The compilation takes its title from the Minor Threat lyric shouted in the song "12XU", included on the album, originally by the English band Wire.

The Teen Idles had broken up in late 1980 and by the time of the release of Flex Your Head members had already started Minor Threat and Youth Brigade.

The Untouchables broke up in 1981 and with the former members joining a multitude of bands including The Faith, The Warmers, Rites of Spring, One Last Wish, Skewbald, Happy Go Licky, Youth Brigade, and The Meatmen.

Minor Threat had disbanded (although they would reunite a few months after the album's release) and since then Ian MacKaye and Jeff Nelson had founded Skewbald while Lyle Preslar joined an early version of Big Black.

A third early cover, designed by Jeff Nelson and used for the British pressing of the album, displayed a black and white version of the flag of Washington, D.C., with the stars replaced by Xs.

Justin M. Norton, contributor at the online magazine Stereogum, was of the view that: "[Flex Your Head is] a Rosetta Stone not just for the DC scene but all of American hardcore ...