Paul Sewald Shows Us Why Vertical Approach Angle Matters

One of the biggest reasons the Mariners have stuck around the fringes of the AL wild card race despite a -60 run differential has been the collective performance of their bullpen. Kendall Graveman had established himself as a bonafide relief ace before being traded to the Astros in late-July, and Drew Steckenrider has revived his career after falling apart in Miami two years ago. But equally unexpected, and perhaps the biggest reason Seattle felt comfortable dealing Graveman at all, has been the fantastic season put together by its other breakout relief ace: Paul Sewald.

Drafted as a reliever by the Mets in the 10th round of the 2012 draft, Sewald came with two years of college experience under his belt at the University of San Diego and quickly moved through the farm system. He reached the majors in 2017 and made 125 appearances out of the bullpen through 2020, but outside of a 23.5% strikeout rate that was just a hair above league average, he was mostly forgettable as a Met; across 147.1 innings in New York, he posted a 5.50 ERA. He was non-tendered this past offseason and signed a minor-league contract with the Mariners in January.

Even though Sewald wasn’t able to make the major league roster out of spring training, he has thrived in Seattle. He was called up on May 13, the same day Jarred Kelenic and Logan Gilbert made their debuts, and while his return to the majors was much less heralded than those two top prospects, he’s arguably been more important to the Mariners this season than either. He’s upped his strikeout rate to 40.3% in 2021, the fifth highest rate among all qualified relievers, and all those strikeouts have helped him drop his FIP to just 1.95. In just 45.1 innings, he’s more than doubled his total career WAR.

Michael Ajeto of Lookout Landing was one of the first to write about Sewald’s breakout, but the reasons for his improvement are tough to spot on the surface. He’s simplified his pitch mix a bit, cutting out his rarely thrown changeup and increasing the usage of his slider to make him a two-pitch pitcher:

Both the fastball and slider are returning better-than-ever results, with the latter generating a 38.6% whiff rate and a .270 wOBA against. But as good as his breaking ball has been, the four-seam fastball has been even better. Its whiff rate is up to 35.3%, the 12th-highest mark among all four-seam fastballs thrown at least 100 times this year, and it boasts the 12th-highest CSW% (35.7) in that group.

Looking at the physical characteristics of Sewald’s fastball, it’s hard to tell why he’s suddenly enjoyed so much success with it:

Nearly every single attribute of the pitch has stayed stable across all five seasons of his career. He is throwing it harder than ever this season, but an increase of less than one mile per hour certainly can’t be the main reason why it’s been such a dominant pitch this year.

Something that does stand out to me is the pitch’s shape, which is unusual for a four-seamer. It doesn’t have the kind of extreme ride that has been the hallmark of the modern fastball; instead, it has a ton of arm-side fade — almost sinker-esque, but without the telltale drop of that pitch type. It’s uncommon for a four-seamer, but it hasn’t changed all that much during his career. There’s obviously something else going on that’s fueling his massive improvement.

This article’s title has probably spoiled what that is at this point, but Sewald’s fastball has benefitted from a huge improvement in vertical approach angle (VAA). That is a relatively new metric that elucidates a pretty intuitive concept, and one for which both Ethan Moore and Alex Chamberlain have written fantastic primers. Essentially, VAA describes the angle at which a pitch crosses home plate. Fastballs with a flat VAA (aka four-seamers) are most effective up in the zone; steep VAAs (aka sinkers) are effective down in the zone.

It’s important to note that VAA doesn’t seem to have a significant impact unless it’s an extreme outlier. But if you ignore submariners like Tyler Rogers and Adam Cimber, Sewald’s four-seamer has the third highest VAA among all pitches thrown at least 100 times this year. That’s a massive change from what he was doing earlier in his career, and it’s born out of two key and interrelated adjustments to his mechanics and approach. First, he’s throwing from a lower slot, helping his VAA play up since it’s starting from a lower point. The other change has been his ability to locate the pitch up in the zone regularly. This table shows how these three characteristics have looked throughout his career.

For the first four years of his career, his fastball had the same release point and nearly the same average vertical location, leading to extremely stable, and disappointingly average, VAAs. While that metric alone can’t explain why a flat fastball is or isn’t effective, its relationship to pitch height and release point are both important aspects that can help us understand why an extreme VAA would help a pitch outperform its raw physical characteristics.

For Sewald, learning how to pitch up in the zone was a point of emphasis for him since spring training. Those adjustments didn’t exactly click immediately, as he told Corey Brock of The Athletic in July:

“From my first meeting in spring, we talked about throwing the ball up in the zone, and while it didn’t work in spring training for me … I kept on working on it. It clicked at the alt site and I’ve finally got the ball where I needed it. That’s been huge for me.”

The difference in his pitch location heat maps is illustrative.

Instead of clustering his fastball locations lower in the zone, he’s done a much better job utilizing the natural movement of the pitch to regularly locate it on the upper arm-side corner this year. Here’s an example of a perfectly located fastball earning a swing and a miss from Joey Gallo.

Hitting that spot up in the zone has improved one end of his raw VAA calculation; his lower arm slot has improved the other. Both manager Scott Servais and catcher Tom Murphy have noticed the huge effect that change in arm slot has had on Sewald’s effectiveness. “What makes Paul so effective is his deception. He’s throwing from a lower slot,” Servais told MLB.com’s Daniel Kramer. “The fastball is up shooting a little bit, and then the slider is coming right out of that tunnel, and it really is effective.” As Murphy told Brock, “He has a very strategic plan of how to use his pitches, and obviously he has found something that’s going to work for a long time. To do what he does with the arm slot he’s at, it’s extremely effective.”

If Sewald hadn’t made an adjustment to both his arm slot and the location of his fastball, it’s likely that his VAA wouldn’t have improved as dramatically. But because both of those adjustments happened together, his fastball is now nearly unhittable. During the offseason, he actually approached the Mariners about signing a contract, because the organization’s development program has gained a reputation for helping pitchers get the most out of their stuff. Between the mechanical adjustments and the honed approach, they clearly unlocked Sewald’s ceiling.