This is a crucial point. Much of the attention in the football world, in the past few years, has been on concussions—on diagnosing, managing, and preventing them—and on figuring out how many concussions a player can have before he should call it quits. But a football player’s real issue isn’t simply with repetitive concussive trauma. It is, as the concussion specialist Robert Cantu argues, with repetitive subconcussive trauma. It’s not just the handful of big hits that matter. It’s lots of little hits, too.