Transcript for The legacy of political protest in sports
Reporter: For Colin Kaepernick, it began a year ago, hoping to send a message. There's a lot of things that need to change. One specifically and police brutality. This conversation started around one quarterback in San Francisco who decided to use his platform to try to draw attention to criminal justice reform, police brutality, and innocent people dying at the hands of police officers. Reporter: This weekend, he saw his message and his means of conveying it, command the attention of the nation. Even Stevie wonder, life-long entertainer and activist, during a concert in New York's central park. Tonight I'm taking a knee for America. But not just one knee. I'm taking both knees. Both knees in prayer for our planet, our future, our leaders of the world, and our globe. Reporter: Unsigned by any NFL team for the 2017 season, Kaepernick could not be found on the field where he said he wants to be, where NFL analysts and players have said he deserves to be. Do I think Kaepernick is better than some of these starting quarterbacks in this league? Absolutely. Should he be on a roster, in my opinion? Absolutely. Reporter: Instead, he could be found here, in cyberspace, retweeting images of players and others who support him, including military widows, as if in response to reaction he told the undefeated that he anticipated. You're going to have that backlash for trying to fight for people. And that was something I was fully prepared for. Without question, he's not in the league because of the protest, because of the knee. The fact is, Colin Kaepernick is the face of this protest in the NFL. Reporter: 70 years ago, this was the face of change and of social activism in sports, when Jackie Robinson broke baseball's color line in 1947, with the Brooklyn dodgers. As he told dick Cabot on ABC in 1972. The whole sitpation in breaking the barrier was done simply because we had a purpose in mind to go out and win. Then you move into a town like Brooklyn and it was just fantastic the way the fans responded and reacted. A great bunch of people. Reporter: His heroism made him a revered American. A true lead in the area of race. At the 1968 summer olympics, this would be one of the golfinizing images of its time. Tommy Smith and John Carlos raising gloved fists. Kareem abdul-jabbar, a presidential medal of freedom recipient in 2016, would boycott those games in 1968. As he told NBC sports -- The voting rights act and the civil rights act had really not taken hold. And( life for black Americans was still touch and go. This is a tradition across the country. We've always had athletes to use their platforms to speak out around issues of social justice. Reporter: Also in the '60s, Muhammad Ali, heavyweight champion of the world, a voice for black empowerment and dissent as on WABC's "Like it is is." You won't even stand up for me in America for my religious beliefs. Reporter: Ali life biographer says activism is Ali's greatest legacy. Ali had the power to speak out because he was a superstar. Now we see athletes taking more responsibility, in part because they have more control over their own careers. Reporter: Ali declared himself a conscientious objector in 1966. Here today, his rejected induction into the military service. What happens now to his title? Reporter: Ali would be stripped of his title and prosecuted. In 1971, the supreme court would rule unanimously in Ali's favor. He said he would give up his life. He would sooner die before he was compromise his convictions and I think that's why he matters today. Reporter: Billie Jean king would advocate for equal rights and opportunity at the Miami open in 2016. Everyone thinks women should be thrilled when we get crumbs. Okay? I want women to have the cake, the icing and the cherry on top. Reporter: In summer 2016, the wnba's Minnesota lynx would bear black lives matter shirts. This is a human issue, and we need to speak out for change together. Reporter: Four off-duty police officers providing security at the game in the twin cities would leave their post in protest. Later that year, Megan rapinoe of the U.S. Women's national team would also take a knee. And an unprecedented role for the best player in the N, Lebron James speaking out on such shows as the espys. It's time to look in the mirror and ask ourselves, what are we doing to create change? He has the right to do so. I think the more voices that are heard, the better off we'll be as a country. Because we need to learn how to respect each other's voices and not retort that statement with anger or divisiveness. The question is, whether or not the NFL community has been changed by this. And that is one only the players themselves can answer. What exactly were they protesting on Sunday? Were they protesting trump's criticism of them protesting? Or were they protesting the things that Colin Kaepernick was protesting, racial inequality and social justice? Reporter: A mid the tumult, a E aclu tweeting page from his 1972 memoir in this American pioneer looks back on his life. As he says, a black man in a white world, and declares, I cannot stand and sing the anthem. For "Nightline," I'm Chris Connelly in Los Angeles.
This transcript has been automatically generated and may not be 100% accurate.