Tuesday, March 5, 2024
AnalysisChicago CubsMLBNational LeagueNL Central

Slow and Steady Wins the Race: Alec Mills’s Surprisingly Devious Curveball

Photo Credit: Daniel Bartel-USA TODAY Sports

With just 10 days until the Chicago Cubs suit up against the Pittsburgh Pirates at beautiful Wrigley Field, the starting rotation’s nearly set in stone. During the offseason, they traded for Zach Davies from the San Diego Padres and signed free agents Jake Arrieta and Trevor Williams. Along with Kyle Hendricks and Adbert Alzolay, this leaves Alec Mills still fighting for a rotation spot and likely a swingman role down the line.

Despite what his being in competition may suggest, Mills’s 11 starts in his 2020 campaign proved to be a much-needed stopgap for a Cubs rotation that was hampered with injuries. In that timeframe, he posted a 4.48 ERA, 1.16 WHIP, 18.3 K%, and 10.7 K-BB%. As expected for a pitcher that relied on contact management over strikeouts, Mills’s advanced stats fared poorly. To go along with those results, he had a 4.61 xFIP (104 xFIP-), 4.81 SIERA, and 5.10 xERA.

What aided Mills’s 2020 contact management-oriented campaign was a very effective curveball. Against a mediocre 33rd percentile spin rate, a mediocre 81% spin efficiency, and a mediocre 21.6 whiff%, opposing hitters posted a .207/.233/.241 slash line, 37 wRC+, and .212 wOBA (.253 xwOBA) against the pitch. It also fared well in Baseball Savant’s run value metric. With a 100-pitch minimum, his curveball’s run value of -2.5 runs per 100 pitches ranked 9th in the league. It’s certainly unconventional compared to the best breaking pitches, yet it’s still a productive pitch in his nuanced arsenal.


His curveball’s primary benefactor is its velocity. I’m not looking at the mid-to-high 80s range that Joe Kelly, Dustin May, and teammate Craig Kimbrel’s power curveballs occupy. I’m looking far, far lower. Mills’s curveball velocity averaged a head-turning 66.6 mph, the slowest of any pitcher with at least 100 curves. He was also the only pitcher on that leaderboard to average a curveball velocity under 70 mph, with the next closest being Zack Greinke at 70.4 mph. So, he ran away (pun intended) with that title.

Mills’s slowest curveball in 2020 clocked in at 60.1 mph as a ball to St. Louis Cardinals outfielder Tyler O’Neill.

This unique quirk gives Mills some powerful opportunities on the mound. One of the most significant ones would be how his curveball pairs with his sinker and four-seam fastball. His fastballs dominated his arsenal, as he used them a combined 58.9 percent of the time compared to his curveball’s 15.0 percent usage rate. Like his curveball, they sported a lackluster profile individually. His average velocity and spin rate on the two clocked in at 89.9 mph and 2197 RPM, both poor with respect to the league.

Despite this, the two pitch groups work in tandem like a regular Bonnie and Clyde against major-league hitters. To start, the pitches have a tremendously distinct pitch velocity difference. With a minimum of 300 fastballs and 100 curveballs, Mills’s difference of 23.3 mph was the highest of 65 pitchers. This eye-popping difference (visualized in the line chart below) was almost unprecedented in the season. The runner-up to Mills was Max Fried’s difference of 18.6 mph, about 20% lower than Mills’s.

Mills has the rare ability to mirror the pitches’ spin directions perfectly as well. By being able to do this, it becomes more difficult for a hitter to identify which pitch is which before each of them breaks given that they’ll spin similarly during a major portion of the flight path. As both pitches act very differently, with fastballs moving much less than breaking pitches, this has the potential to seriously fool some hitters. Spin direction is quantified as time on a clock, with perfect mirroring being a 6:00 difference. With the previous minimum for velocity difference in effect, Mills was one of four pitchers with this magic number. The other three were Aaron Nola, Shane Bieber, and Mike Fiers.

Mills’s deception is cemented further by his ability to keep the distribution of his release points tight. By detecting minor differences in a pitcher’s release points between different pitches, it becomes much easier for a hitter to predict which pitch is coming and thus gives them a significant advantage. When comparing his standard deviation of release points between the two pitches, Mills’s value comes out to 0.134, placing 19th with the previous minimum in effect. On a leaderboard with 65 players, it’s less impressive than his other feats but still noteworthy nonetheless.

So, what comes of a slow curveball velocity, perfect spin mirroring, a cavernous velocity difference, and similar release points between his curveball and fastballs? Well, in Mills’s case, the answer was a lot of poor contact. Poor contact, from Baseball Savant, is classified as a ball hit under, topped, or weak. Most of them never amass to anything substantial for a hitter and are very beneficial for a pitcher. In 2020, hitters averaged a mere .128 wOBA and .136 xwOBA on batted balls in these classifications.

In 2020, an astounding 77.8% of batted ball events that resulted from Mills’s curveball ended in poor contact. With a minimum of 25 batted ball events off of curveballs, Mills’s percentage fares as the second-best in the game behind fellow contact manager Julio Urías (78.6%). He also garnered a lot of contact from his breaking ball. Out of 51 swings, 78.4% of them were hit by opposing hitters, good for the fourth-highest in the league with a minimum of 50 swings. It’s an ideal number to pair along with the bounties of poor contact the pitch gets. Now it’s easy to see where his aforementioned .212 wOBA and .253 xwOBA on the pitch came from.


In one of the most active offseasons in recent memory, it’s clear that the Cubs are trying to prioritize sentiments shared in the days of Fergie Jenkins and Greg Maddux. The days where getting groundouts and having swift games were the norm throughout the sport. The aforementioned offseason acquisitions to the team’s rotation (Davies, Arrieta, and Williams) had mediocre K% of 22.8, 16.8, and 19.4 respectively. Hendricks, the new anchor of the Cubs’s rotation, has a career 3.12 ERA and 20.8 K% in 1,047.1 IP.

This isn’t to say that the Cubs don’t have guys with the potential to rack up punch-outs, but it certainly wasn’t as focused on as much as in prior years. This transition marks a significant, confident effort for the team to re-establish themselves from their closing championship window. And, standing at the entrance of this change in direction, is Mills’s 2020 season and his devastating curveball. With more familiar styles to lean on for support and an invigorating return to 162 games, his pitch is finally gonna get the opportunity it deserves to shine in the spotlight.

Steven Pappas

Hello! My name is Steven Pappas, and I'm a high school junior. I love to analyze and write about baseball data as a huge Chicago Cubs fan and lifelong follower of the sport. I use large databases such as Baseball Savant, basic coding knowledge in RStudio, and my inquisitive mindset to always scour the infinite data available. I really enjoy watching and following basketball and am a Chicago Bulls fan, actively going to their games at the United Center. I love the study of filmmaking, and it's a passion that I've begun to explore as a career opportunity. My favorite works come from the minds of Stanley Kubrick, Yorgos Lanthimos, Alejandro G. Iñárritu, the Coen brothers, and Wes Anderson. As a deft and passionate writer, I use my proficiency to create works from baseball data, for films, and my ideas in the form of short stories and little nuggets. I'm also a libertarian socialist in training and an active Greek Orthodox Christian.