UPDATED
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 [1] 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 [2], were found to have measurable impairment of neurocognitive function (primarily visual working memory) on computerized neurocognitive tests [3], 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 [4] they sustain in a sports season, over the course of a year, and during a career.
Pop Warner responded by instituting rule changes [5] in 2012 designed to limit contact during practices.
In 2013, state high school athletic associations in Arizona [6], Washington State [7], Iowa, and Texas [8] moved to impose some limits on full-contact practices.
In June 2013, the Pac-12 announced [9] 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 [10] 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 [11] 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 [12], 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.
In advance of the 2014 season, the Wisconsin Interscholastic Athletic Association mandated new limits on the amount and duration of full-contact activities during team practices, prohibiting full contact during the first week of practice, limiting full contact to 75 minutes per week during week 2, and capping it at 60 minutes thereafter.
In November 2014, the National Federation of State High School Associations (NFHS) recommended [13] to its member associations that they adopt limits on full-contact practices in high school football. The recommendations, contained in a position paper [14] issued by the NFHS Concussion Summit Task Force in July 2014,[42] were approved by the NFHS Sports Medicine Advisory Committee and the NFHS Board of Directors, and discussed by the 51-member state associations at the NFHS Winter Meeting in early January 2015.
n the months leading up to the 2015 fall season, some state associations adopted the NFHS recommendations exactly, while others altered them to more closely fit the needs of their member schools:
The state associations in Iowa, Kansas, Georgia and Tennessee opted to limit full-contact practice to 90 minutes a week.
Other states, such as Ohio, chose to limit full-contact to 60 minutes a week instead.
The Kansas State High School Activities Association (KSHSAA) was one of many state associations to make pro-cactively make changes to its football rules that went beyond the recommendations of the NFHS Task Force. For example, players in Kansas are no longer be able to participate in "Live Action" the day after a game. And, effective with the 2016 season, they will not be allowed to participate in games on consecutive days, a change was made to address the issue of student-athletes playing a varsity game followed by a junior varsity game the next day.
Many states have also enacted rules changes establishing a progression up to full-contact in preseason practices, similar to the heat acclimatization schedules [15] integrated into preseason workouts in recent years. For example, the Illinois High School Association (IHSA) limits equipment to helmets only during the first two days of practice, helmets and shoulder pads the next three days, with full pads only being introduced on the sixth day of the acclimatization period. Similar progressions have also been adopted in Alabama, Minnesota and Kansas, among others.
Interestingly, a recent study by Purdue researchers provides support for such a progression. finding that cerebrovascular reactivity (CVR) - a measure of the ability of blood vessels in the brain to dilate to compensate for increased levels of carbon dioxide in the blood, such as occurring during exercise - was significantly reduced in almost all football athletes during the first six weeks of the contact season, findings which the researchers viewed as demonstrating that the onset of subconcussive blows had "at least a transient effect on the brain, but also suggest[ing] that the brain can adapt to [the contact] with an eventual return to baseline."
The researchers expressed concern that athletes may be at risk of incurring symptomatic injury during period their brains were trying to adapt to contact at the beginning of the season. Noting that in most states football teams typically switch from limited contact levels during the preseason to two practices a day, at least one of which includes contact, they expressed concern that, based on their findings, "the brain may not be able to adjust quickly to this change, leaving players at increased risk for injury" at the beginning of the football season. They thus suggested that it might be better for teams to increase the amount of contact more gradually to allow players' brains to adapt so as to reduce the risk of serious injury. This is what the new rules in Illinois, Alabama, Minnesota, and Kansas appear to address. Whether they go far enough is another question.
Prompted by the NFHS Task Force recommendations, member state associations have been reviewing there policies concerning offseason football. In states such as Ohio and Illinois, there were already rules in place to limit contact during the offseason, with teams prevented from participating in full gear or in full-contact practices. Other states that previously had no restrictions in place for offseason football have begun to adhere to the NFHS task force's guidelines as well.
