2024 aStuff+ Recap
Analyzing top aStuff+ gainers and notable performances from the regular season.
Earlier this season, I released the first version of my pitch quality model, aStuff+, to evaluate pitching performance based on the velocity, movement, and release characteristics of each individual pitch. Pitch quality models, such as aStuff+, are valuable tools to utilize when evaluating the effectiveness of a given pitcher’s arsenal due to their predictive power and ability to stabilize quickly, providing insight into how well a pitcher will perform long-term in a small sample size. These models can also be utilized to design pitch arsenals, providing players and coaches information on whether a given pitch shape will be effective if added to a pitcher’s arsenal.
With the 2024 Major League Baseball regular season concluded, I figured it is a good opportunity to review the aStuff+ leaderboards for 2024 and evaluate the changes that certain pitchers made to their arsenals to improve their aStuff+ from last season.
aStuff+ Scaling Update:
Before proceeding with the review of the aStuff+ leaderboards from 2024, I would like to make a quick announcement regarding an adjustment to the scaling method used for my pitch quality model. Since unveiling my model in May, I have utilized a 100 mean/10 standard deviation scale, scaled relative to the average expected run value per 100 pitches of the average pitcher or pitch type. For example, a pitcher whose pitch arsenal quality was graded one standard deviation better than the average pitch arsenal quality would be graded as a 110 aStuff+. A pitcher who throws a four-seam fastball that is graded as one standard deviation worse than the average pitch type quality would be graded as a 90.
When reviewing the aStuff+ leaderboards for 2023 and 2024, I realized that scaling the pitch quality model on two separate models (one when grouped by pitcher, another when grouped by pitch type) could become complicated and skewed by outliers at both high and low extremes. In response, I have decided to adjust the scaling of aStuff+ (when grouped by pitcher and when grouped by pitch type) to be relative to the average pitch thrown during a given season, ensuring a clearer comparison across pitchers and pitch types without skewing due to outliers. For example, since Emmanuel Clase has a 116 aStuff+, this means that his entire pitch arsenal grades as 1.6 standard deviations better than the average pitch thrown during the 2024 season. This scaling update does not impact the descriptive and predictive capabilities of the model (since this analysis was conducted on expected run values). I hope that this update will enhance the quality of aStuff+’s interpretability.
https://x.com/drivelinekyle/status/1844583115039387934
2024 aStuff+ Leaderboards:
The tables above depict the top 10 starting pitchers and top 10 relief pitchers during the 2024 Major League Baseball regular season, by measure of aStuff+. To little surprise, Tyler Glasnow and Jacob deGrom rank as the top two starting pitchers by measure of aStuff+, grading as 109 and 108 respectively this season. Keaton Winn grades as the 5th best starting pitcher by measure of aStuff+, largely due to his splitter that grades as 115. Yonny Chirinos graded as the 6th best starting pitcher by measure of aStuff+, which can be attributed to the drastically improved slider shape in 2024, increasing the pitch’s aStuff+ grade by two full standard deviations from 95 to 115.
Among relief pitchers, Emmanuel Clase once again leads Major League Baseball with a 116 aStuff+ during the 2024 season. “Stuff+ staples” such as Ryan Helsley, Devin Williams, Ryan Pressly, and Craig Kimbrel all make appearances amongst the top 10 relief pitchers as well. Despite pitching only 9.2 innings at the Major League level this season, Mason Montgomery grades as the 8th best reliever by measure of aStuff+, largely due to a velocity increase he experienced once being moved to the bullpen before his promotion to the Majors.
As shown by the table above, the best individual pitch thrown in Major League Baseball this season (minimum 100 pitches thrown) was Clay Holmes’s sweeper, grading as 129 aStuff+, nearly three standard deviations better than the average pitch thrown during the season. Two other sweepers (thrown by Griffin Jax and Lucas Sims) rank among the top three, while Hayden Birdsong’s kick change is not far behind Devin Williams’s airbender as it pertains to the best changeup in Major League Baseball by measure of aStuff+.
Among four-seam fastballs and sinkers, Grant Anderson possesses the best fastball in Major League Baseball, with his sinker from a negative 3-degree arm angle grading as 112 aStuff+. Tyler Glasnow’s four-seam fastball grades just below Anderson’s sinker as the second-best fastball in Major League Baseball, while Josh Hader’s rising sinker grades as the league’s third-best fastball by measure of aStuff+.
Among pitchers with at least 250 pitches thrown in both the 2023 and 2024 Major League Baseball seasons, Drew Rasmussen, Michael Kopech, and Luke Weaver experienced the three biggest year-to-year aStuff+ increases. In this next section, to better understand how these pitchers improved their pitch arsenal quality, I’ll take a detailed look at each pitcher’s arsenal to further understand the year-to-year adjustments that they have made to become the three biggest aStuff+ gainers of the 2024 season.
Drew Rasmussen - Tampa Bay Rays:
Coming off of an elbow injury that required Tommy John surgery and caused him to miss most of the 2023 season, Drew Rasmussen had a resurgent 2024 season after returning to the Majors in August, producing a 30.2% strikeout rate, 5.2% walk rate, and 25.0% K-BB over 28.2 innings pitched for Tampa Bay. From a usage standpoint, Rasmussen ditched his curveball in 2024 and increased the usage of his sinker from 12.2% in 2023 to 29.2% in 2024. This adjustment turned Rasmussen into a “multiple fastball” pitcher, with his combined four-seam and sinker usage increasing from 41.7% in 2023 to 66.9% in 2024, and the movement differences between these two pitches at similar average velocities (97.4 MPH for the four-seam fastball, 96.2 MPH for the sinker) likely contributed to the four-seam fastball generating above-average swing-and-miss (30.8% whiff rate) and the sinker generating plenty of weak contact (.190 xwOBA).
