Browns receiver Andrew Hawkins wore a T-shirt on the field that read "Justice for Tamir Rice and John Crawford." Both were shot by police while holding BB or pellet guns.
LeBron James and other athletes have donned shirts emblazoned with the phrase "I Can't Breathe" -- a reference to the last words of Eric Garner, a New York man who died as police took him into custody.
And five members of the St. Louis Rams held their hands in the air before a recent game, emulating the gestures of protesters in Ferguson, Missouri.
Athletes have long used the spotlight to send social and political messages, New York University Professor Jeffrey Sammons said.
"The most famous of all time seems to be the 1968 Olympics," he said.
Tommie Smith and John Carlos, two African-American medalists, raised their fists in a Black Power salute. Jesse Owens, the black Cleveland Olympian who had won gold medals in Hitler's Berlin, was among the critics of the symbolic action.
Boxer Muhammad Ali famously objected to being drafted to fight in Vietnam.
Sammons said while he sees some of that 60s spirit of protest in recent demonstrations, he also sees some of the same criticism.
"I think there's still this notion that sport is sacrosanct, there shouldn't be this mixture of politics and race," he said.
It's a criticism Sammons said he doesn't find to be fair.
Cleveland's police union president has called on Hawkins to apologize. Hawkins has said he feels at peace with his decision.
