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MLB Curves And Sliders Have Gotten Alarmingly Nasty

Major League Baseball seems to be on an unstoppable pathway to more and more strikeouts. After a record share of plate appearances ended in a strikeout last season (22.3 percent), this season is winding up to set another record: If the to-date strikeout rate of 23 percent holds or increases over the season, it would mark the 12th straight year of a record set for strikeout rate.

Fastball velocity is often cited as the source of the strikeout surge, along with hitters willing to trade contact for power. The constantly increasing fastball velocities of recent years are such a concern that MLB is partnering with the independent Atlantic League next year to move the mound back by 2 feet — to 62 feet, 6 inches from home plate. Still, there may be another culprit behind all of the K’s: Breaking balls have never moved more. According to a FiveThirtyEight analysis of PITCHf/x and Statcast data at Baseball Prospectus, sliders and curveballs this season are darting away from bats at their greatest levels in the pitch-tracking era (since 2008).

Breaking balls typically have greater whiff rates than fastballs, and breaking balls are being thrown more frequently.

On sliders, pitchers are often looking for more horizontal break — and less vertical movement — than on a curveball. They’re getting both this season:

Velocity is still certainly at play in these nastier breaking balls: Increased velocity generally leads to an increase in spin rate. (The average curveball spin rate this year is 2,536 rpms, up from 2,315 rpms in 2015). More spin (specifically, transverse spin) creates more of the phenomenon known as Magnus Effect, which plays a key role in determining a pitch’s movement.

But velocity hasn’t surged as dramatically as pitch movement has in recent seasons. And it isn’t just that fresh faces with killer breaking balls have replaced older, craftier pitchers: 86 pitchers who threw at least 50 curves in 2017 and 2019 averaged an increase of 1.4 inches of greater vertical break, according to a FiveThirtyEight analysis. Something else must be at work.

One possibility is that new methods of training, and new technologies, are playing a role in creating better breaking balls. For example, when Cleveland Indians pitcher Trevor Bauer revamped his slider in the 2017-18 offseason, he wanted a pitch with zero inches of vertical movement and an elite level of horizontal movement. At Driveline Baseball, a data-based training center, he used new pitch-tracking technology, allowing for improved feedback of the pitch-in-progress. Specifically, he wanted to create a certain spin axis, which determines the direction the ball moves, while spin rate determines how much the pitch can move.

The technology allowed him to monitor the spin rate and spin axis of the pitch. High-speed cameras showed him how his grip was imparting spin on the ball. The work paid off: In 2017, Bauer saw just 2.83 inches of horizontal movement on his slider, which ranked him 131st in baseball. But this season, he’s up to 11.1 inches, putting him third.

Detroit Tigers starter Matt Boyd, who also designed a slider at Driveline, has quietly become one of the top pitchers in baseball. His slider ranks as the game’s seventh best since 2018, in terms of runs saved compared to the major league average per pitch type. “I’ve worked on it at Driveline a ton in recent years,” Boyd told FanGraphs. “At Driveline, we were on the Rapsodo [pitch-tracking tech] and the Edgertronic [high-speed camera] a lot. … Again, we were working that thing, working that thing.”

White Sox left-handed reliever Aaron Bummer became yet another pitcher to look into designing a pitch this winter. In a Driveline-like facility called Dynamic Velocity in Omaha, Nebraska, he created a cut fastball to better combat right-handed hitters. After righties hit .338 against him last year, they are batting .095 this season. Bummer has allowed only one earned run in his first 11⅓ innings this season. His cutter and fastball rank among the best in the game.

“It’s actual proof that your ball is moving, not someone sitting behind you and saying, ‘Oh, yeah, that’s a great pitch. That looks really good,’” Bummer said of the tech-aided pitch development. “You have instant feedback that tells you, ‘Did that pitch do what it’s supposed to do?’ To me that was a huge deal. The belief behind it.”

While the effort to create better pitches began with curious (and/or motivated) players going outside the game to independent facilities, teams are hiring more and more outsiders to pitch-design roles within their organizations. Over the offseason, the San Francisco Giants hired former Driveline pitch-design guru Matt Daniels, and the Philadelphia Phillies hired another design specialist in Eric Jagers. The industry is so interested in pitch design that the Edgertronic SC1, the most common high-speed camera, is selling more quickly than Sanstreak can produce it. This year, the Edgetronic and Rapsodo devices were seen in spring training bullpen mounds all over Arizona and Florida. The Orioles held “mandatory group spin axis seminars” this spring.

Pitchers’ pitches will likely keep getting better, moving faster and with more movement. There is seemingly no end to the increasing frequency of strikeouts. While the home run surge has masked other areas of offensive decline, technology and training is giving pitchers a new edge.

Check out our latest MLB predictions.

Travis Sawchik is a sportswriter for FiveThirtyEight.

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