Tim McGraw started it, but soon Garth Brooks was on Twitter snapping a flex selfie. Could Florida Georgia Line be next?

Country stars, athletes and celebrities nationwide are flexing their biceps for a good cause. Flex 4 Forces (#Flex4Forces) is a USO sponsored digital initiative to show support for the United States military. The campaign "underscores the USO’s historic mission to strengthen service members by keeping them connected to family, home and country," a description at the Flex 4 Forces Facebook page reads.

McGraw got involved when celebrity chef Robert Irvine challenged him. He in turn challenged former NFL quarterback Brett Favre via Twitter, who then turned it on Brooks. More are sure to come.

Taste of Country's Ania Hammar challenges Tyler Hubbard and Brian Kelley of Florida Georgia Line in the above video. You boys up for it?

WKSR reports that the first #Flex4Forces photos began to appear on May 20. It's not clear if there's a deadline or end date for the initiative. The idea is similar to the Ice Bucket Challenge of 2014, when hundreds of celebrities and dozens of country stars poured cold water on their heads to show support for ALS. Forbes reports that more than $115 million was raised during that fundraiser.

Of course, McGraw is no stranger to flexing. His workouts are nearly legendary, and he has the physique to prove it. Fans are not used to seeing Brooks in gym clothes, but he proves he knows how to throw the weights around, as well. We look forward to watching more and more country stars and celebrities flex for the troops this year.

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