TAKING HIT AFTER HIT CAN EXACT A HEAVY TOLL well after a player has left the gridiron: in a 2009 study of retired NFL players, Boston University researchers found that multiple concussions often lead to chronic traumatic encephalopathy, which is marked by paranoia, depression and aggression. Quarterbacks are at particularly high risk of concussion, the brain forced against the skull at a force as great as 100 Gs (the equivalent of being slammed against the windshield of a car that’s traveling at 25 mph—twice).