How a Lower Arm Angle Affected Jhoan Duran

Duran’s release was slightly different, which made impacts throughout his arsenal

By most metrics, and the eye test, Twins’ reliever Jhoan Duran was not as effective in 2024 as he had been. His 3.64 ERA, 1.66 win probability added (WPA), .231 batting average against, and 28.9% strikeout rate were all the worst of his career. It’s not that those are necessarily *bad* numbers — among 169 qualified relievers in 2024, Duran tied for 29th in WPA and 36th in strikeout rate, and his ERA and batting average allowed ranked 85th and 88th, respectively — but they weren’t the kind of league-dominating metrics that we’ve grown accustomed to from him. They were more above-average reliever than this:

I wrote late last season about Duran’s first rough patch as a Twin, a summer swoon through July and August that I pinned mostly on negative movement and shape changes affecting his high-velocity fastball.

His September was much better, with a slight return of some of the vertical movement on his heater. Then he threw five scoreless innings in the postseason, including getting the critical outs that secured the AL Wild Card Series for the Twins.

It Starts with the Heater

This season, when we dig into Duran’s results by pitch type, it’s again clear that his four-seam fastball was not as effective as it had been:

Data from Baseball Savant

Despite again averaging triple-digit velocity, Duran’s fastball was a net negative in terms of run value and allowed a higher weighted on-base average (wOBA) for the second year in a row.

For context, in 2022, Duran’s four-seamer ranked in the 87th percentile in run value. In 2023, it was 71st. This past season, it was in the 30th percentile, a dramatic fall for a pitch that continues to lead all of baseball in average velocity (100th percentile).

Where batters hit .245 and .212 against Duran’s fastball in 2022 and 2023, they hit .296 last season.

Much of that difference can be chalked up to some poor batted-ball fortune. He allowed a .465 batting average on balls in play (BABIP) off his fastball last season, and the average exit velocity of those batted balls was 87.6 mph, a lower mark than either of the past two seasons.

To that point, the expected batted average and slugging percentage against Duran’s express last season were both slightly better than in 2023. At the same time, opposing hitters whiffed on 36.1% of their swings against Duran’s fastball last season, which was a touch better than 2023’s 35.7% mark and significantly better than 2022’s 26.3%.

Given those marks, we might have expected Duran’s four-seamer to perform better, or at least more in line with the past.

A Running Fastball

However, it’s also true that his four-seamer was different. You saw in the table above that its average velocity, while still incredible, was the lowest of his career. The pitch also had a different movement profile, which continued a trend from last season:

Data from Baseball Savant

You can see he lost two inches of induced vertical break (i.e., rise, resisting the effects of gravity) and added horizontal movement.

I explained in my analysis last year that Duran’s heater is not the type of high-spin vertical moving fastball that stays above barrels at the top of the zone. Instead, it has much less vertical movement and much more horizontal movement than other fastballs at comparable velocities, making it increasingly a side-to-side, running fastball. That’s evident in the table columns comparing his movement to the averages.

Statcast recently released new data and metrics about pitchers’ arm angles that help to explain these changes. Those data reveal that Duran’s arm slot dropped slightly last season, down to 34° from the upper 30s the previous two seasons:

Graphics from Baseball Savant

That change, intentional or not, affected how Duran’s fastball behaved primarily because it subtly changed the direction in which the ball spins out of his hand.

Think of a clock face. The 12 o’clock position represents perfect backspin, which contributes to vertical movement on four-seam fastballs. As you move clockwise toward the 3 o’clock position, the spin direction becomes less backspin and increasingly side-spin, which contributes to horizontal movement.

Duran’s fastball spin direction shifted from 1:30 on the clock face in 2022 and 2023 when his release angle was higher, to 1:45 (toward more side spin) as his arm angle declined last year. That explains the lesser vertical movement and the increased horizontal break.

We can visualize that change with this excellent chart from Max Bay, who created the Stuff+ metric and partnered with Eno Sarris, that shows the movement profile of Duran’s fastball in 2024 compared to the previous two years:

Graphic courtesy of Max Bay, accessed from: https://dynamic-dead-zone.streamlit.app/

You can see in the plot that the pink dot and circled area are lower and more to the right (his arm side, toward third base) than they had been previously.

Something to Fix or Embrace?

It’s hard to parse how much of this stems from the spring training oblique injury that Duran sustained. It’s also a change that is not necessarily a bad thing on its own. It may not even be a problem that needs to be “fixed.” In pitching, it’s often useful to be different from others and different than expected, especially if you can capitalize on that difference and deploy it to your advantage.

In that regard, there might be some low-hanging tweaks that Duran could take advantage of, particularly in where he throws the pitch.

You can see in the animation below that the most common locations for his four-seamer have shifted toward his glove side over time:

Courtesy of Baseball Savant

That’s the opposite direction from which his fastball has gained movement and also not in the areas where the pitch has best performed in his career, as you can see here:

Courtesy of Baseball Savant

Up and to his arm side (up and right in the chart above) is where he’s had the best success with that pitch, and it’s where we would typically expect a pitch with this shape to play best. As the pitch has migrated across the plate, it hasn’t performed as well.

That location change has also likely had some negative impact on Duran’s curveball and splitter, which he threw more often last season, no doubt in some measure a response to the struggles with his fastball.

While both his secondaries were still net positive in run value, they were also less effective than the previous two seasons.

That could be in part because of the arm slot change affecting their movement (especially the horizontal shape of the curveball, which increased). But it could also be that the fastball being located more on the glove side is a less effective tunnel from which to deceive hitters.

To that point, Duran has gotten less chase and less swing and miss on his out-of-the-zone curveballs and splitters over time:

Data from Baseball Savant

This offseason and heading into next season, it will be interesting to watch if Duran works to raise his arm angle in an attempt to regain his vertical movement, or if he embraces his new pitch shapes and optimizes where to best deploy them.

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John writes for Twinkie Town, Twins Daily, and Pitcher List with an emphasis on analysis. He is a lifelong Twins fan and former college pitcher. Follow him on Twitter @JohnFoley_21.