April + May 2025 aStuff+ Recap
Analyzing top aStuff+ gainers and notable performances from the start of the season.
This past March, I released the updated version of my pitch quality model, aStuff+ v2, to evaluate pitching performance based on the velocity, movement, acceleration, and release characteristics of each 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 2025 Major League Baseball season just over one-third complete, I believe that it is an appropriate time to review the aStuff+ leaderboards for this season and evaluate the changes that certain pitchers have made to their arsenals to improve their aStuff+ from last season.
Changes - Altitude Adjustment, MiLB leaderboards:
Before proceeding with reviewing the aStuff+ leaderboards for the 2025 season, I would like to discuss a couple of changes that I have made to improve the performance quality of the leaderboards. First, I have refined the altitude adjustment in aStuff+ v2, now directly adjusting the horizontal and vertical accelerations of each pitch, based on the altitude where the park is located. Altitude can play a significant role in determining the movement profile of a given pitch, particularly in Colorado, where the high-altitude environment reduces the vertical acceleration/movement on four-seam fastballs and horizontal acceleration/movement on sliders. This causes some pitches (four-seam fastballs, in particular) to produce much lower aStuff+ scores when utilized at Coors Field, compared to other environments. While Rockies pitchers (most notably, Chase Dollander and Seth Halvorsen) generally experience a boost in aStuff+ due to this adjustment, I have found that this adjustment is just as important for refining the aStuff+ scores when visiting pitchers pitch in Colorado. For example, Hunter Greene produced a 107 pre-adjusted aStuff+, due to the 94 pre-adjusted aStuff+ four-seam fastball he utilized when pitching in Colorado on April 26th. After the altitude adjustment re-evaluated the four-seam fastball he utilized in Colorado to 108, his overall altitude-adjusted aStuff+ is now 109. All Major League aStuff+ values on the leaderboards are altitude-adjusted, while the College aStuff+ (Statcast games) leaderboards remain altitude-agnostic.
I have also created Minor League aStuff+ leaderboards for the 2025 season, which are now publicly available. These leaderboards contain data from games where Statcast is available, which are games played at the AAA level and the Florida State League. These values are altitude-adjusted, which is critical given the high-altitude environments of numerous Pacific Coast League parks. It should also be noted that the Florida State League, similar to NCAA, uses a different baseball than at AAA or MLB, so pitchers at the amateur level and the lower levels of the Minors may experience elevated movement profiles on their offerings. aStuff+ does not account for this effect (pitches are graded as if they are thrown with an MLB baseball), and pitchers at these levels may be awarded elevated pitch quality grades as a result. MiLB leaderboards will be updated at least once a week throughout the remainder of the season. With that being said, onto the leaderboards!
2025 aStuff+ Leaderboards:
The tables above depict the top 10 starting pitchers and top 10 relief pitchers during the 2025 Major League Baseball regular season, by measure of aStuff+. Freddy Peralta grades as the best starting pitcher by measure of aStuff+, largely driven by a 120 aStuff+ four-seam fastball, which he utilizes ~60% of the time. Framber Valdez and Cristopher Sánchez continue to be highly valued by the model, with each pitcher possessing a 109 aStuff+, and Tarik Skubal’s dominance to begin the 2025 season is largely driven by the third-highest aStuff+ among starting pitchers.
Mason Montgomery continues to be the league leader in aStuff+, building upon his 120 aStuff+ performance in 2024 with a 121 aStuff+ so far this season. “Stuff+ staples” such as Aroldis Chapman and Félix Bautista continue to appear on the Top 10 leaderboard while, despite his slow start, Devin Williams is still graded highly by the model, with a 113 aStuff+ (compared to the 112 aStuff+ he produced in 2024).
The tables above depict the top 10 starting pitchers during the 2025 Major League Baseball season, minimum 400 pitches thrown, by measure of aLocation+ and aPitching+. Tarik Skubal utterly dominates all of the pitch quality models, with a 105 aLocation+ and 109 aPitching+, ranking him as the best starting pitcher in all of Major League Baseball (by measure of aPitching+) by a considerable margin. “Command artists” such as Kyle Hendricks, Joe Ryan, Trevor Williams, and Nick Martinez all appear on the aLocation+ leaderboards, with Shota Imanaga displaying the best location ability according to the model, with a 106 aLocation+.
Alongside Skubal, Jacob deGrom, Logan Gilbert, and Shota Imanaga are among the upper-echelon of starting pitchers in Major League Baseball according to aPitching+. The models still value Clay Holmes after the changes he made to his arsenal to become a starting pitcher, awarding him a 105 aPitching+. The Miami Marlins sneakily have two of the top 10 starting pitchers in Major League Baseball on their pitching staff, with Edward Cabrera and Max Meyer both grading as a 105 aPitching+.
