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Robo-umpiring has arrived in the minor leagues. Here’s how it’s been working.

At Polar Park in Worcester, a message is displayed on the large video screen when a challenge has been requested by the batter, catcher, or pitcher.Tayla Bolduc/Worcester Red Sox

WORCESTER — At first glance, the difference at WooSox games in Polar Park is almost indiscernible. A pitch is thrown. An umpire signals a ball or strike. The game moves forward.

Yet across Triple A this year, Major League Baseball has conducted a massive experiment with significant implications for how the game is played, how players develop, and how developing players are evaluated.

Robo-umpiring has arrived — or at least it has on weekdays.

Automatic balls and strikes

Since April 25, MLB has employed an Automatic Ball-Strike system in all Triple A ballparks for games played Monday through Thursday. The system is a slight variation on one that was last year in Triple A throughout the Pacific Coast League as well as in Charlotte of the International League.


With ballparks outfitted with an array of Hawk-Eye cameras, a computer determines whether a pitch clips any part of the strike zone or misses it entirely. An umpire is informed in an earpiece whether the pitch was a ball or strike and signals accordingly.

But the strike zone used by ABS in Triple A is not the same one that governs the big leagues. The major league strike zone is a three-dimensional prism above the plate that runs from the midpoint between the shoulders and top of the pants to the bottom of the kneecaps. The ABS system used in Triple A is a two-dimensional rectangle (closer to what you see on a TV broadcast), set 8.5 inches from the front edge of the plate.

Minor leaguers were measured for their precise heights in spring training, with the top of the zone set at 51 percent of a batter’s height and the bottom of the zone at 27 percent of his height. By all accounts, that two-dimensional shape with those specifications has created a strike zone that is meaningfully smaller than the one used by umpires in Triple A last year and likewise smaller than the zone in the big leagues.


Camera equipment is installed around Polar Park in Worcester to power the automated ball/strike system as well as challenge calls made by the home plate umpire.Major League Baseball

“It’s like throwing to a shoebox,” said Red Sox starter James Paxton, who made a rehab start for the WooSox on the first day the ABS system was used at Polar Park, an outing in which he walked four batters and threw just 51 percent of his pitches for strikes. “It basically feels like you’ve got to throw the ball down the middle of the plate.”

That sense has been particularly profound among pitchers like Paxton whose pitch mixes are designed to work at the top of the zone with four-seamers and at the bottom with curveballs. On the other hand, east-west pitchers who lean on sinker/slider/cutter mixes have been pleased to clip the inner or outer edge of the zone by a seam and still get a called strike.

“It is definitely tighter, but there are some ways to exploit the zone a little bit,” said WooSox pitching coach Paul Abbott. “The biggest thing for me is that high strike. It’s not getting called. I feel like we’re still in beta form and they’ll get some bugs out of it, but there’s some strikes that we’re not getting.”

Challenge system

While the ABS system is used Monday through Thursday (and there has been only one week this year when Triple A teams played on Monday), from Friday through Sunday the plate umpire calls balls and strikes. However, on those days, each team can challenge up to three calls, with the pitcher, catcher, or batter permitted to seek an overrule.


The player taps his head to challenge a ruling. The umpire calls time, an announcement is made that the call has been challenged, and on the scoreboard, a three-dimensional Hawk-Eye-generated animation — similar to those seen in tennis — shows whether the pitch was in or out of the rectangular, two-dimensional strike zone.

The process takes roughly 15 seconds.


“It adds a little bit of theatrics to the game,” said Red Sox shortstop Trevor Story, who experienced both ABS and the challenge system during his rehab assignment in Worcester. “They put it up on the board and the whole crowd reacts to it. Initially I was very against it, but I can see that being something [good] for the game.”

There’s a strategic element regarding the risk/reward of when to burn a challenge. For instance, when Ceddanne Rafaela led off the bottom of the first inning of a recent game for the WooSox by challenging a 1-and-1 fastball that was well within the strike zone, it drew a few head shakes.

“There were borderline pitches where maybe I thought about challenging but I was like, ‘This scenario doesn’t really call for this and you don’t really want to risk it,’ ” said Red Sox catcher Reese McGuire, who also spent time in Worcester during a rebab stint.

Interestingly, players and coaches in Triple A largely agree that umpires have adapted their interpretation of the strike zone to conform relatively closely to the ABS system, in part because they want to be accurate in case of challenges.


“We very quickly saw umpires making the zone more true [to the ABS zone],” said WooSox pitcher Chris Murphy, who speculated that at the least, an ABS system could be employed in spring training to calibrate zones for umpires entering the season.

The average Triple A game under the challenge system has seen both teams use their full complement of three challenges each. (Some would prefer a system in which teams had three challenges on both offense and defense per game.) Challenges have resulted in overturns only about 46 percent of the time, with umpires’ initial rulings upheld a majority of the time.

The effect

At least as implemented in Triple A this year, the impact of the ABS system has coincided with a significant change in the offensive environment — a seemingly natural development with a tighter zone.