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 [29] 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)."
An August 2015 editiorial in the British Journal of Sports Medicine, [41] said that autopsy studies - many conducted in Boston at the Center for the Study of Chronic Traumatic Encephalopathy - and a study reporting that retired NFL players who began playing football before age 12 demonstrated greater levels of cognitive impairment in their 40s-60s than those who started later, [40] "raises concern that an accumulation of undiagnosed subconcussive head trauma may lead to (or be a leading factor) for CTE."
In a 2012 study,[8] 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 [19] 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 [16] 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 [16].[17,18]
Another 2013 study by researchers at URMC and the Cleveland Clinic [9] 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 [26] 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).
There are also emerging data that football players are more frequently diagnosed with sport-related concussion on days with increased frequency and higher magnitude of head impact (greater than 100g linear acceleration).(43-45)More recently, a remarkable series of eight studies [17] [31-38] by Purdue scientists as part of an ongoing study of brain changes in high school football players, made a host of significant findings:
Perhaps most concerning, four of the Purdue studies found that damage to the brain from RHI persisted after the football season was over, as did a 2014 study by Bazarian and his URMC colleagues, [23] which found changes in brain white matter in a small group of college football players which persisted six months after the season was over. They found a strong correlation between the white matter changes and the number of head hits with a peak rotational acceleration exceeding 4500 rad/sec2 and the number of head hits with a peak rotational acceleration exceeding 6,000 rad/sec2, and an especially strong correlation where the number of the former exceeded 30-40 for the season, and the number of the latter exceeded 10-15 for the season. (For reference, a person nodding his head up and down as fast as possible produces a rotational acceleration of approximately 180 rads/sec2).
That six months off may not be long enough for the brains of football players to completely heal after a single season, putting them at even greater risk of head injury the next season, was concerning, said Bazarian.
"I don't want to be an alarmist, but this is something to be concerned about. At this point we don't know the implications, but there is a valid concern that six months of no-contact rest may not be enough for some players," he said. "And the reality of high school, college and professional athletics is that most players don't actually rest during the off-season. They continue to train and push themselves and prepare for the next season."
The findings of the first Purdue study alone were troubling, said Larry J. Leverenz, PhD, ATC, a Clinical Professor in the school's Department of Health and Kinesiology, shortly after the study was published, because it meant that players were:
Commenting at the time on the 2010 Purdue study for Sports Illustrated [20] [15], Randall Benson, a neurologist at Wayne State University in Detroit, speculated that the Purdue researchers may have taken what amounted to a "real-time snapshot" of the early stages of the corrosive creep that wears away at the frontal lobe, a part of the brain involved in navigating social situations. Too much erosion and victims reach a breaking point - like former Steelers offensive lineman Terry Long, who died in 2005 from drinking antifreeze. "It's an insidious progression," Benson said, "and it's not obvious when you talk to [players]."
Four years later, Benson's speculation was echoed in eerily similar comments by Bazarian and his colleagues in the 2014 URMC study: "[i]f RHIs are related to neurodegeneration many years later, a long clinically silent period between the onset of neuronal injury and overt symptoms of dementia would not be unexpected." During this clinically silent period, however, there may be indicators of dysfunction on a cellular level, such as the elevated levels of S100B antibody found in the cerebral spinal fluid in the football players in the study, even six months after the end of the season, which he said, could "potentially herald[ ] the early stages of [chronic traumatic encephalopathy] or CTE."
"Pending confirmation in a long term longitudinal study tracking athletes prospectively for years to decades looking for manifestations of early cognitive dysfunction and dementia," writes Bazarian, "we believe our results suggest that these persistent DTI changes are likely detrimental. If borne out in future research, the long-term persistence of these [white matter] changes would mean that athletes returning to play the following season would be at risk for expanded RHI-related WM changes, undetectable by conventional assessments. Could the lack of WM recovery we observed result in cumulative WM damage with subsequent football seasons of RHI exposures? If so, could this cumulative WM damage be related to the long-term development of CTE?"
