Several country stars are showing their strength as part of the USO’s new Flex 4 Forces campaign, which brings attention to the USO’s mission of keeping service members connected to family, home and country. The campaign calls on the public to demonstrate appreciation to service members and their families by striking a biceps flex and sharing it on social media using the hashtag #Flex4Forces. That person can then nominate four additional people to flex next, much like the Ice Bucket Challenge did in 2014 to bring awareness to ALS.

• Tim McGraw flexed his muscles for the campaign over the weekend, nominating NFL great Brett Favre to take the challenge next. Favre then flexed his stuff and nominated Garth Brooks, who posted a gym selfie showing off his big guns. Garth, in turn, nominated tennis player Serena Williams to take the challenge next.