The Rasmussen Riddle: A study in super weird ground balls

The thoroughly modern baseball fan knows what a frontline starter looks like.

He’ll throw a high-spin four-seam fastball at the top of the zone, getting it up above the hitter’s hands and racking up whiffs and lazy fly balls. Or maybe he’ll throw a wake-shifted sinker in under the hitter’s hands, getting whiffs and weak contact. He’ll pair one or more breaking pitches with mirrored movement planes off that primary offering.

Modern pitching optimization is directional — pitches are located where their movement plays up, and pitch systems work together to make the batter wrong in opposite and complementary directions. We say things like “north-south” and “east-west.” We care about launch angle and occasionally think we understand it. And we expect good pitchers to miss bats and get strikeouts.

On paper, Drew Rasmussen should be this type of pitcher. His fastball has 96th percentile spin, and he pairs it with three average or better breaking balls. But his 22% whiff rate is slightly below average, and his 20% strikeout rate is nearly half a standard deviation below average.

Rasmussen this year has an ERA of 2.80, a FIP of 3.36, an xFIP of 3.68, and a SIERA of 3.90. The more advanced and more predictive the statistic gets, the worse it thinks Rasmussen is.

The thoroughly modern baseball fan knows what to do with a pitcher like this. We say that he is lucky and move on.

The Rasmussen Riddle

Bur stay with me. The most common formulation of “The Rasmussen Riddle” is “Where are the whiffs?”

It’s a complicated question with a many-faceted answer. Ben Clemens answered part of it, in his discussion of Rasmussen’s cutter-heavy pitch mix. There’s also a dimension of command here. Rasmussen’s go-to strikeout pitch is his sweepy slider, which is brand new this year. I don’t think his command of the pitch is at the same level as his command of his fastball or cutter, and the result is that he often executes less precisely in two-strike counts than when behind or even.

But with Rasmussen, maybe more interesting is a secondary formulation of the question: “What’s happening instead of the expected bunches of strikeouts?”

And specifically, what’s happening to those high-spin 95+ mph fastballs that batters aren’t swinging under?

Here’s a fastball up and on the outer third.

And here’s a fastball on the inner third.

And here’s a fastball at the top, center of the plate.

And yes, occasionally the fastball is down.

So on the one hand yes, maybe Rasmussen leads a charmed life wherein he’s been magically (read: luckily) able to sequence a groundball just exactly when he needs it to get out of a jam, and we shouldn’t think too much more about it. But on the other hand there’s something super weird here going on here — too weird to simply accept and move past.

Drew Rasmussen is a legitimate groundball pitcher (0.7 standard deviations above average) but unlike most of the starters ahead of him on the leaderboard, he gets his grounders without leaning on a classic groundball pitch, instead posting one of the highest groundball percentages in baseball on four-seam fastballs. The better question is really, “Why are all these four-seam fastballs ending up on the ground?”

Here’s the GB% leaderboard, pulled from Baseball Savant, just for four-seam fastballs (minimum 100 fastballs put into play):

Most of these names are who we should expect to be on a list like this. Let’s step through it quickly.

Unique Angles

Tyler Rodgers is a submariner with a fastball wholly unlike any other in baseball. It comes from below and moves like a lefty slider.

Data from Pitch Info

Low Spin Rate, High Active Spin

Adon and Gray have extremely pedestrian fastballs with a high percentage of active spin (96% and 95% respectively, meaning that most of their spin is translated into Magnus force movement), an unremarkable spin angle, and a very low spin rate (10th and 36th percentile). The result is a fastball that’s on the way to a sinker but not all the way there. The low spin may fool some batters occasionally, but these are probably not pitches to emulate.

Data from Pitch Info

Low Spin Rate, Low Active Spin

The more interesting group of low-spin pitchers is lefty Fried and righties Senzatela and Pallante, who combine their lack of fastball spin (19th, 22nd, and 44th percentile respectively) with a relatively low proportion of active spin (75%, 82%, and 72%). Low active spin means that some of their spin is gyrospin, which does not produce a Magnus force. Instead, when combined with a particular seam orientation, the gyrospin can help create seam-shifted wake effects. In the case of these fastballs, all three pitchers generate both significant drop relative to a normal fastball and some amount of armside cut.

Data from Pitch Info

Unlike Gray’s and Adon’s, these pitches have a unique shape, departing from the cluster of normal four-seam fastballs and acting somewhat like both a cutter and a splitter, but with higher velocity. Basically, their low spin rate and low active spin create a deceptive lack of rise (that’s the part that’s like a splitter), while they also use seam-shifted wake effects to eliminate armside run and push their movement armside, to the center of the graph.

This is an intriguing trio (Senzatela’s lack of quality secondary offerings notwithstanding), and it makes sense that a fastball with this shape would have clear groundball tendancies.

High Spin Rate, Low Active Spin

Those groundball tendencies become less self-evident for the final group on this leaderboard, which includes righties Rasmussen and Ohtani and lefties Steele and Keller. These four starters throw fastballs with low active spin (82%, 76%, 61%, and 72% respectively) but with average or even high spin rates (96th, 53rd, 94th, or 71st percentile).

They still use seam-shifted wake to generate armside cut, but their high spin rate keeps the movement profile of the pitch in contact with the main clump of four-seam fastballs (albeit on the bottom edge).