Commenting on the recent series of Purdue studies for Purdue News,[30] Eric Nauman, a professor of mechanical engineering, basic medical sciences and biomedical engineering, and author or co-author of all 10 of the Purdue studies, said he was "particularly disturbed that when you get to the offseason, - we are looking somewhere between two and four months after the season has ended - the majority of players are still showing that they had not fully recovered."
One approach to the problem of sub-concussive blows that escape detection via conventional means is to find new enhanced detection methods: If functional impairment could be detected on the sports sideline, a player, like those exhibiting more obvious concussion signs or complaining of symptoms consistent with concussion, could be removed from play.
As Dr. Leverenz told MomsTEAM after publication of the first Purdue study, the limitation of screening tools currently being used to assess neurocognitive function on the sports sideline, such as the Standardized Assessment of Concussion (SAC) [21] and the Sports Concussion Assessment Tool 3 (SCAT3) [22], is that they test verbal memory, not the visual memory which he and the Purdue researchers found impaired in the functionally, but not clinically impaired, players who experienced at least short-term neurologic trauma from RHI.
All too often, even hits hard enough to cause an athlete to display signs of concussion that can be observed by sideline personnel, or which cause the athlete to experience symptoms of concussion, go undetected, either because the signs are too subtle to be seen or are simply missed by sideline personnel or because the athlete fails to report them (a 2010 study[7] of Canadian junior hockey players, for example, found that, for every concussion self-reported by the players or identified by the coaches or on-the-bench medical personnel, physician observers in the stands picked up seven) - a persistent problem that, given the "warrior" mentality and culture of contact and collision sports, is not going to go away any time soon, if ever.
One way to increase the chances of detection may be to equip players with impact sensors [23] to alert sideline personnel to head impact exposure, either from a single, forceful hit, or from less forceful but repetitive blows, that has the potential to result in brain injury, which could help medical staff identify athletes who should be removed for evaluation on the sports sideline and, if found to have a suspected brain injury, referred for further evaluation and banned from a return to play.[6]
But better detection does nothing to prevent such brain trauma, or at least reduce the risk, in the first place. No matter how good the technology, no matter how good we get at identifying suspected concussions, the essential problem remains: the hits themselves.
As a result, an increasing number of experts are urging that the focus be on reducing the risk of concussions and sub-concussive brain trauma by reducing exposure to concussive and sub-concussive hits [24] that athletes sustain during contact and collision sports.
Limiting contact practices in football to one session per week, or eliminating contact practices altogether, for example, would, according to a 2013 study[10] by researchers at the University of Michigan, result in an 18% to 40% reduction in head impacts respectively over the course of a high school football season.
The Michigan study pointed to recent research suggesting that the number of head impacts sustained may play a more important role in putting an athlete at risk of developing CTE than clinically evident concussions. Among them were the Purdue and Rochester studies of athletes in high school and college football [1,8,9,12,13, 31-38] and ice hockey, [8] which, as noted above, found subtle changes in cerebral function in the absence of concussion symptoms or clinically measurable cognitive impairment which researchers linked to the volume of head impacts, and a much publicized case-study autopsy of a collegiate football player, Owen Thomas, with no reported history of concussions, which revealed early signs of CTE. [14]
"If verified," lead author, Steven P. Broglio, PhD, ATC, of Michigan NeuroSport and Director of the NeuroSport Research Laboratory at the University of Michigan, writes, these reports "would support the use of head impact numbers to limit the head trauma volume experienced by an athlete each season."
Broglio recognized that "contact sport athletes appear to be at a greater risk for developing CTE," but was careful to note the absence of studies "indicating the relationship between head impacts, concussions, and other factors (eg. genetic profile) that may trigger the disease pathway."