Rasmussen’s aStuff+ increase in 2024 was primarily driven by the improved quality of his fastballs, with his four-seam fastball improving in quality by nearly two standard deviations from 89 to 108, and his sinker improving by a full standard deviation from 98 to 108. These improvements largely come from an increase in velocity that Rasmussen experienced this season, with the average velocity on his four-seam fastball increasing from 95.7 MPH to 97.4 MPH and his sinker increasing from 95.1 MPH to 96.2 MPH. It is also interesting to note that Rasmussen has been generating more arm-side run on his four-seam fastball and sinker this season, increasing the movement differentials of these pitches from his breaking balls, which is likely a contributor to the increase in overall swing-and-miss he experienced in 2024.
Heading into 2025, it will be interesting to see if these velocity gains hold if/when Rasmussen makes his return to the rotation as a full-time starting pitcher. If these velocity gains hold, Rasmussen has the potential to be one of the best starting pitchers in all of Major League Baseball next season. Even if these gains do not sustain, and the average velocity of Rasmussen’s arsenal sits near his averages of the past two seasons, Rasmussen projects to be a productive starting pitcher for the Tampa Bay Rays in 2025.
Michael Kopech - Los Angeles Dodgers:
Traded to the Los Angeles Dodgers from the Chicago White Sox at this season’s trade deadline, Michael Kopech has had one of the best seasons of his career in 2024, producing a 31.5% strikeout rate, 12.2% walk rate, and 19.4% K-BB over 67.2 innings pitched between Chicago and Los Angeles. Known for his ability to throw for high velocity since his time as a prospect, Kopech has unleashed his four-seam fastball in his first full season as a reliever, increasing the average velocity on the offering from 95.2 MPH in 2023 to 98.7 MPH in 2024. Kopech has also been very confident in utilizing the four-seam fastball, with the offering possessing a 78.4% (!) usage rate, and the pitch has been exceptional at generating swing-and-miss this season with a 34.0% whiff rate.
With the move to the bullpen, Kopech dropped his underperforming curveball and changeup from his pitch arsenal and added a cutter which grades very well by measure of aStuff+ (112). Not only does the addition of the cutter allow him to generate swing-and-miss and weak contact (as evident by the 33.3% whiff rate and .235 xwOBA that opponents have produced against the offering), but the plus velocity on the pitch provides Kopech with essentially a “second fastball” that hitters must be prepared for, allowing his four-seamer to “play up” and be even more effective. While Kopech has only utilized the pitch 10.5% of the time this season, I would expect the usage rate on this pitch to only go up in the future, as increased usage of this offering would limit opposing hitters' ability to simply sit on the four-seamer when facing Kopech.
Throwing a hard cutter with a similar movement profile to a slider is a recipe for creating a pitch that grades as well above average, and the addition of this offering to pitch arsenal, combined with the bullpen velocity bump that he has experienced, has allowed for Kopech to have an excellent first season in the bullpen and he projects to be a prominent member of the Los Angeles Dodgers bullpen in 2025.
Luke Weaver - New York Yankees:
Claimed off waivers in September of 2023 after being designated for assignment by the Seattle Mariners, Luke Weaver has had an impressive first season with the New York Yankees, producing a 31.1% strikeout rate, 7.9% walk rate, and 23.3% K-BB over 84.0 innings pitched for New York.
The increased pitch quality of his four-seam fastball and changeup have been a major contributor to Weaver’s success in 2024, with the four-seam improving from 96 to 106 aStuff+ and the changeup improving from 95 to 102 aStuff+. These “stuff” improvements can be largely attributed to seam orientation changes that Weaver made on these pitches over the offseason, which has allowed for the four-seam fastball to generate more ride (16 inches of induced vertical break in 2023 to 18.6 inches of induced vertical break this season) and the changeup to generate more drop (8.2 inches of induced vertical break in 2023 to 4.6 inches of induced vertical break in 2024), creating more movement seperation between these two offerings. Weaver also experienced increased velocity on all of his pitches this season, which can likely be attributed to the velocity bump that occurs when a pitcher moves to the bullpen. These changes have allowed Weaver to generate more swing-and-miss this season, increasing his strikeout rate, and allowing him to become the Yankees de facto closer by the end of the 2024 season.
Concluding Thoughts:
Creating a pitch quality model to evaluate the pitching performance based on the velocity, movement, and release characteristics of each individual pitch has been a rewarding experience this season, and has allowed me to deepen my understanding and analysis of pitching beyond simply utilizing existing public metrics. Over the offseason, I am looking forward to developing and unveiling Location+ and Pitching+ style metrics, as well as a pitching dashboard, which will only improve our understanding of each pitcher’s arsenal. I have also been intrigued by models that evaluate arsenal effects, such as Driveline’s Mix+ and Match+, and I believe that these effects are the “third pillar” of analysis, alongside stuff and location, that should be used when evaluating the quality of a pitcher’s arsenal (a topic that will be discussed more in-depth in future blog posts). The continued evolution of pitch arsenal evaluation has greatly increased our understanding of how to evaluate and develop Major League pitchers over the past couple of seasons, and the continued development of analytical tools and resources, such as aStuff+, on both the public and private side promises to further revolutionize how we understand pitching performance in 2025 and beyond.
Thanks for reading!
Follow @MLBDailyStats_ on X and Adam Salorio on Substack for more in-depth MLB analysis. Photo credits to New York Post.