Best of BP: Luis Castillo and the Pronator’s Triangle

In baseball, we frequently use the term ‘motor preference’ to refer to an athlete’s natural movement patterns—his default setting, if you will. No two human bodies are the same, which means no two pitchers will move in the same exact way. You can see this with basic pitching mechanics; there are a wide variety of ways to throw 95 mph, for example, and each high-level athlete arrives at that destination in a unique way that fits what their body is naturally good at. And as pitcher training becomes more of an individualized process, understanding each hurler’s strengths and weaknesses is at the very core of successful player development.

As it pertains to the scope of this piece, we’re looking at movement bias in the sense of how a pitcher tends to apply pressure to the ball at release. There are lots of factors that play into this, from how a pitcher’s limbs are built to his mechanics, and the spectrum of motor preference is very wide in this regard. But for simplicity’s sake, we’re going to divide these preferences into three categories: supination bias, backspin bias, and our main topic for today; pronation bias.

Motor Preferences

In simple terms, pronation bias is the tendency to get on the inside part of the baseball at pitch release. Backspin bias is what I personally call the tendency to stay behind/through the ball, and supination bias applies to being on the outside of the ball. Each of these three categories leads to really drastic differences in the kind of pitch movement a pitcher is naturally good at imparting on the baseball. With the disclaimer that most pitchers tend to fall somewhere in between the extremes, let’s put each motor preference in context and see what kind of pitch mix it lends itself to.

Supination Bias

Supination-dominant pitchers are often breaking-ball masters. Their natural tendency to get on the outside of the ball means they usually have an easier time putting all sorts of spin on it. It’s common for these guys to cut their fastball as well, leading to a pitch mix dominated by glove-side movement. In contrast, they can struggle to pronate and lack the ability to create arm-side run, sometimes leading to poor shape on traditional changeups. Supinators are almost always candidates for seam-shifted wake, most often applied to sinkers, changeups, and sliders. You can likely picture this kind of pitcher with ease: Corbin Burnes, Kyle Bradish, etc.

Backspin Bias

Backspin-dominant pitchers are the kings of the rising fastball. Their natural tendency to stay behind the baseball allows them to impart efficient backspin on the ball, creating elite vertical movement on a four-seamer. They often have access to the full array of breaking balls, but can sometimes run into issues with their changeups. Because they stay behind the ball to such a degree, it can be difficult for them to remove backspin from their cambio, leading to floating changeups without impact dive. This is part of why splitters are often such good fits for pitchers of this kind, as their low-spin nature allows them to kill that vertical movement. Logan Gilbert and Justin Verlander are two good examples of this kind of hurler.

Both supinators and backspin-dominant pitchers are instinctive profiles to understand. On top of that, both of these variations often feature eye-popping metrics for pitches we all associate with modern strategy: high-spin breaking balls or high-carry four-seamers. But pronators are different.

What Pronators are All About

Pronation-biased pitchers as the masters of arm-side movement. Their natural motion frequently leads to fastballs with significant run, and they parlay that motor preference into natural changeup feel. In contrast to most backspin-dominant pitchers and their issues turning over a classic changeup, pronators are excellent at eliminating vertical movement on their cambio.

Here you can see Mariners ace Luis Castillo in action. Check out how that running fastball ties up a right-hander in on his hands.

That’s not all he does, of course. Castillo mixes in a sinker as well, using that natural ability to pronate and his slingin’ arm slot to create excellent run and sink on his best ground-ball pitch.

As is often the case nowadays, his sinker is deployed almost exclusively against right-handed batters, but Castillo is far from helpless against lefties. Watch him pull the string on a lefty trying to pull a fastball.

Castillo’s changeup is a high RPM sidespinner, and the low three-quarters slot helps him kill some of its vert. This fastball-changeup pairing gives a pronator like Luis at least one pitch to tackle same-handed and opposite-handed hitters, respectively. Given how often we hear “if ____ can develop a changeup, look out!” in regard to young starting-pitcher prospects, having this built-in talent to create changeup movement seems like a cheat code. But of course, that’s not the full story, and pronators are not only unique because of their changeup mastery. In fact, it’s their main limitation that makes them so fascinating.

Breaking Ball Profile

What sets pronators apart is their natural disadvantage when it comes to breaking-ball manipulation. Because of the way the pitch moves, many athletes of this kind will find it challenging to get around a breaking ball. This limits the kind of glove-side movement they can create, and seems like a massive hurdle to clear in today’s game. We’ve never had a better grasp on how to manipulate the ball to create movement, and it seems as if a pronator is shut off from a large part of that area. But is that true?