He described the goal of reducing the overall number of head impacts that high school football players sustain in a season as "logical" and "appealing," but noted that, "until the risk factors for chronic traumatic encephalopathy [25] (CTE) are better defined by carefully designed and controlled research," and research determines "what the advisable limit to head impact exposure should be," employing contact limits or establishing "hit counts [4]" will remain "educated guesses, at best."
Indeed, the 2014 University of Rochester study [23] suggests that, "rather than monitor total head hits, as [was initially suggested [by Sports Legacy Institute in its much publicized Hit Count program], it may be more effective to monitor those hits that are most likely to produce [white matter] changes, which Bazarian and his colleagues found were when the number of helmet impacts resulting in a peak rotational acceleration of 4500 rads/sec2exceeded 30-40 for the season, and when the number of helmet impacts resulting in a peak rotational acceleration .6000 rads/sec2 exceeded 10-15 for the season.
Two of the Purdue studies [36,37] suggested that it might be possible to reduce risk of brain trauma by gradually increasing the amount of contact in the football pre-season to allow time for players' brains to adjust, and one, by finding that players who sustained more than 50 hits per game, were much more likely than those who sustained fewer hits to be "flagged" by ImPACT and/or fMRI results as having neurocognitive deficits or altered brain activity, suggested that players be limited to a certain number of plays per game (a hard rule to implement, given the prevalence of two-way players in the high school game).
A 2016 study by Broglio [46] found that a rule change limiting full-contact high school football practices appears to have been effective in reducing head-impact exposure for all players, with the largest reduction occurring among lineman. The study found that impacts were reduced from an average of 592 impacts per player per season before the rule change to an average of 345 impacts per player per season, or a 42% decline in impact exposure
Although Broglio and his colleagues viewed the results as "promising", they were careful to note that the restrictions on full-contact practices in football were being implemented despite a "lack of clarity surrounding the relationship between repeated head impacts in high school athletes and long-term neurocognitive dysfunction.
Finding a way to reconcile two competing demands - minimizing contact in practice in order to reduce the number of concussions sustained and the number of hits players sustain over the course of a week and a season that emerging science, now more than ever, suggests may have a deleterious cumulative effect [26] on a player's cognitive function over the long term, while at the same time maximizing the amount of time in practice learning how to tackle and block without head-to-head contact - time that is needed to maximize the protective effect of proper tackling on the number of head-to-head hits players sustain in game action, which can not only result in concussion, but catastrophic neck and spine injuries - is challenging, but clearly not impossible.
"As a scientist, I am not in a position to make policy," Broglio told MomsTEAM, but "we can't just reduce [the number of contact practices] without looking at the whole picture. We don't know if 18% means anything, or how much less [in terms of the number of impacts] is meaningful."
If he were making policy, however, Broglio would "lean more towards the cautious side" in limiting contact practices, which is not to say that he doesn't think "that a football program could be successful" with some limits on full-speed contact practices. Pointing to rugby, where players practice tackling without helmets without increased risk of head injury in games, he "didn't necessarily buy" the argument advanced by some experts that limiting contact practices would expose football players to increased injury risk in games.
As Broglio writes in his 2013 study, however, his view comes with a very important caveat: only if "extra emphasis on the appropriate tackling technique [is] put in place to ensure that the highest level of safety was maintained during games."
While the recent movement to limit full-contact practices is intended to make the game safer, some experts agree with Broglio that caution should be the byword [27]. A March 2013 review of current risk-reduction strategies in the British Journal of Sports Medicine [11] reminds state high school athletic associations and legislatures that, in enacting rules, such as limits on full-contact practices, they "need to carefully consider potential injury 'trade-offs' associated with the implementation of injury-prevention strategies, because every change may have certain advantages and disadvantages. That is, by reducing one risk or danger, additional risks may be created." In other words, as the Michigan study points out, limits on full-contact practices could create additional risk of injury to players because they haven't spent enough time learning to tackle properly.