Walks in the International League have jumped 16.5 percent.

“I can’t remember being around a baseball level that has seen as many walks as this season,” said WooSox lefthander Brandon Walter.

With the top and bottom of the zone smooshed, pitchers have been forced to throw strikes in hitters’ nitro zones, contributing to a 12 percent jump in homers per game. Runs per game have shot up; those Triple A games (not just International League) being governed by the ABS have seen about 4.5 percent more runs per game through Aug. 15 than those contests in which the challenge system is in play, according to a league source.


The scoring increase creates challenges in player evaluations. A poor line might belie a pitcher who throws with good command. Abbott noted that after Paxton struggled with the strike zone in Worcester’s first ABS game, he and the pitcher reviewed the outing and identified roughly half a dozen pitches that would have been well-located strikes.

Meanwhile, many hitters have gleefully adjusted to the dimensions where pitchers must attack. With a narrower zone funneling pitches into players’ wheelhouses, hitters have been emboldened to swing from their heels in pre-two-strike counts.

The result? The leaguewide slash line in the International League of .263/.358/.444 roughly approximates that of Mets leadoff hitter Brandon Nimmo, who received an eight-year, $162 million contract from his team.

The camera equipment used in the ABS system is similar to that used by the Hawk-Eye systems in tennis.Major League Baseball

“You do see offense really going up in Triple A,” said Red Sox chief baseball officer Chaim Bloom. “We can and do adjust for it, but it is hard to rewire your brain to see hitters going off or pitchers getting shelled.”

Of course, MLB can alter the zone to make it larger — or make it three-dimensional, or position the rectangle closer to or farther from the plate. However, one thing that seems unalterable with ABS is any value attached to how catchers frame pitches. As such, McGuire — who prides himself on how he presents pitches to umpires — is no fan.

“I didn’t like it at all, honestly,” he said. “It just took away from the natural feel of the game. It was almost like I didn’t even really need to try my hardest to make a pitch look good. I could almost be a little bit lazy.

“If it’s automated, it kind of doesn’t really matter. You can be almost a first baseman back there and stand up if you want to. It’s just very unnatural.”

For players trying to reach the big leagues, there are developmental benefits and detriments to the automated system.

Historically, the strike zone has shrunk steadily from level to level, with minor league umpires getting increasingly close to big league standards as they move up the ladder. The strike zone in Single A was larger than that in High A, which was larger than that in Double A … and so on.

But the zone used in Triple A this year has jolted that gradual progression with a zone smaller than that employed in the big leagues. As such, some pitchers are actually finding greater freedom to execute pitches in the big leagues — even against more advanced hitters — than they did in Triple A.

Murphy’s walk rate dropped from 12 percent in Triple A to 9 percent in the big leagues. Walter likewise saw his walk rate go from 9 percent in Worcester to 6 percent in Boston.

On the other hand, pitchers going from Double A to Triple A have found an already significant jump to be at times exasperating with a smaller zone. Lefthander Shane Drohan saw his walk rate jump from 7 percent in Double A to 14 percent with Worcester.

“You just kind of have to brush it off and not let it affect you,” said Drohan. “I feel like there’s some good in it, too. You can get a lot out of it, really being in the strike zone. So I think there’s a skill development aspect to it.”

The video board at Polar Park shows the ruling after a challenge decision is determined by the automated balls-strikes system.Tayla Bolduc/Worcester Red Sox

Hitters, on the other hand, might find the jump to the big leagues — already daunting — even harder if they’re adjusting not only to better arms but also a larger zone.

For pitchers and hitters alike, the eventual adoption of a standardized automated zone across all levels could smooth out the level-to-level (and game-to-game) adjustments to varying zones, potentially creating a better environment for both player development and evaluation.

The future

So, will an automated strike zone come to the big leagues?

Quite possibly — but the likelier next step appears to be the adoption of the challenge system at the highest level. Players who experienced both in Triple A widely prefer the challenge system. At the All-Star Game, commissioner Rob Manfred described MLB as “sort of big on that [challenge system] idea,” though it ultimately will be up to a joint committee of MLB representatives, Players Association members, and an umpire to set a course.

“The joint committee is going to have to get to some kind of consensus on whether and when,” said Manfred.

Still, the strong reactions — and the significant change in Triple games — underscore the sensitivity of adjusting a central feature of baseball.

“People want things to complain about,” said Paxton. “We’re human. We want to have some things that aren’t perfect about this game or else it’s going to get boring. People like coaches being tossed once in a while and guys arguing. I understand that [missed calls] change the outcomes of some games, but that’s been happening forever.

“It’s great that we’re making these steps forward, but at the same time, we need to be careful about the steps that we are making so we don’t change the game too much and make it boring baseball to watch.”

Justyn-Henry Malloy (right), a Detroit Tigers prospect, waited along with the umpire and catcher for a ruling on a challenge of a ball/strike call during the Futures Game held as part of MLB’s All-Star Game events.Daniel Shirey/MLB/MLB Photos via Getty Images

Alex Speier can be reached at Follow him @alexspeier.