Well, yes and no. Here’s Castillo again—does this look like a bad slider to you?

That’s what’s frequently called a gyro slider, or gyro ball. The premise of this pitch is for the hurler to throw it hard while “spiraling” the ball like a football, which induces a high degree of gyroscopic spin on the ball. In simple terms, gyro spin effectively removes the effects of most spin-induced movement on the ball, enabling gravity to play a big role in bending the offering’s movement. This is why gyro sliders have such late, sharp, downward action. The one we saw from Castillo just now is a textbook example of what a gyro slider looks like.

This kind of slider is extremely common among pronators, because you don’t really need to supinate very well in order to throw it. And because it doesn’t feature big horizontal movement, it frequently tunnels better with the pitcher’s arm side-oriented repertoire than a big, horizontal sweeper. Gyro sliders can also be thrown quite hard while still generating just enough depth to miss bats, giving pronators a fantastic weapon to draw swing-and-miss from righties. It pairs with the fastball and changeup to create what I always like calling the “Pronator’s Triangle”, because of the shape it creates on a scatter chart. Let’s check that out.

The Pronator’s Triangle

Here’s Castillo’s movement plot from 2023, from the catcher’s POV:

The Mariners’ ace is just about the perfect example of the pronator’s triangle you’ll see in the majors today. You can tell his breaking ball has heavy gyro spin, since it sits right around the zero mark for both vertical and horizontal movement. No two pitchers are the same, of course, but this kind of triangle tends to appear in many of the more prominent pronators in today’s game. Here’s Sandy Alcantara:

Alcantara’s slider and changeup tend to stay up slightly more than Castillo’s, because he throws them harder. His two secondaries rarely drop below the 88-mph mark, whereas Castillo’s tend to be more mid-high 80s on average. The extra velocity means more backspin and less time to drop, but aside from that, the two pitchers are remarkably similar in terms of movement profile. How about someone slightly different? Here’s Alcantara’s teammate, Jesús Luzardo:

Luzardo is different from Castillo and Alcantara because his changeup is not as polished yet, and stays up more than you’d probably like. As far as the breaking ball goes, though? Luzardo throws one of the purest gyro balls in the majors, one that generated a whopping 51.8% whiff rate in 2023. Not too shabby!

I’d like to point out a few more examples. First, postseason hero Nathan Eovaldi:

I didn’t draw a triangle on Eovaldi’s, and that’s entirely on purpose, because you can interpret his chart in a variety of ways. Eovaldi’s unique because while he does have a relatively standard slider, he uses his cutter far more. And unlike the previous pitchers we’ve looked at, Eovaldi does mix in a curveball—except that it’s not a great one in a vacuum. Eovaldi’s curve is a relatively loopy 76-mph breaking ball without impressive movement, but it works for him, and gives him another way to mess with hitters’ timing. This is a reminder that there’s always a wide array of pitcher profiles, even within the broad categories we like to establish in order to have a better grasp of how arsenals work. Not every pronator is destined for just the triangle.

And finally, here’s Blue Jays ace Kevin Gausman:

A near-textbook practitioner of the triangle, Gausman’s fastball profile is biased more toward vertical movement than Castillo’s, in large part because of his higher arm slot. Gausman also doesn’t throw a sinker, while Castillo does, which drives his average fastball carry down when looking at his fastballs as a unit.

The triangle applies to a lot of different pitchers in the majors and minors today, and I find it to be a very helpful tool to understand how a certain pitch mix is put together. The reason the hard gyro slider is such a good fit for these kinds of pitchers is that it gives them an offering that manages to create swing and miss without asking their bodies to move in a way they don’t necessarily lean toward. And on top of that, it’s a pitch type that tends to have less drastic platoon splits than the super popular sweepers, as their velocity and downward action make them prime candidates for back-foot breaking ball shenanigans. There are other ways to create breaking ball movement without extreme supination, such as a spiked grip. But this kind of profile is very popular for a reason.

And now, onto the next big part of this study. Over the last few years, Gausman has earned a reputation as a great pitcher who allows more hits than you might expect and whose fastball can get beat up at times. The latter statement also applies to the 2023 version of Alcántara, whose four-seamer is often shockingly hittable for a pitcher who averages 98 mph. Meanwhile, Castillo’s four-seamer has been one of the most devastating in baseball for a few years now. Just what is it that sets Castillo apart from the rest?

