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Five years ago this month, then-NFL quarterback Colin Kaepernick "took a knee" during the U.S. national anthem. His polarizing protest of police brutality against Black Americans would effectively end his professional football career. But it sparked a wave of athlete activism across the country. Author Dave Zirin's new book, The Kaepernick Effect, collects interviews with athletes from high schools up to pro leagues who followed the NFLer's lead by taking a knee – often in spite of great pressure not to. These protests, Zirin says, helped pave the way for the rise of the historic Black Lives Matter movement.