Why Is the #Flex4Forces Challenge Inspiring Chris Pratt, The Rock and Jessica Biel to Show Off Biceps?

The movement, which is set up to raise awareness for the United Service Organization (USO), has already attracted the attention of Dwayne "The Rock" Johnson, Chris Pratt, Jessica Biel, Jamie Lynn Sigler and Elizabeth Banks.

Ashley Camac, Director of San Diego's USO unit, told NBC News the social media campaign is "very simple."

"It's just to raise awareness and show the troops overseas that we're supporting them," she said. "I think sometimes an overseas service member can feel disconnected, but when they can see their country is supporting their efforts, it makes a huge difference in their mind."

Biel first nominated The Rock on July 4th, saying "Join me and @TheUSO as we honor and support our servicemen and women! I hereby nominate @theRock to join in on the #gunshow #Flex4Forces"

The Rock took on her offer, adding a name for his guns. "Introducing 'Jack' of the 'Hammer Bros' to @JessicaBiel & @The_USO. U don't wanna meet 'Sledge'. Inominate @prattprattpratt #Flex4Forces"

The "Guardians of the Galaxy" star hilariously replied to Johnson on Monday, saying he too has a name for his left arm, "Melissa."

"Thank you to @therock for nominating me for the #flex4forces challenge and introducing me to 'Jack and Sledge Hammer," he wrote. "The Rock's not the only one who names his guns by the way. Allow me to introduce you to my left arm, Melissa. She's less of a blunt force tool and more of an emotionally supportive friend."