As mentioned in the aStuff+ v2 release article, I am currently working on a model titled “arsenal-infused aPitching+” (also referred to as aArsenal+), which incorporates arsenal breadth into the aPitching+ model. Preliminary results indicate that including this arsenal breadth feature in the model improves descriptive performance for starting pitchers. The tables above depict the top 10 starting pitchers during the 2025 Major League Baseball season, minimum 400 pitches thrown, by measure of aArsenal+, along with some notable pitchers who experience increases in aArsenal+ compared to aPitching+.
Tarik Skubal and Jacob deGrom continue to be ranked as the two best starting pitchers in Major League Baseball according to arsenal-infused aPitching+. Max Fried is valued particularly highly by the arsenal-infused model, with his 106 aArsenal+ more accurately describing his dominant start to the season than his 103 aPitching+, in my opinion. Landen Roupp rises to the Top 10 leaderboard by measure of arsenal-infused aPitching+, with Nick Pivetta also joining the leaderboard with a 104 aArsenal+. Other notable aArsenal+ risers include Paul Skenes, Chris Sale, Clarke Schmidt, Miles Mikolas, and Nick Martinez. There is still more work to be done before releasing arsenal-infused aPitching+, however, these leaderboards display the value that the addition of additional “arsenal-based” features can provide when evaluating starting pitchers.
Notable aStuff+ Improvements
The table above depicts six players who have, so far, notably improved their aStuff+ grade from last season. Pitchers who transition from the starting rotation to the bullpen tend to experience an increase in aStuff+ upon acclimating to their new role, due to the associated velocity increase that occurs when appearing in shorter outings, so I did not include any pitchers who belong to this archetype in the table. Regardless, among all pitchers with at least 100 pitches thrown in each of the 2024 and 2025 seasons, Ben Casparius has seen the largest year-to-year increase in aStuff+, from 99 in 2024 to 109 in 2025. This increase in pitch quality has largely been driven by the ~1 MPH increase in average velocity on nearly all of his pitches. Corresponding with a 4-degree increase in his arm angle, Casparius has also increased the induced vertical break on his four-seam fastball by ~3 inches and the horizontal movement on his slider by ~3 inches, and these changes in movement profile are likely positively received by the model as well (via their acceleration components).
Aroldis Chapman is another notable aStuff+ gainer this season, with some improvements he made to the shapes of his four-seam fastball and sinker. While keeping the same arm angle of 52 degrees, Chapman has added ~1.5 inches of induced vertical break on both his four-seam fastball and sinker. This has resulted in the aStuff+ grade of the four-seam fastball improving from 114 in 2024 to 120 in 2025, and the sinker improving from 116 in 2024 to 123 in 2025. Given that his arm angle has remained constant year-to-year, it appears likely that Chapman has generated this improvement movement profile via an orientation change on his four-seam fastball and sinker.
Zebby Matthews and Tarik Skubal have each improved their aStuff+ grades by 6 points compared to 2024. Both pitchers appear to have improved their pitch quality grades by increasing their average velocity compared to 2024, as they each haven’t made significant changes to their movement profiles, relative to their arm angle. Skubal has increased his average velocity on all of his pitches by at least 1 MPH (with his changeup increasing from 86.3 MPH to 88.5 MPH), while Matthews has increased the average velocity on all of his pitches by at least 1 MPH as well. Simply put, Tarik Skubal is the best pitcher in all of Major League Baseball right now (as indicated by his appearance on nearly every leaderboard in this article), however, I am intrigued by Zebby Matthews over the rest of the season, as he is currently displaying a 106 aStuff+, 104 aLocation+, and 107 aPitching+ over his first 152 pitches of the season.
Ryan Weathers and Max Meyer have also made notable improvements to their pitch quality grades compared to last season. I discussed Meyer’s adjustments in detail in the article “How Max Meyer Revamped His Arsenal” on Pitcher List a few weeks ago, and he has largely improved his pitch quality grades by revamping his entire arsenal around his plus slider, adding a sweeper and revamping the shapes of his sinker, changeup, and four-seamer. Ryan Weathers has also increased the average velocity on nearly all of his pitches, with the added 1.6 MPH increase in his average four-seam fastball velocity increasing the pitch’s aStuff+ grade from 99 in 2024 to 109 in 2025.
Concluding Thoughts:
Creating a pitch quality model to evaluate the performance of Major League Baseball pitchers has been a rewarding experience over the past couple of seasons, and has allowed me to deepen my understanding and analysis of pitching beyond simply utilizing existing public metrics. As shown by this article, the use of aStuff+, aLocation+, and aPitching+, can be valuable tools to contextualize the factors that generate pitching production, and I am hopeful that continued research into arsenal effects, such as arsenal breadth, tunneling effects, and dynamic dead zone, will provide further insight into how to maximize a pitcher’s potential. 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!
Statistics through the end of play on May 28th, 2025.
Follow @MLBDailyStats_ on X and Adam Salorio on Substack for more in-depth MLB analysis. Photo credits to Junfu Han / USA TODAY.