Nearly two years ago, National Football League (NFL) player Colin Kaepernick sparked a wave of protests in the US against racial injustice when he refused to stand during the pre-game US national anthem.

The former San Francisco 49ers quarterback silently took a knee during the Star-Spangled Banner to condemn police brutality, including the killings of unarmed African Americans.

"I am not going to stand up to show pride in a flag for a country that oppresses black people and people of colour," the biracial athlete from Wisconsin, said at the time.

Since opting out of his contract with San Francisco for the 2017 season, the 30-year-old free agent has failed to land a contract with any team.

"To me, this is bigger than football and it would be selfish on my part to look the other way," Kaepernick said.

The snub spotlighted a familiar sight of athlete activism in US sport.

For decades, athletes, particularly African Americans, have used professional sports as a platform to highlight social and political issues.

According to sports writers and race relations researchers, sporting events have provided a platform that would have been otherwise unavailable for African Americans.

"Athletes traditionally have access to an audience and admiration from white America that regular black people don't have," said writer and podcaster Michael Harriot.

Daniel Grano, a professor who teaches spots, politics and race at the University of North Carolina, Charlotte, added that "recently in the US, sports has become one of the most important areas for people of colour to publicly respond to all the various racial issues that are defining our political moment, including police violence and the effort of the [Donald] Trump campaign and presidency to capitalise upon white grievance".

As the US marks Black History Month in February to celebrate the achievements and contributions of African Americans, Al Jazeera takes a look at the role sports and professional athletes have played in fighting for racial equality.

John Carlos and Tommie Smith

In the summer of 1968, American sprinters John Carlos and Tommie Smith orchestrated a silent protest on the Olympic podium.

During the medal ceremony for the 200-metre race at the Summer Olympics in Mexico City, gold medalist Smith and bronze medalist Carlos famously raised black-gloved fists and bowed their heads to signal black power and black unity in the US.The photographic image of protest still resonates today.

The two athletes wore black socks without shoes to show black poverty, while the black scarf around Smith's neck represented black pride.

The overtly political statement in an international arena was booed by the crowd, widely denounced back home and the two were subsequently expelled from the Games.

However, neither man said he regretted the move.

"Black America will understand what we did tonight," Smith later said at a press conference.

Fifty years later, analysts draw similarities between Smith and Carlos' act of protest and that of Kaepernick today.

"Tommie Smith and John Carlos are really interesting parallels to the current anthem protests, because a lot of the calculations they made about both the risk to their reputation and also the means for themselves, profaning a sacred patriotic ritual, parallels pretty nicely to the anthem protests of today," said Grano.

Muhammad Ali

The late boxer Muhammad Alipacked a heavy punch in the ring, but also stepped up the political and social fight outside it.

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Over a 21-year career, the three-time heavyweight champion from Louisville, Kentucky won 56 bouts, as well as an Olympic gold medal in 1980.

He also made headlines with his outspoken critique of racism in the US, conversion to Islam and his refusal to serve in the military in the Vietnam War.

"Why should they ask me to put on a uniform and go 10,000 miles from home and drop bombs and bullets on brown people in Vietnam while so-called Negro people in Louisville are treated like dogs and denied simple human rights?" Ali said at the time.

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Following the controversy, Ali was stripped of his titles, had his boxing licence suspended and was convicted of draft evasion, which was later reversed.

"Ali was at the height of his career and he decided to toss it all away to do what he thought was right and he never wavered," Harriot said.

He is considered by many sports writers as arguably the greatest athlete of all time. He himself proclaimed: "I am the world's greatest!"

Harriot argued that Ali is the "greatest black man to have ever lived."

"You really can't find examples like that anywhere else, in not just black history but American history," he told Al Jazeera in a phone interview from Birmingham, Alabama.

Jesse Owens

American track and field star Jesse Owens caused a major stir, both at home and in Germany, when he won four gold medals and broke two world records at the 1936 Berlin Olympic Games.

German dictator Adolf Hitler, who later went on to carry out the Holocaust, condemned the US for including black athletes on its roster. Hitler had instituted an "Aryans only" policy in German athletic organisation prior to the Games.

Owens' success dealt a blow to Hitler's Aryan race superiority myth, and also helped ignite the civil rights movement debate in the US.

"There was quite a bit of activism during the civil rights era, of course, [but also] well before that in 1936, [when] Jesse Owens repudiated Hitler's eugenic programme at the Olympics," Grano said.

Despite his Olympic success, Owens was not given the hero's welcome back home, or a customary White House invitation in a segregated US.

There are conflicting reports of Hitler refusing to shake Owens' hands after his victories, but it was the reaction by then US President Franklin D Roosevelt that appeared to leave the Alabama native more dismayed.

"Hitler didn't snub me; it was our president who snubbed me," Owens said months after the Berlin Games. "The president didn't even send a telegram."

