National Football League commissioner Roger Goodell issued a 1 minute and 21 second apology on 5 June, following several years of debate within the NFL as well as among fans, Donald Trump and countless pundits who criticised Colin Kaepernick and other players' silent protests against police brutality.

Mr Goodell's message came nearly two weeks after the police killing of George Floyd, whose Memorial Day death and the killings of other black Americans have galvanised global protests against police violence and racism.

But in the commissioner's message, Kaepernick — the former San Francisco 49ers quarterback who protested police brutality against black Americans by taking a knee during the national anthem — was notably absent.

"We, the National Football League, admit we were wrong," the commissioner said. "We, the National Football League, believe Black Lives Matter. ... The protests around the country are emblematic of the centuries of silence, inequality and oppression of black players, coaches, fans and staff. We are listening."

A wave of players in the NFL and in other sports followed Kaepernick’s example, morphing into a direct resistance against the president after he condemned it. The NFL responded by announcing that teams would be fined if their players continued to do so.

But the move to take a knee during the national anthem before kick-off — or stand with arms locked in silent protest — follows in a long tradition of sports stars standing up for human rights, despite pressure from fans and public figures telling athletes to "stick to sports".

When did this all start?

Taking a knee has a larger legacy within the Civil Rights Movement. A famous photograph captured Martin Luther King Jr and other civil rights marchers kneeling in prayer in Selma, Alabama in 1965.

The photo was taken after the arrests of dozens of demonstrators amid three nonviolent protest marches fighting for voting rights for black Americans, ultimately leading to the landmark passage of the Voting Rights Act.

On "Bloody Sunday" on 7 March 1965, as hundreds of marchers prepared to cross the Edmund Pettus Bridge, they were met by state troopers and deputised white residents, who brutally attacked dozens of marchers.

By 2016, America was reeling from the deaths of Eric Garner, Michael Brown, Laquan McDonald, Tamir Rice, Sandra Bland, Alton Sterling and Philando Castile, among other black Americans killed by police.

Many of their deaths were captured on video and broadcast on social media, igniting a mass movement to condemn police violence.

NFL players weren't typically on the field for "The Star-Spangled Banner" until 2009. Between 2011 and 2014, the US Department of Defence gave the NFL millions of dollars to promote patriotic displays, including on-field flag ceremonies and tributes to veterans. Soon on-field anthem ceremonies became expected pregame rituals.

Kaepernick first protested at the start of the 2016 season on 26 August, but his quiet demonstrations were not immediately noticed. At that point, he simply sat on benches.

On 1 September, he transitioned to taking a knee in protest instead, following advice from Retired Army Green Beret Nate Boyer, who suggested kneeling during the ceremony would be more respectful towards veterans.

“I am not going to stand up to show pride in a flag for a country that oppresses black people and people of colour,” Kaepernick said in a press conference after first sitting out during the anthem. “To me, this is bigger than football, and it would be selfish on my part to look the other way. There are bodies in the street and people getting paid leave and getting away with murder.”

Several other players joined Kaepernick's protest against criticism from football fans, and he had the support of then-president Barack Obama.

Commissioner Goodell, however, said he didn't "necessarily agree" with his method of protest.

Kaepernick played his final NFL game on 1 January 2017. Many players continued to kneel.

