(Jeff Curry/USA Today Sports)

The past quarter-century has been great for athletes as corporations and “brands,” with salaries exploding, endorsement opportunities multiplying and awareness of how to groom their images for maximum marketability growing. But it has not been a very good period for the notion of athletes as activists. Political expression and social activism, hallmarks of the sports landscape of the 1960s and ’70s, had come to be seen as too risky, bad for business.

“The era of the activist athlete is dead,” a Duke University cultural anthropologist wrote in 2008.

But that accepted wisdom appears to be changing. As the United States confronts anew its racial polarization, prominent athletes are no longer avoiding these sensitive topics — and in some cases are leading the public discourse.

On Sunday, five St. Louis Rams players took the field together during pregame introductions with a “Hands Up, Don’t Shoot” gesture meant to show solidarity with Michael Brown, the black teenager shot dead by a white police officer in August in the St. Louis suburb of Ferguson.

Three days later, following another controversial non-indictment by a grand jury of a white police officer who killed an unarmed, 43-year-old African American man with a chokehold in Staten Island, University of Maryland wide receiver Deon Long took part in a peaceful protest on the College Park campus, holding up a handwritten sign — widely disseminated on social media — that said, “Are we still ‘thugs’ when you pay to watch us play sports?” The sign had the popular Twitter hashtag “#blacklivesmatter.”

In an image posted to LeBron James' Twitter page, James and his Miami Heat teammates wear team hoodies. (Associated Press)

“It’s extremely tough to take a stand one way or another [as an athlete]. In our position, it’s very easy to not have an opinion on anything — because if you don’t have an opinion on anything, you draw no scrutiny to yourself,” said Washington Redskins safety Ryan Clark, speaking of the Rams players.

“I think it was really cool that those guys felt strongly enough and felt the solidarity among one another — whether you agree with their stance or not — to go out and actually try to say, ‘Okay, this is how we feel.’ ”

Clark spoke from experience. Three months earlier, in the immediate aftermath of Brown’s shooting in Ferguson, he and safety Brandon Merriweather led a group of Redskins teammates onto FedEx Field before a preseason game in a similar “hands-up” gesture of support for Brown. A thousand miles from Ferguson, months before the grand jury’s non-indictment reignited the protests, the gesture drew little media attention.

At least in St. Louis, the Rams’ gesture had a polarizing effect. The St. Louis Police Officers Association criticized the players involved, demanding punishment for them, and a bitter public debate ensued over whether the team had apologized to the police department for the players’ actions.

But in any case, the days when athletes were expected to “shut up and play” appear to be over.

“It’s a sensitive subject right now,” Cleveland Cavaliers star LeBron James told reporters during practice in New York on Thursday. “Violence is not the answer, and retaliation isn’t the solution,” he said, discouraging violent protests. “As a society, we just have to do better. I pray for the families of the lost ones.”

On Thursday night, Amare Stoudemire’s New York Knicks played a home game at Madison Square Garden against James and the Cavaliers, while more than 200 people were arrested in the city during protests of the Staten Island grand jury decision. Stoudemire said he would have been outside with the protesters if he didn’t have a game to play.

“I think it’s something that’s . . . very alarming in our country,” Stoudemire told reporters. “We have to be more conscientious of what the law enforcement’s job is, and that’s to protect and serve. Those two words are very strong when you think about that. Your first job is to protect, and your second job is to serve. Obviously it’s not happening that way.”

Watching from afar, one man who understands like few others the intersection of sports and activism has been watching the current events and smiling.

“I’m delighted,” John Carlos said. “I always knew it was going to come.”

Carlos, 69, is considered the godfather of modern-day athlete activism. The gesture he and U.S. teammate Tommie Smith made on the medal stand at the 1968 Mexico City Olympics — a single black-gloved fist raised in the air, meant to signal “Black Power” — became one of the decade’s iconic images.

Cultural anthropologist Orin Starn, the one who wrote in 2008 that athlete activism was dead, said there are “echoes” of Carlos and Smith’s 1968 protest today, though it is still too early to say whether there has been a revival of that tradition. “To me, these still feel like isolated incidents rather than a shift in sporting culture,” Starn said.

Carlos and Smith, who were kicked out of the Olympics for their gesture, were hardly alone in using the platform of sports stardom to advocate for social change during those decades of upheaval. Boxer Muhammad Ali protested against the Vietnam War. NBA star Bill Russell spoke out against racism. Tennis icons Arthur Ashe and Billie Jean King advocated for equality for blacks and women.

By the 1990s, generally speaking, that sense of activism among athletes was all but gone, replaced by a sense of the athlete as a marketing vessel. This stance was most vividly seen when Chicago Bulls star Michael Jordan reportedly explained his refusal to endorse a Democratic Senate candidate in his home state of North Carolina by saying, “Republicans buy shoes, too.”

Jordan “was the alpha model for the new kind of athlete who never took controversial stands,” Starn said, “and Tiger Woods followed in his footsteps.”

Perry Wallace, an American University law professor who in 1967 became the first black basketball player in the Southeastern Conference, believes the rise in social activism among athletes is in part a reaction to the criticism heaped on African American superstars such as Jordan and Woods for not “contributing to the cause.”

“Jordan was criticized a lot inside and outside the African American community for not playing a more active role in helping the community,” said Wallace, the subject of a new biography, “Strong Inside.” “Some of that criticism has had an impact on some of these younger players. . . . I’m glad to see it because, among other things, I hope it reflects that they’re looking around and thinking about the broader society rather than just themselves and their sneaker sales.”

If there was one turning point in returning social activism to big-time sports, it may have come in 2012, when James and his Miami Heat teammates donned black “hoodies” and posed together for a team picture with their heads bowed and their hands in their pockets. The gesture, which James tweeted along with the hashtag “#WeWantJustice,” was meant to show solidarity with Trayvon Martin, the black teenager shot and killed by a white/Hispanic man in Florida while wearing a similar hooded sweatshirt.

“When the Heat stood up and made that statement, it was saying to everybody, ‘Enough is enough,’ ” Carlos said. “Young people are losing their lives and feeling like there’s no justice, and this [gesture] was meant to say, ‘We’re tired of it.’ ”

A year later, punter Chris Kluwe was released by the Minnesota Vikings, allegedly because of his outspoken support for same-sex marriage. Although the team denied that allegation, as well as Kluwe’s claims he was subjected to homophobic behavior by team officials and coaches, the team reached a settlement with Kluwe that included a donation to charities that support lesbian, gay, bisexual and transgender causes.

Also in 2013, after Los Angeles Clippers owner Donald Sterling was secretly recorded making racist comments about African Americans, the NBA forced him to sell the team, largely because of the outpouring of criticism and the demands of prominent players as well as the team’s coach, Doc Rivers.

But the spate of recent high-profile and polarizing police killings — in Ferguson, in Staten Island and in Cleveland, where a white police officer shot a 12-year-old African American boy after mistaking his pellet gun for a more lethal one — appears to have pushed the notion of athletes-as-activists to a new threshold.

In part, this is because social media has made it easier than ever for athletes to voice their opinions. In the wake of the Ferguson grand jury’s decision not to indict officer Darren Wilson for the killing of Brown, many high-profile athletes took to Twitter and Instagram to speak out. They included James, Los Angeles Lakers legend Magic Johnson, Redskins wide receiver DeSean Jackson and tennis superstar Serena Williams.

Perhaps the most pointed statement of all, though, was made by the Lakers’ Kobe Bryant, who tweeted, “The system enables young black men to be killed behind the mask of law.”

Staff writer Liz Clarke contributed to this report.