Read the (lengthy)
New Yorker piece on the subject. It's not just concussions, it's normal play with no noticeable injuries that may be leading players to dementia in old age. Some samples:
The other major researcher looking at athletes and C.T.E. (chronic traumatic encephalopathy) is the neuropathologist Bennet Omalu. He diagnosed the first known case of C.T.E. in an ex-N.F.L. player back in September of 2002, when he autopsied the former Pittsburgh Steelers center Mike Webster. He also found C.T.E. in the former Philadelphia Eagles defensive back Andre Waters, and in the former Steelers linemen Terry Long and Justin Strzelczyk, the latter of whom was killed when he drove the wrong way down a freeway and crashed his car, at ninety miles per hour, into a tank truck. Omalu has only once failed to find C.T.E. in a professional football player, and that was a twenty-four-year-old running back who had played in the N.F.L. for only two years.
“There is something wrong with this group as a cohort,” Omalu says. “They forget things. They have slurred speech. I have had an N.F.L. player come up to me at a funeral and tell me he can’t find his way home. I have wives who call me and say, ‘My husband was a very good man. Now he drinks all the time. I don’t know why his behavior changed.’ I have wives call me and say, ‘My husband was a nice guy. Now he’s getting abusive.’ I had someone call me and say, ‘My husband went back to law school after football and became a lawyer. Now he can’t do his job. People are suing him.’ ”
McKee got up and walked across the corridor, back to her office. “There’s one last thing,” she said. She pulled out a large photographic blowup of a brain-tissue sample. “This is a kid. I’m not allowed to talk about how he died. He was a good student. This is his brain. He’s eighteen years old. He played football. He’d been playing football for a couple of years.” She pointed to a series of dark spots on the image, where the stain had marked the presence of something abnormal. “He’s got all this tau. This is frontal and this is insular. Very close to insular. Those same vulnerable regions.” This was a teen-ager, and already his brain showed the kind of decay that is usually associated with old age. “This is completely inappropriate,” she said. “You don’t see tau like this in an eighteen-year-old. You don’t see tau like this in a fifty-year-old.”
McKee is a longtime football fan. She is from Wisconsin. She had two statuettes of Brett Favre, the former Green Bay Packers quarterback, on her bookshelf. On the wall was a picture of a robust young man. It was McKee’s son—nineteen years old, six feet three. If he had a chance to join the N.F.L., I asked her, what would she advise him? “I’d say, ‘Don’t. Not if you want to have a life after football.’ ”
There's also information about a study using helmet-mounted accelerometer in practices. Remarkably, many of the concussions occur on the less violent hits (as measured by accelerometers). And the sheer number of high readings during normal play is startling.
Very scary stuff.