Fans cheer with their shirts off after heading to upper deck outfield seating in the ninth inning of a game against the Cleveland Guardians on Tuesday in Detroit. (AP photo)

Major League Baseball is experiencing an epidemic of (mostly) guys being dudes.

At ballparks across the country, groups consisting of mostly young men are joining in on the “Tarps Off” trend that’s loud, goofy, infectious and new to the baseball world. Joining in on the fun is simple: Go to the section where the party is happening, take off your shirt and start twirling it above your head.

Soccer-like chants or singing usually follows — injecting a jolt of energy for a sport that occasionally is chided for its lack of energy inside the stadium.

After getting its start in St. Louis last Friday, it has spread across the league to places like Detroit, Tampa Bay, Philadelphia, Seattle and Anaheim.

Chad Bitzer, who has been coming to Mariners games for about 13 years, was among the shirtless fans in Seattle. His reasoning was simple: “Cause everyone else was taking it off. Why not?”

“It’s fresh. It’s a beautiful night. Take it off,” Bitzer said. “Great Northwest night. We live for the summers. We live for the good weather.”

Ground zero for the shirtless outbreak was in St. Louis last Friday, when a club baseball team affiliated with Stephen F. Austin State University was in Alton, Illinois, for the National Club Baseball Division II World Series. The Cardinals offered tickets to the team, and 17 players attended.

That group started the fun, dozens of others joined and suddenly there were a couple hundred fans creating a ruckus in right field that helped propel the Cardinals to a 5-4 victory in 11 innings over the Kansas City Royals. Cardinals manager Oliver Marmol loved the energy so much that he bought tickets for the shirtless revelers for Saturday’s game and they returned.

A similar outbreak of shirtless fans broke out at a Tampa Bay Rays game Monday and again Tuesday. Another small group celebrated in Philadelphia as the Reds and Phillies played in the rain. Angels fans celebrated with a mix of joy and irritation, chanting for owner Arte Moreno to sell the team.

MLB certainly won’t complain about the attention. Attendance is up at big league stadiums so far this season, averaging roughly 1,000 more fans per game than a year ago through Monday’s contests.

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AP Sports Writer Andrew Destin in Seattle and AP freelance writer Warren Mayes in St. Louis contributed to this report.

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AP MLB: https://apnews.com/MLB