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MLB report ties historic home run surge to ball seam height, batter behavior

The baseballs weren’t juiced, so says a new research study commissioned by Major League Baseball.

A new study released December 11 by the league concluded that a record-setting number of home runs hit during the 2019 season was due in part to inconsistent seam height for the Rawlings-produced balls and “changes in player behavior” that favored higher launch angles and higher exit velocities for batters.

The 27-page report, written by a four-person committee of scientists, concluded the league did not intentionally alter the ball as the league in 2019 hit 6,776 total home runs, 11 per cent higher than the previous record set in 2017. 

But the study did say there was “manufacturing variability” that helped create differences in the seam height on the balls. The lower a seam is on the ball, the less drag is encounters in the air after being hit by a batter. Pitchers across the league have long argued anecdotally that balls of recent vintage have flatter seams than their historical counterparts.

“No evidence was found that changes in baseball performance were due to anything intentional on the part of Rawlings or MLB and were likely due to manufacturing variability,” the report said.

The study later went on to say about the launch angles of balls in flight, “lacking strong evidence that the change in launch conditions are due to changes in the baseball, we conclude that they are due a change in player behavior.”

The “manufacturing variability,” however, is in direct opposition to the stated goals last year when a private equity fund led by San Diego Padres general partner Peter Seidler and MLB itself acquired Rawlings Sporting Goods Co., in part to gain greater control over the ball manufacturing process.

While Rawlings would appear to have a growing quality control issue, company chief executive Michael Zlaket said at baseball’s Winter Meetings that he has never been pressured to alter the ball during manufacturing.

“We have never been asked to juice or de-juice a baseball,” Zlaket said. “And we’ve never done anything of the sort. Never would.”

League executives similarly said that complete consistency in the ball is impossible given its construction using natural materials and hand labor at the Rawlings factory in Costa Rica.

“We choose to use a piece of equipment in our game that is made of natural materials and is hand-stiched,” said Morgan Sword, MLB senior vice president. “That introduces a lot of variability.”

The study also offered a series of recommendations, including subsequent research on how rubbing mud applied to new balls before they are used contributes to drag, the installation of atmospheric tracking systems at ballparks, and an inquiry into the possibility of using humidor systems in all 30 MLB stadiums to create consistency in ball storage conditions. 

Questions about whether the ball is juiced and is responsible for the historic home run surge has dominated league discussion throughout the year, and was a major topic at both the All-Star Game and World Series.

During the postseason, there was also widespread talk of the ball flying significantly less far than during the regular season, and the report confirmed that anecdotal suspicion, though the scientists could not determine a specific reason for that.