Brain trauma among football players (and athletes in other sports such as soccer and ice hockey) may be less the result of violent collisions that cause concussions as the cumulative effect of repetitive head impacts (RHI). The discovery has lead to increased calls by experts to take steps at all levels of sports, from professional down to the youth level, to limit exposure to such repetitive trauma, while a shrinking minority (including the NCAA) continue to urge a more cautious approach until more is known.
Although scientists have long suspected that RHI caused brain damage, especially in boxers, a 2010 study of high school football players by researchers at Purdue University [1,13] was the first to identify a completely unexpected and previously unknown category of players who, though they displayed no clinically-observable signs of concussion, were found to have measurable impairment of neurocognitive function (primarily visual working memory) on computerized neurocognitive tests, as well as altered activation in neurophysiologic function on sophisticated brain imaging tests (fMRI).
Indeed, researchers found, the players with the most impaired visual memory skills were not those in who had been diagnosed with concussions but were in the group which, in the preceding week, had experienced a large number of RHI - around 150 hits - mostly in the 40 to 80 g range of linear acceleration.
Publication of the Purdue study sent shock-waves reverberating through the football world, with the findings cited by concussion experts calling on youth sports organizations to take more aggressive action to minimize exposure to RHI, including sub-concussive blows, by changing the way contact and collision sports are played and practiced, and reducing the amount of brain trauma a child incurs by limiting the number of hits they sustain in a sports season, over the course of a year, and during a career.
Pop Warner responded by instituting rule changes in 2012 designed to limit contact during practices.
In June 2013, the Pac-12 announced that it would adopt a policy limiting full-contact practices as well, although it did not state what those limits would be, only that they would be less than allowed by the NCAA. The next month, the conference announced that the limit would be two a week, the same as the Ivy League put in place before the 2011 season. No other major college football conference has followed suit, at least so far.
In July 2014, the NCAA issued new guidelines recommending that full-contact practices during the season be limited to two per week. The NCAA guidelines also recommend no more than four contact practices per week during the preseason and no more than eight of the 15 sessions during spring football.
That same month California governor Jerry Brown signed into law AB 2127, limiting middle and high school to two full-contact practices - each no more than 90 minutes long - per a week during the 30 day period before the regular season and during the regular season itself, and banning off-season contact practices completely.
Cause for concern
Since publication of the first Purdue study, similar findings about the effects of RHI, both in the short- and medium-term, have been reported by researchers [8,9,16,19,21,22,26,27,28, 29-38], with several of the studies finding changes which persisted for weeks and even months after a football season ended.
Summarizing the state of the research in 2015, a study by researchers at the University of Virginia found that, in the short term, RHI has been "linked to increased susceptibility to concussion, decreased cogntitive function, altered gray matter functional connectivity, and changes in white matter microstructure," and that, in the long term, "retired football players who have sustained high levels of subconcussive impact over their careers have been hypothesized to have an increased risk of developing neurodegenerative disorders, like amytrophic lateral sclerosis [e.g. Lou Gehrig's disease], Alzheimer's disease, Parkinson's disease, and chronic traumatic encephalopathy (CTE)."
In a 2012 study, researchers at the University of Rochester Medical Center (URMC) measured before-and-after data from the brains of a group of nine high school football and hockey players using an advanced form of imaging similar to an MRI called diffusion tensor imaging (DTI). They found subtle evidence of axonal injury at the cellular level in six athletes who had not been diagnosed with concussion but sustained RHI during the normal course of play. The abnormalities disclosed on post-season DTI scans among the players were closer to the scan of the one player with diagnosed concussion than to the normal brains in the control group. Axons, which are like cables woven throughout brain tissue, swell up when traumatic brain injury occurs.
The imaging changes also strongly correlated with the number of head hits (self-reported by the athletes in a diary), the symptoms experienced, and independent cognitive tests, said lead author Jeffrey Bazarian, M.D., M.P.H., associate professor of Emergency Medicine at URMC.
Another 2012 study  found that new learning on a sophisticated pencil-and-paper neurocognitive test declined over a single season of RHIs among college football and hockey players who did not experience concussions. The study found that the players had poorer post-season reaction time and scores on a test of visual attention and task switching, which deficits were associated with greater head impact exposures.
Using DTI imaging technique, researchers at Indiana University School of Medicine and the Geisel School of Medicine at Dartmouth College, found in a 2013 study  significant differences in brain white matter of varsity football and hockey players compared with a group of non-contact-sport athletes, with the number of times they were hit correlated with changes in the white matter. They also found that some of the athletes, none of whom suffered diagnosed concussions, didn't do as well as predicted on tests of learning and memory at the end of the season, although the study did not find "large-scale, systemic differences" in the brain scan measures, which the authors found "somewhat reassuring" and consistent with the fact that millions of athletes play contact sports for many years without developing progressive neurodegenerative disorders.[17,18]
Another 2013 study by researchers at URMC and the Cleveland Clinic  also found evidence of brain damage in college football players from RHI in the form of elevated levels of S100B, a protein in the blood usually present only in the brain. The presence of the S100B protein triggers the release by the body of antibodies which can then leak back into the brain through the damaged blood-brain barrier, where they are thought to attack brain tissue. The highest protein levels were found among players who sustained the most hits to the head during games and practices.
Using DTI, researchers at Wake Forest found in a 2014 study that a single season of high school football can produce changes in the white matter of the brain of the type previously associated with mTBI in the absence of a clinical diagnosis of concussion, and that these impact-related changes in the brain are strongly associated with a postseason change in the verbal memory composite score from baseline on the ImPACT neurocognitive test. "Taken together, these data add to the growing body of literature providing evidence that a season of play in a contact sport can show brain changes in the absence of concussion or clinical findings," they wrote.
In a research paper presented at the annual meeting of the Radiological Society of North America in December 2014 analyzing the same data,[27,28] the Wake Forest researchers found that players experiencing greater levels of RHI (heavy hitters) had more changes to specific areas of their brains compared to players with lower impact exposure (light hitters).