Data from Pitch Info

So on the one hand, there seems to be an obvious conclusion to draw. Seven of the top-10 fastballs most likely to generate a groundball in 2022 all featured significant armside cut driven by seam-shifted wake effects. But on the other hand that conclusion becomes very unsatisfying if you dig at it a little bit more.

Beware of Obvious Conclusions

For starters, I’m not actually sure that the two lefties, and especially Steele, belong in the same grouping as Ohtani and Rasmussen. Steele’s remarkably low active spin actually gives him true armside movement, and the lefties live in the low-to-mid 90s while Ohtani and Rasmussen sit mid-to-high 90s, with the lower speed creating more real-world absolute drop.

Alex Chamberlain has a wonderful Tableau viz tool that calculates the similarities between pitches, computing a Euclidean distance. That’s a fancy way of saying that it puts speed, horizontal movement, and vertical movement in as three dimensions then figures which pitches are closest to each other in the theoretical space, using the Pythagorean Theorem.

Chamberlain includes some statistics for each pitch alongside the Euclidean distance, and most helpfully for us, one of them is “LA Influence,” which is a fancy way of asking whether hitters seem to be induced to hit the pitch in the air or on the ground.

Alex Chamberlain

The problem is that of the set of pitches throughout baseball most similar to Rasmussen’s 2022 fastball . . . they mostly seem to induce fly balls. Some of them, like Jameson Taillon’s and Chad Green’s fastballs, are even pretty extreme fly ball creators. Ohtani, Rasmussen, and to a lesser extent Keller are actually the only ones on this list with clear groundball tendencies.

So What’s Going On Here?

These two things are true.

1. Of the pitchers who generate the most groundballs on their four-seam fastballs, many of them have a pitch shape similar to Rasmussen’s: relatively high armside cut and below average rise.
2. When you look at all the pitchers with similar four-seam fastball pitch shape to Rasmussen’s, only a select few of them actually tend to generate groundballs.

How are we to reconcile? I don’t know, but I can speculate wildly.

Theory 1: Pitch Interactions

Rasmussen leans heavily on another hard pitch, his cutter, which can be thrown to the same locations as the fastball and tunnels with it well. Ohtani throws both a hard cutter and a splitter, which perhaps have similar interactions. If I squint and I can suppose that Keller’s and Fried’s sinkers and sliders (small movement, mid-to-high-80s) might pair with their fastballs in a similar way.

By throwing other pitches with less rise in the zone for strikes, do these pitchers create a comparison to their four-seam fastball that makes hitters perceive the pitch to have more rise than it actually does?

Theory 2: Variable Pitch Locations and Movement

Against left-handed batters, Rasmussen’s 36 groundball fastballs rose 1.24 inches (Baseball Savant numbers) while his average rise on the fastball was 1.34 inches. The groundball fastballs also cut more than the average, and were located lower in the zone and closer to the batter.

Against right-handed batters, the location in the zone was more distinct, but there was slightly less rise on the grounder-inducing pitches than on the others as well.

Expressed in simple numbers like this, the relationship seems obvious and intuitive. Looking at the mass of individual pitches though, it’s obvious that this is a complex and noisy system.

I think that the impact of movement variation within a pitcher’s single pitch type is a fertile area for further research.

Theory 3: Perception Tricks

Related to the impact of movement variation within pitch types is the study of batter perception. There’s a set of factors that impact the shape of any given pitch, such as release point including extension, release angle, spin rate, spin angle, and velocity. It’s unlikely that batters are able to perceive differences in each of these pitch characteristics evenly, meaning that some pitch characteristics impart more deception to the pitch than others.

For instance, studies have already show pitches with high amounts of seam-shifted wake to be more deceptive than similarly-moving pitches generating their movement primarily through Magnus forces. We also know that mirrored spin between two pitch types is more deceptive than differentiated spin. And we know that there doesn’t appear to be a benefit to pitchers with little extension (except perhaps with curve balls), indicating that extension is primarily acting through perceived velocity, rather than being perceived in its own right. Ben Lindbergh wrote last year about how teams are trying to qunatify many facets of deception.

Given a comprehensive understanding of hitter perception, it would be possible to create a physics model for what shape they expect a pitch to have given what they see, and some pitchers would have actual pitch shapes that differentiate more or less from that model. Both Rasmussen’s and Ohtani’s fastball have fairly unique physical characteristics, and it’s reasonable to wonder if their fastballs are unusually different than the assumed shape based on what hitters see.

Theory 4: It Really is Mostly Luck

Also it could be nothing.

Every time I watch Drew Rasmussen pitch, I wonder if today will be the day his fastball stops generating groundballs. Will it become a normal flyball pitch and never look back? Will the Rasmussen Riddle fade into the dustbin of baseball history, the way countless other random variations in the noisy system of the batter-hitter matchup have?

Or might Rasmussen’s true talent change in such a way that eliminates the groundball tendencies, leaving us unable to ever fully decide whether there was meaning to what we’ve already seen.

There’s always going to be meaningless outliers and this might be one. Or it might not.

So instead of a conclusion, I give you an invitation to wonder. When you watch Drew Rasmussen or Shohei Ohtani pitch, wonder why they get groundballs. Wonder if it will continue. Try to embrace the outliers and learn from them without becoming seduced into fully believing them.

Baseball is weird. Weird is good.