The results of at least two recent studies, however, suggest that reductions in full-contact practices can be accompished safely without putting players at additional risk, while researchers continue looking for the head trauma "holy grail": a threshold - whether it is number of hits per week, over the course of the season, of a certain force, or to a certain part of the helmet (e.g. facemask, top of the head) above which players are at an unacceptably high risk of permanent brain injury.
A 2013 study [28] by researchers at Wake Forest Baptist Hospital and Virginia Tech [24] showed that reducing the number of head hits in practice did not, as some had predicted [29] [25] lead to higher force impacts during games.
And, more recently, a 2015 study [39] reported that comprehensive coach education in teaching "heads up" tackling and practice contact restrictions, such as implemented by Pop Warner, can be effective in reducing the rate of concussions in youth football.
The challenge is to determine whether a critical number of head hits exists above which this type of brain injury appears, and then to get players and coaches to agree to limit play when an athlete approached that number.
As the recent Purdue studies demonstrate, science is coming closer than ever before to determining that number,
2. Crisco JJ, Fiore R, Beckwith JG, et al. Frequency and location of head impact exposures in individual collegiate football players. J. Athl Train 2010;45:549-559.
3. Field M, Collins MW, Lovell MR, Maroon J. Does age play a role in recovery from sports related concussion? A comparison of high school and collegiate athletes. J Pediatr. 2003;414:546-553.
4. Pullela R, Raber J, Pfankuch T. et al. Traumatic injury to the immature brain results in progressive neuronal loss, hyperactivity and delayed cognitive impairments. Dev Neurosci 2006;28:396-409.
5. Stern R, Riley D, Daneshvar D, Nowinski C, Cantu R, McKee A. Long-term Consequences of Repetitive Brain Trauma: Chronic Traumatic Encephalopathy. Phys. Med. & Rehab. 2011;3;S460-S467. DOI:10.1016/j.pmrj.2011.08.008.
6. Greenwald R, Chu J, Beckwith J, Crisco J. A Proposed Method to Reduce Underreporting of Brain Injury in Sports. Clin J Sport Med 2012; 22(2):83-85.
7. Echlin P, Tator C, et al. A prospective study of physician-observed concussions during junior ice hockey: implications for incidence rates. Neurosurg Focus 2010;29(5):E4.
8. Bazarian JJ, Zhu T, Blyth B, Borrino A, Zhong J. Subject-specific changes in brain white matter in diffusion tensor imaging after sports-related concussion. Magnetic Resources Imaging. 2012; 30(2): 171-180.
9. March N, Bazarian JJ, Puvenna V, Janigro M, Ghosh C, et. al. Consequences of Repeated Blood-Brain Barrier Disruption in Football Players. PLoS ONE 2013;8(3): e56805. doi: 10.1371/journal.pone.0056805.
10. Broglio SP, Martini D, Kasper L, Eckner JT, Kutcher JS. Estimation of Head Impact Exposure in High School Football: Implications for Regulating Contact Practices. Am J Sports Med 2013;20(10). DOI:10.1177/036354651302458 (epub September 3, 2013).
11. Benson B, McIntosh A, Maddocks D, et. al. What are the most effective risk-reduction strategies in sport concussion? Br J Sports Med 2013;47:321-326.
12. Breedlove EL, Robinson M, Talavage TM, et al. Biomechanical correlates of symptomatic and asymptomatic neurophysiological impairment in high school football. J Biomech. 2012;45(7):1265-1272.
13. Talavage TM, Nauman EA, Breedlove EL, et al. Functionally-detected cognitive impairment in high school football players without clinically diagnosed concussion. J Neurotrauma. 2014;31(4):327-338. doi:10.1089/neu.2010.512.
14. McKee AC, Stein TD, Nowinski CJ, et al. The spectrum of disease in chronic traumatic encephalopathy. Brain. 2013;136(Pt 1):43-64.
