NFL: The long-term cost

Former American football player and professional wrestler Chris Nowinski says head injuries can have fatal consequences for retired sportsmen later in life.

"Chronic traumatic encephalopathy [CTE] is a neuro-degenerative disease that we're finding in the brains of a lot of contact sport athletes," says Nowinski, who works with the nonprofit Sports Legacy institute at the University of Boston.

"It's become so prevalent in the brains of athletes that they are suffering from what we sometimes call early-onset dementia in their 30s and 40s - hearing voices, paranoia. A lot of them have already had memory problems and have stopped working."

Nowinski says CTE can leave former players suffering terrible depression. At its worst, they can become violent.

"If you are a high school football player, you will hit your head at least 1000 times this fall, especially if you're a lineman or a linebacker.

Much - in fact probably most - of that contact is happening in training. And a lot of the really major hits [in games] are from illegal practices that have not been flagged until recently."