I'm sure you could find that if you found the study rather than an article about it (though I'm not sure if there is a scientific journal paper attached to this

I'm sure you could find that if you found the study rather than an article about it (though I'm not sure if there is a scientific journal paper attached to this or where or how it will be published). The article says they compared to "healthy control subjects of similar ages". Presumably, what they are controlling the for is the occupation of being a football player, since that is the entire point of the study. Also this part "Noseworthy was asked if there’s any chance the players’ results aren’t connected to football. “No, this is football,” he said. “That’s the common denominator.”"