15. David Epstein. The Damage Done: While Concussive Hits Dominate the Debate, A Groundbreaking New Study Suggests That Minor Blows - And There Can Be Hundreds Each Game - Are Just As Traumatic. Sports Illustrated. November 1, 2010 (accessed October 14, 2013 at http://sportsillustrated.cnn.com/vault/article/magazine/MAG1176377/2/ [30]).
16. McAllister TW, Ford JC, Flashman LA, et al. Effect of head impacts on diffusivity measures in a cohort of collegiate contact sport athletes. Neurology 2013; doi:10.1212/01.wnl.000438220.16190.
17. Savica R, Parisi JE, Wold LE, Josephs KA, Ahlskog JE. High School Football and Risk of Neurodegeneration: A Community-Based Study. Mayo Clin Proc 2012;87(14):335-340.
18. Jordan BD, et al. Head trauma and participation in contact sports as risk factors for Alzheimer's disease. Neurology 1990;40:347.
19. McAllister TW, Flashman LA, Maerlender A, Greenwald RM, Beckwith JG, et al. Cognitive effects of one season of head impacts in a cohort of collegiate contact sports athletes. Neurology 2012;78:1777-1784.
20. Bazarian JJ, Zhu T, Zhong J, Janigro D, Rozen E, Roberts A, Javien H, Merchant-Borna K, Abar B, Blackman EG. Persistent, Long-term Cerebral White Matter Changes after Sports-Related Repetitive Head Impacts. PLoS ONE. 2014;9(4): e94734 DOI: 10.1371/journal.pone.0094734
21. Koerte IK, Ertl-Wagner B, Reiser M, Zafonte R, Shenton ME. White matter integrity in the brains of professional soccer players, without a symptomatic concussion. JAMA. 2012;308:1859-1861.
22. Koerte IK, E Hartl, Bouix S, Pasternak O, Kubicki M, Rausher A, Li D, et al. A prospective study of physician-observed concussion during a varsity university hockey season: white matter integrity in ice hockey players. Part 3 of 4. Neurosurgical Focus. 2012;33:E3.
23. Bazarian JJ, Zhu T, Zhong J, et al. Persistent, Long-term Cerebral White Matter Changes after Sports-Related Repetitive Head Impacts. PLOS ONE. 2014;9(4):e94734.
24. Cobb BR, Urban JE, Davenport EM, Rowson S, Duma SM, Maldjian JA, Whitlow CT, Powers AK, Stizel JD. Head Impact Exposure in Youth Football: Elementary School Ages 9-12 Years and the Effect of Practice Structure. Ann Biomed Eng. ( 2013): DOI: 10.1007/s10439-013-0867-6 (online ahead of print)
25. Kontos P, Fazio V, Burkart S, Swindell H, Marron J, Collins M. Incidence of Sport-Related Concussion among Youth Football Players Aged 8-12 Years. J Pediatrics 2013. DOI 10.1016/j.jpeds.2013.04.011
26. Davenport EM, Whitlow CT, Urban JE, et. al. Abnormal White Matter Integrity Related To Head Impact Exposure in a Season of High School Varsity Football. J Neurotrauma 2014;31:1617-1624.
27. High School Football Players Show Brain Changes after One Season. Radiological Society of America. December 1, 2014. (accessed at http://www2.rsna.org/timssnet/media/pressreleases/14_pr_target.cfm?id=78... [31])
28. Bahrami N, Davenport E, Whitlow CT, et. al. Head Impacts and White Matter Changes in High School Football: A TBSS Analysis. (research paper). Radiological Society of America. December 1, 2014.
29. Reynolds BB, Patrie J, Henry EJ, Goodkin HP, Broshek DK, Wintermark M, Druzgal TJ. Practice type effects on head impact in collegiate football. J Neurosurg. Published online August 4, 2015; DOI:10.3171/2015.5.JNS15573.
