Denver: Broncos linebacker Brandon Marshall took to a knee during the playing of the US national anthem before Denver’s NFL season opener against Carolina.

Marshall is a college teammate of Colin Kaepernick, the San Francisco 49ers’ quarterback, who refused to stand for the anthem during his team’s preseason games, saying it was to protest racial oppression and police brutality in the United States.

It opened a wide debate and put the microscope on a pregame tradition in the US that has, for decades, been routine.

Marshall made no mention of his plans before Thursday’s game, but shortly after the large American flag was unfurled, he knelt down on one knee.

Marshall is not been the only American athlete to kneel during the national anthem in solidarity with Kaepernick. Megan Rapinoe, who plays for the Seattle Reign in the National Women’s Soccer League, also took a knee during a game last weekend.