike rate is up over this period—so pitchers are feeling less pressure to get into the zone with a fastball when they do need that quick strike behind in the count. Think about Kershaw burying a curve out of the zone in hope of a whiff from an eager batter, even behind in the count.
Still, that's just a guess. But I do know this: If you combine the demonstrated change in opportunity (a 4.4 percent decline in pitches thrown behind in the count) with the demonstrated change in behavior (a 5.8 percent decline in the rate at which pitchers throw fastballs in those situations) since 2008, you get something that looks rather a lot like the overall 5.4 percent decline in fastballs thrown.
Is that conclusive? No. Is it interesting? I think so. Those missing fastballs aren't disappearing into a vacuum. They're being replaced, as a percentage of all pitches thrown, by other pitches—in fact, by definition, by non-fastballs. And non-fastballs might, on the whole, be more taxing to throw than fastballs, given the biomechanical demands many breaking pitches make on arms (although fastball velocity also seems to have negative effects; this is a complicated area).
Now, to be clear, I'm not in a position to say that the increase in strikeouts is tied to an increase in the wear and tear on Kershaw's arm in particular, pitchers' arms in general, or to an increased risk of injury overall. Hell, I'm barely in a position to say that the increase in strikeouts is tied to this observed decrease in fastballs (although I think there's fairly convincing evidence for that position). That said, early in the season, this is a trend I'm watching closely, as a thousand little moments begin to come together into a bigger story.
Big-league teams, bitten by injuries to key arms, are working overtime to better understand the elements that combine to determine their pitchers' health. The mechanisms here are far from clear—velocity, high-impact pitches, and overuse might all play a role—but pitch mix is also clearly a part of the story. This year, as it's been for the last decade, that mix is evolving away from fastballs, and toward pitches that, like the arms that throw them, tend to break.
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