American sportspeople kneel in protest Show all 14 1 /14 American sportspeople kneel in protest American sportspeople kneel in protest Terrance Smith #48, Eric Fisher #72, Demetrius Harris #84, and Cameron Erving #75 of the Kansas City Chiefs is seen taking a knee before the game against the Los Angeles Chargers at the StubHub Center on 24 September, 2017 in Carson, California Sean M. Haffey/Getty American sportspeople kneel in protest Members of the Dallas Cowboys link arms and kneel during the National Anthem before the start of the NFL game against the Arizona Cardinals at the University of Phoenix Stadium on 25 September, 2017 in Glendale, Arizona Christian Petersen/Getty American sportspeople kneel in protest Bruce Maxwell of the Oakland Athletics kneels as teammate Mark Canha #20 places his hand on Maxwell's shoulder during the singing of the National Anthem before their MLB American League baseball game against the Seattle Mariners at Oakland-Alameda County Coliseum in Oakland, California on 25 September 2017 Reuters/Stephen Lam American sportspeople kneel in protest Members of the Oakland Unified School District Honor Band kneel as they perform the national anthem before a MLB American League baseball game between the Oakland Athletics and the Seattle Mariners at Oakland-Alameda County Coliseum in Oakland, California on 25 September, 2017 Reuters/Stephen Lam American sportspeople kneel in protest Samaje Perine #32 and Chris Carter #55 of the Washington Redskins hold hands as they stand and kneel in unison during the national anthem before playing against the Oakland Raiders at FedExField on 24 September, 2017 in Landover, Maryland Patrick Smith/Getty American sportspeople kneel in protest Meghan Linsey, a former contestant on "The Voice," kneels after singing the national anthem before the start of the Tennessee Titans and Seattle Seahawks football game at Nissan Stadium in Nashville, Tennessee on 24 September, 2017 Ryan Hermens/The Paducah Sun via AP American sportspeople kneel in protest The Los Angeles Chargers are seen during national anthem before the game against the Kansas City Chiefs at the StubHub Center on 24 September, 2017 in Carson, California Jeff Gross/Getty American sportspeople kneel in protest Nick Kyrgios of Team World joined NFL stars from Wembley by dropping his knee before his Laver Cup match with Roger Federer today during his mens singles match against Roger Federer of Team Europe on the final day of the Laver cup on 24 September, 2017 in Prague, Czech Republic Julian Finney/Getty American sportspeople kneel in protest Laremy Tunsil #67, Maurice Smith #27 and Julius Thomas #89 kneel with Jarvis Landry #14 of the Miami Dolphins during the National Anthem prior to an NFL game against the New York Jets at MetLife Stadium on 24 September, 2017 in East Rutherford, New Jersey Steven Ryan/Getty American sportspeople kneel in protest Tampa Bay Buccaneers Wide Receivers Mike Evans #13, and DeSean Jackson #11, take a knee during the national anthem before the game against the Minnesota Vikings on 24 September, 2017 at U.S. Bank Stadium in Minneapolis, Minnesota Hannah Foslien/Getty American sportspeople kneel in protest Denver Broncos players kneel during the American National Anthem before an NFL game against the Buffalo Bills on 24 September, 2017 at New Era Field in Orchard Park, New York Brett Carlsen/Getty American sportspeople kneel in protest Buffalo Bills players kneel during the American National anthem before an NFL game against the Denver Broncos on 24 September, 2017 at New Era Field in Orchard Park, New York Brett Carlsen/Getty American sportspeople kneel in protest Colin Kaepernick #7 and Eric Reid #35 of the San Francisco 49ers kneel in protest during the national anthem prior to playing the Los Angeles Rams in their NFL game at Levi's Stadium on September 12, 2016 in Santa Clara, California. The kneel began as a protest against police killing people of colour Thearon W. Henderson/Getty American sportspeople kneel in protest Megan Rapinoe #15 kneels during the National Anthem prior to the match between the United States and the Netherlands at Georgia Dome on 18 September, 2016 in Atlanta, Georgia Kevin C. Cox/Getty

Mr Trump has frequently criticised kneeling during the anthem by framing Kaepernick's protest as disrespect for US troops without addressing the reasons for the protest in the first place.

In 2016, Kaepernick responded by saying: "He always says make America great again. Well, America has never been great for people of colour. That's something that needs to be addressed. Let's make America great for the first time."

Mr Trump became a catalyst for the protest in September when he said during a campaign rally in Alabama that he wished that NFL players would be fired for kneeling during the national anthem.

“Wouldn’t you love to see one of these NFL owners, when somebody disrespects our flag, to say, ‘Get that son of a b**** off the field right now, out, he’s fired. He’s fired,’” Mr Trump said.

“You know, some owner is going to do that. He’s going to say, ‘That guy that disrespects our flag, he’s fired.’ And that owner, they don’t know it [but] they’ll be the most popular person in this country.”

Nike made Kaepernick the face of a campaign ("Believe in something, even if it means sacrificing everything") in September 2018, prompting debate over his protests and boycotts against the company.