30. 'Deviant brain metabolism' found in high school football players. Purdue University News. August 19, 2015. (accessed at http://www.purdue.edu/newsroom/releases/2015/Q3/deviant-brain-metabolism... [17])
31. Breedlove KM, Breedlove EL, Robinson M, Poole VN, King JR, Rosenberger P, Nauman EA. Detecting neurocognitive & neurophysiological changes as a result of subconcussive blows in high school football athletes. Ath. Tr. & Sports Health Care. 2014;6(3):119-127.
32.Poole VN, Breedlove EL, Shenk TE, Abbas K, Robinson ME, Leverenz LJ, Nauman EA, Dydek U, Talavage TM. Sub-concussive HIt Characteristics Predict Deviant Brain Metabolism in Football Athletes. Developmental Neuropsychology 2015;40(1):12-17, DOI:10.1080/87565641.2014.984810.
33. Nauman EA, Breedlove KM, Breedlove EL, Talavage TM, Robinson ME, Leverenz LJ. Post-Season Neurophysiological Deficits Assessed By ImPACT and fMRI in Athletes Competing in American Football. Developmental Neuropsychology. 2015;40(2):85-91. doi:10.1080/87565641.2015.1016161.
34. Robinson ME, Shenk TE, Breedlove EL, Leverenz LJ, Nauman EA, Talavage TM. The Role of Location of Subconcussive Head Impacts in fMRI Brain Activation Change. Developmental Neuropsychology 2015;40(2): 74-79. doi:10.1080/87565641.2015.101224.
35. Shenk TE, Robinson ME, Svaldi DO, Abbas K, Breedlove KM, Leverenz LJ, Nauman EA, Talavage TM. fMRI of Visual Working Memory in High School Football Players. Developmental Neuropsychology 2015;40(2):63-68. doi:10.1080/87565641.2015.1014088.
36. Svaldi DO, Joshi C, Robinson ME, Shenk TE, Abbas K, Nauman EA, Leverenz LJ, Talavage TM. Cerebrovascular Reactivity Alterations in Asymptomatic High School Football Players. Developmental Neuropsychology 2015;40(2):80-84. doi:10.1080/87565641.2014.973959.
37. Abbas K, Shenk TE, Poole VN, Robinson ME, Leverenz LJ, Nauman EA, Talavage TM. Effects of Repetitive Sub-Concussive Brain Injury on the Functional Connectivity of Default Mode Network in High School Football Athletes. Developmental Neuropsychology 2015;40(1):51-56. doi:10.1080/875665641.2014.990455.
38. Chun IY, Mao X, Breedlove EL, Leverenz LJ, Nauman EA, Talavage TM. Detection of Longitudinal WM Abnormalities Due to Accumulated Head Impacts. Developmental Neuropsychology 2015;40(2):92-97. doi: 10.1080/87566541/2015.1020945. .
39. Kerr ZY, Yeargin S, McLeod TC, Nittoli VC, Mensch J, Dodge T, Hayden R, Dompier TP. Comprehensive Coach Education and Practice Contact Restriction Guidelines Result in Lower Injury Rates in Youth American Football. Orthopaedic J Sports Med. 2015;3(7). doi:10.1177/2325967115594578.
40. Stamm JM, Bourlas AP, Baugh CM, et al. Age of first exposure to football and later-life cognitive impairment in former NFL players. Neurology 2015 Jan 28 [Epub ahead of print]
41. Asplund CA, Best TM. Brain damage in American Football Inevitable consequence or avoidable risk? Br J Sports Med 2015;49:1015-1016.
42. National Federation of State High School Associations. Recommendations and Guidelines for Minimizing Head Impact Exposure and Concussion Risk in Football. Report from the July 2014 NFHS Concussion Summit Task Force. (accessed at http://www.nfhs.org/articles/concussion-task-force-recommendations-to-be... [32])
43. Mihalik JP, Bell DR, Marshall SW, Guskiewicz KM. Measurement of head impacts in collegiate football players: an investigation of positional and event-type differences. Neurosurgery 2007; 61:1229-1235.
