Back in his playing days with the Boston Celtics, Bill Russell was one of the most socially active players in the NBA. He was involved in the Civil Rights Movement, and had a front row seat in Washington D.C. to Martin Luther King Jr.'s famous "I Have A Dream" speech. 

Unsurprisingly, Russell's willingness to take a stand for what he believes in has carried into his post-playing days. In 2011, he was awarded the Presidential Medal of Freedom -- the highest civilian award in the United States -- by Barack Obama. More recently, he has been vocal in his support of Colin Kaepernick and other players in the NFL who protested last season. Last fall, he tweeted a picture of himself kneeling, as many of the NFL players did, while wearing that Presidential Medal. 

Now, Russell has once again tweeted the photo of himself kneeling -- this time in memory of Stephon Clark, the man who was killed by police in Sacramento earlier this month. 

Protests in Sacramento following Clark's death have migrated to the Kings' arena prior to two of their recent games. The first time, the game was delayed, and played in front of a very sparse crowd after most fans were not able to get into the arena. Kings owner Vivek Ranadive gave a speech following that game, and the Kings -- along with the Celtics -- subsequently made a PSA and wore special shirts pregame. 

Since then, the Kings have announced a partnership with the Black Lives Matter group in Sacramento. 

Russell, who was the first African-American coach in the NBA with the Celtics, also coached the Kings from 1987-88.