Chris Pratt Shows Some Muscle for the 'Flex 4 Forces' Challenge

Chris Pratt is showing off his muscles for a good cause!

The 38-year-old actor took to his Instagram to take on the “Flex 4 Forces” after being nominated by his friend Dwayne Johnson.

The new social media challenge is to raise awareness for the USO, an organization which provides services to military members and their families.

“Thank you to @therock for nominating me for the #flex4forces challenge and introducing me to ‘Jack and Sledge Hammer’ (his arms),” Chris wrote.

He jokingly added, “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.”

Check out the photo below…