It's a moment football fans relish: A running back breaks through the line and heads up the field for a big play. A defender streaks toward the ball carrier, intent on stopping his forward progress as quickly as possible. The spectacularly violent collision that follows brings the cheering crowd to its feet. That hit—and the dozens more like it in any given game—have helped make American football enormously popular worldwide.
Such electrifying plays have also placed the sport in a great quandary because the concussive forces at work, particularly when helmets collide, put players at risk for traumatic brain injury. Head injuries are nothing new to football—the sport has been grappling with its brutal nature since its inception nearly 150 years ago. Now research is connecting several high-profile former players' repeated gridiron head impacts with the emergence of chronic traumatic encephalopathy (CTE), a neurodegenerative disorder characterized by atrophied, abnormal brain tissue.