NBA legend Bill Russell said his viral photo of him taking a knee was to let NFL players "know they are not alone."

NBA legend Bill Russell told ESPN's Jackie MacMullan he posted a photo of himself taking a knee to let NFL players "know they are not alone."

Russell posted the photo to his recently created Twitter account Monday afternoon. In the picture, Russell is kneeling while wearing his Presidential Medal of Freedom—the highest civilian award in the United States, which he received from former President Barack Obama in 2011.

"What I wanted,'' Russell told ESPN, "was to let those guys know I support them. I wanted them to know they are not alone.''

The caption read "Proud to take a knee, and to stand tall against social injustice."

Russell, 83, was an 11-time champion and five-time NBA MVP with the Boston Celtics. He has also been one of the leading advocates for civil rights in the sports world, including his support for Muhammad Ali after he refused induction into the U.S. Army and his participation in the 1963 March on Washington with Martin Luther King Jr.

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During the 1950s and 1960s, Russell regularly spoke out against racial injustice, despite backlash from white Boston residents.

Russell said he decided he wanted to do something after hearing President Donald Trump's criticism of NFL players and seeing the response from teams.

"Just tell those NFL players, I'm with them," Russell told ESPN.