Who protested?

Some football teams chose not to come out onto the field at all after Mr Trump’s comments, while other teams allowed their players to protest at their own discretion.

In addition to most, if not all, of the NFL teams seeing some players protesting the weekend after Mr Trump’s remarks, baseball and basketball players also joined in.

Notably, New England Patriots quarterback Tom Brady called Mr Trump’s comments “divisive” and locked arms with his teammates during his game following the president’s remarks.

How did Black Lives Matter protests change the NFL?

Ahead of an uncertain season disrupted by the coronavirus pandemic and propelled by the urgency of nationwide protests, it's difficult to say.

In 2018, NFL owners unanimously approved a national anthem policy requiring players to stand if they are on the field during the performance of the song.

But its status is unclear, and it hasn't been enforced over the last two seasons.

Under the rule, players have the option to remain in the locker room during the anthem if they prefer. If a player or other employee of a team kneels or sits during the anthem, the teams themselves are fined. The teams then have the option to fine the individual players or personnel for the infraction.

The vote was unanimous, but the owner of the San Francisco 49ers — the team Kaepernick played for when he started the protest — abstained from the vote.

Kaepernick's kneeling protest also has a grim resonance following the death of Mr Floyd.

Widespread footage of now-former Minneapolis police officer Derek Chauvin shows him digging his knee into the back of Mr Floyd's neck for nearly nine minutes as he struggled to breathe.

At protests around the world, demonstrators have kneeled to echo Kaepernick's protest. Many police officers and elected officials have joined them.

Black NFL players issued a viral message following Mr Floyd's death to demand NFL officials condemn systemic racism, admit that the league has "wrongfully silenced" peaceful protests among players, and say that "Black Lives Matter."

"We will not be silenced," players say in the message. "We assert our right to peacefully protest."

New Orleans Saints quarterback Drew Brees, among several NFL players who disagreed with kneeling during the anthem as a sign of protest, faced a wave of backlash after he said he will "never agree with anybody disrespecting the flag of the United States of America or our country."

Several other Saints players posted messages urging Brees to reconsider his statements.

He later apologised to his teammates and fans, saying: "I realise this is not an issue about the American flag. It has never been. We can no longer use the flag to turn people away or distract them from the real issues that face our black communities."

His initial comments drew support from Mr Trump, who said Brees "should not have taken back his original stance on honouring our magnificent American Flag."

But in his statement aimed directly at the president, the quarterback said: "I completely missed the mark ... We must stop talking about the flag and shift our attention to the real issues of systemic racial injustice, economic oppression, police brutality, and judicial [and] prison reform. We are at a critical juncture in our nation's history! If not now, then when?

In a pair of tweets, the president said: "OLD GLORY is to be revered, cherished, and flown high ... We should be standing up straight and tall, ideally with a salute, or a hand on heart. There are other things you can protest, but not our Great American Flag - NO KNEELING!"

But is it abnormal for sports stars to make their political opinions known during events?

No. There is a rich history of American sports stars highlighting injustice in their country.

American Olympians John Carlos and Tommie Smith made headlines across the world in 1968, months after the murder of Martin Luther King Jr, when they raised their gloved fists as a black power salute on the podium after winning in that year's summer games. Their iconic protest brought them death threats, and they were expelled from the games. Smith later said: "If I win, I am American, not a black American. But if I did something bad, then they would say I am a Negro. We are black and we are proud of being black. Black America will understand what we did tonight."

Muhammad Ali is perhaps one of the best known American athletes to take a major political stand. The boxing champion refused to be drafted into the Vietnam War — a refusal which involved jailtime. He did so on the basis of his faith, he said, but did note the cruel irony of asking black men to fight in Vietnam for a country that has treated them as subhuman.

NBA stars have also worn their support for the Black Lives Matter movement.

What happened to Kaepernick?

Kaepernick is still not currently with any NFL team, though he held an open workout with the NFL in November 2019, stirring speculation that he was being groomed for a return to the league.

While he's been off the field, he has remained busy with charity work and as a prominent civil rights activist. In his NFL career, he made a $1 million pledge to charitable organisations.