I have understood that in general any type of sudden acceleration and deacceleration is enough to cause potential damage. So it is just not the hard hits it is any situations which brain moves against the skull. Over long enough period these will add up and cause cumulative damage.
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replyFrom the article: The bulk of the head impacts that these former NFL players experienced didn’t occur at the NFL level. They occurred at the collegiate level ... and at the high school level and in many cases the youth level. And all of those cumulative hits to the head added up to result in an increased risk,” said Daneshvar.
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