The sight of football players kneeling during the national anthem is the continuation of a tradition as old as the song itself.

That's according to University of Michigan musicology professor Mark Clague. He says "The Star-Spangled Banner" has been a channel for protest since at least the 1840s.

The lyrics have been recast repeatedly over many years to push for an end to slavery, racial equality, women's suffrage and labor rights.

Former San Francisco 49ers quarterback Colin Kaepernick began taking a knee during the anthem last year to make a statement about social inequality and police treatment of blacks.

After President Donald Trump suggested that the NFL should fire athletes who protest during the song, more than 200 players knelt or took other action during last weekend's games.

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