Jackie Robinson

Hall of Famer Jackie Robinson broke the colour barrier in Major League Baseball (MLB) when he stepped out on the field to play for the Brooklyn Dodgers on April 15, 1947, becoming the first African American to play in the MLB,

Over a decade-long career, strife with racial abuse and harassment, the second baseman won the 1955 World Series, was named Rookie of the Year in his first season, and the league's Most Valuable Player (MVP) two years later.

After his retirement from the sport in 1956, Robinson devoted his life to the civil rights movement, serving as a board member for the National Association for the Advancement of Colored People (NAACP).

"A life is not important except in the impact it has on other lives," Robinson once said.

For his achievements and efforts, Robinson received two of the highest American awards: the Presidential Medal of Freedom and the Congressional Gold Medal.

Ken Reed, sports analyst and author, believes Robinson is "probably the most famous and influential African American social activist to challenge inequality" in sports.

"Robinson was a man of great integrity and courage," Reed, who is the sports policy director at the League of Fans told Al Jazeera in an email.

"He was an MVP and world champion on the baseball field, but more importantly, he was an MVP and world champion in the game of life for his work in fighting for social justice in the US."

Jack Johnson

Before Muhammad Ali, there was Jack Johnson.

As the first African American to win the world heavyweight championship in 1908, Johnson paved the way for other black boxers.

In what was then called the "Fight of the Century", the Texan challenged former heavyweight champion Jim Jeffries, before knocking out the white boxer after 15 rounds in Nevada, Las Vegas.

In doing so, the flamboyant Texan angered white America and sparked race riots across the country.

Johnson in many ways is an embodiment of the African American struggle to be truly free in this country – economically, socially and politically. Ken Burns, documentary filmmaker

Filmmaker Ken Burns, in his 2004 documentary, Unforgivable Blackness: The Rise and Fall of Jack Johnson, said the boxer "fought for freedom not just as a black man, but as an individual".

He added: "Johnson in many ways is an embodiment of the African American struggle to be truly free in this country - economically, socially and politically."

He absolutely refused to play by the rules set by the white establishment, or even those of the black community."

Althea Gibson

What Jackie Robinson did for baseball, Althea Gibson did for tennis.

After intense lobbying, the 23-year-old from Clarendon County, South Carolina broke the colour barrier in international tennis in 1950.

Six years later, she became the first black tennis player to win a Grand Slam trophy, paving the way for the likes of Arthur Ashe and the Williams sisters.

"I have all the opportunities today because of people like Althea," Venus Williams, a five-time Wimbledon champion, has said. "Just trying to follow in her footsteps."

During her amateur career, Gibson won a total of 11 Grand Slam titles.

In 1964, the trailblazer made more history on the golf course when she became the first African American woman to join the Ladies Professional Golf Association (LPGA) tour.

Arthur Ashe

As a black tennis player, Arthur Ashe is renowned for a series of firsts in a predominantly white sport.

In 1963, he became the first-ever black player to represent the US in the Davis Cup competition and the first African American man to be ranked number one in the world.

He is also the first black, male tennis player, and, to date, the only one, to win the singles titles at the US Open and Wimbledon.

Off the court, the Virginia native raised millions of dollars for the United Negro College Fund and launched the African American Athletic Association.

He was arrested for protesting against the US treatment of Haitian refugees. as well as the South African apartheid.

Ashe died of AIDS in 1993.

Serena Williams

With 23 Grand Slam titles - an Open era record - former world number one Serena Williams has cemented her place in tennis' history.

In doing so, the 36-year-old Olympic gold medalist has not been shy at calling out racism both on and off the court.

"Growing up, I was told I couldn't accomplish my dreams because I was a woman and, more so, because of the colour of my skin," she wrote in an essay for Fortune, marking Black Women's Equal Pay Day.

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"In every stage of my life, I've had to learn to stand up for myself and speak out," she went on to write.

"I have been treated unfairly, I've been disrespected by my male colleagues and, in the most painful times, I've been the subject of racist remarks on and off the tennis court."

Analysts say after Ashe and Gibson, Williams has played a pivotal role in challenging white norms in tennis.

Grano said: "She's a really important figure not only because of the things that she has said and the identity that she's crafted for herself, but also because she's done so in a sport that has been dominated by a white aesthetic, white players and a set of privileges and preferences that have not always afforded a space for black expression and black activism."

Curt Flood

St Louis Cardinals centre-fielder Curt Flood sacrificed his baseball career in an attempt to bring institutional change.

Unhappy with his trade to the Philadelphia Phillies for the 1970 season, Flood waged a legal war against the MLB, challenging the league's reserve clause, which prevented players from changing teams unless they were traded.

In a letter to Bowie Kuhn, the MLB commissioner, Flood wrote: "After 12 years in the major leagues, I do not feel that I am a piece of property to be bought and sold irrespective of my wishes."

Despite losing the Supreme Court battle, Flood's persistence and the lawsuit opened the door for free agency that exists in modern baseball today.

Follow Saba Aziz on Twitter: @saba_aziz