There’s a variety of reasons for Castillo’s development into one of the best and most reliable starters in baseball, but the ascension of his four-seam fastball ranks very high on that list. If we don’t count his first taste of the big leagues in 2017, 2023 would mark a career-high in four-seam fastball usage for the right-hander. He tossed it 44.4% of the time last season and got a mind-bending 33% whiff rate on it, ridiculous for a fastball. Opponents put up just a .278 wOBA against this offering, and believe it or not, this was a significant downgrade from his 2022 mark, when they only managed a pitiful .203 wOBA. But what is it that makes it so dominant?

Is it velocity? Well, Castillo is indeed one of the hardest-throwing starters in baseball, averaging 96.6 mph on that fastball over the last two seasons. But velo itself won’t make a heater great—just ask Hunter Greene. So while it’s definitely a plus, it’s not the only factor in play here.

Is it elite vertical movement? Absolutely not. Castillo’s four-seamer features a middling 13.6 inches of induced vertical break (IVB), which is right in the “dead zone” for a fastball. You would expect a hard fastball that doesn’t move to get crushed quite often, but Castillo’s doesn’t. Clearly, we need to dig deeper.

Is it elite extension? No, that’s not it either. Castillo is a very rotational pitcher with a short stride down the mound, and his 5.6 feet of extension is actually well below major-league average. This means his velo is likely playing down compared to some of his peers, which makes the lack of movement even more confounding.

So we have a pitcher who throws quite hard, but whose velo likely plays down. Oh, and he doesn’t make his fastball “hop” like a typical dominant heater does. What main variable do we have left? That’s right: vertical approach angle.

The Power of Being an Outlier

Vertical Approach Angle (VAA) is the angle at which a pitch crosses home plate. There are quite a few ingredients baked into this pie, many of which you’ll be able to deduce right away. Obviously, a fastball that enters the plate at knee height will have a steeper VAA than a pitch thrown at the letters, but there’s more. Pitchers who throw from lower release points can create flatter VAAs, as the ball travels in a less downhill manner than usual. Picture Justin Verlander, and then picture Steve Cishek. Which of the two is creating a steeper angle on average?

For a four-seam fastball, the flatter the approach angle, the better. Flat VAAs have an incredibly strong correlation with swing-and-miss rates, and this is where Castillo truly shines. Among the 179 starting pitchers to toss at least 500 four-seamers over the last two seasons, Castillo has the eight lowest release point among them, at 5.23 feet of height… but he’s also the 11th-hardest thrower! The end result is an extreme outlier profile that’s really easy to see down below.

I’ve taken the liberty of marking some other outliers, such as Castillo’s Mariners teammate Bryan Woo, the unicorns known as Jacob deGrom and Justin Verlander, and the incredibly funny appearance of Trevor Williams. I’ve also marked the four other pitchers we talked about in the triangle section, and I think you can start to see why Gausman’s fastball gets hit so often: He falls dangerously close to the middle, the area you don’t want to live as a pitcher. Castillo, on the other hand, rides his excellent velo and outlier release point into a totally unique profile that makes his fastball overwhelming. There are more variables to fastball success than just velo and VAA, of course, but those two are what make the Mariners righty so different from everyone else.

Castillo’s dominant fastball helps him tackle a frequent problem for pronators—how to effectively keep opposite-handed batters off their secondaries. Because a pronator’s fastball tends to run, it’s frequent for pitchers of this profile to run into problems when they don’t have the platoon advantage. It’s common for fastballs with heavy run and middling carry to have severe platoon splits, as the pitch tends to run into barrels. And this has applied to Castillo as well; for his career, lefties have a wOBA against his fastball that is 63 points higher than righties.

Over the last two seasons, righties have a miserable .209 wOBA against the heater, but against lefties it jumps all the way to .283. Because he’s starting from such a low point, however, Castillo has a fastball that still handles lefties just well enough to keep them honest. He’s even begun throwing more sliders to lefties too, as his breaking ball’s heavy gyro action enables it to dip beneath barrels at a reasonable rate—something Mikey Ajeto picked up on in March of last year.

Some Final Thoughts

As I delve deeper and deeper into pitching data, I keep coming back to a certain dynamic that really sticks out. The value of outliers in the art of pitching is such an important concept when it comes to both scouting and development. But the more I learn, the more I realize archetypes exist for a reason—because they provide a good, stable blueprint to follow in a pitcher’s journey to be the best version of himself. The pronator’s triangle is a great example of this.

I also really hoped I conveyed just how much I enjoy these kinds of pitchers. In an era where breaking-ball manipulation is king and we’ve all gone crazy with the sweeper, I find it important to remind people that there’s a million ways to get batters out. Pronators are a walking, breathing reminder of that—and variety is a huge part of what makes baseball so interesting.