44. Trulock S, Oliaro S. Practice contact. Safety in College Football Summit. Presented January 22, 2014, Atlanta, GA.
45. Crison JJ et al. Frequency and location of head impact exposures in individual collegiate football players. J Athl Train 2010; 45:549-559
46. Broglio SP, Williams RM, O'Connor KL, Goldstick J. Football Players' Head Impact Exposure After Limiting of Full-Contact Practices. J Ath Tr. 2016;51(7):000 (e-published ahead of print July 2016).
Originally posted April 6, 2012; most recently July 30, 2016
Links:
[1] https://momsteam.com/health-safety/purdue-study-first-find-subtle-cognitive-deficits-in-high-school-football-players-from-repetitive-head-impacts
[2] https://momsteam.com/node/149
[3] https://momsteam.com/node/801
[4] https://momsteam.com/node/4406
[5] http://www.popwarner.com/About_Us/Pop_Warner_News/Rule_Changes_Regarding_Practice___Concussion_Prevention_s1_p3977.htm
[6] http://www.prweb.com/releases/2013/4/prweb10637189.htm
[7] http://www.bellevuereporter.com/sports/204181201.html
[8] http://www.dallasnews.com/sports/high-schools/headlines/20130421-uil-committee-recommendation-limits-in-season-full-contact-high-school-football-practice.ece
[9] http://www.usatoday.com/story/sports/ncaaf/2013/06/03/pac-12-limiting-contact-football/2384731/
[10] https://momsteam.com/health-safety/rules-to-reduce-concussions-subconcussive-hits-in-place-for-2011-ivy-league-football
[11] http://www.ncaa.org/health-and-safety/football-practice-guidelines
[12] http://leginfo.legislature.ca.gov/faces/billNavClient.xhtml?bill_id=201320140AB2127
[13] https://momsteam.com/health-safety/nfhs-approves-concussion-task-force-recommendations-discussion-with-state-associations
[14] http://www.nfhs.org/media/1014079/2014-nfhs-recommendations-and-guidelines-for-minimizing-head-impact-final-october-2014.pdf
[15] https://momsteam.com/health-safety/pre-season-heat-acclimatization-guidelines
[16] https://momsteam.com/node/6773
[17] http://www.purdue.edu/newsroom/releases/2015/Q3/deviant-brain-metabolism-found-in-high-school-football-players.html
[18] https://momsteam.com/node/4002
[19] https://momsteam.com/node/2839
[20] http://sportsillustrated.cnn.com/vault/article/magazine/MAG1176377/index.htm
[21] https://momsteam.com/node/215
[22] https://momsteam.com/node/1335
[23] https://momsteam.com/node/4120
[24] https://momsteam.com/node/4679
[25] https://momsteam.com/node/3289
[26] https://momsteam.com/node/5481
[27] https://momsteam.com/node/6041
[28] https://momsteam.com/health-safety/sports-concussion-safety/concussions-by-numbers/head-hits-can-be-reduced-in-youth-football-study-says
[29] https://momsteam.com/health-safety/youth-football-concussion-study-generates-controversy-over-suggestion-that-limiting-contact-practices-mistake
[30] http://sportsillustrated.cnn.com/vault/article/magazine/MAG1176377/2/
[31] http://www2.rsna.org/timssnet/media/pressreleases/14_pr_target.cfm?id=781
[32] http://www.nfhs.org/articles/concussion-task-force-recommendations-to-be-implemented-in-2015/
[33] https://momsteam.com/health-safety/pediatrics-group-refuses-to-endorse-outright-ban-on-tackling-in-high-school-youth-football
[34] https://momsteam.com/head-impact-exposure-in-youth-football-surprisingly-high-limits-in-contact-practices-urged
[35] https://momsteam.com/sports/study-finds-head-impacts-among-high-school-football-players-greater-than-collegiate-level
[36] https://momsteam.com/health-safety/seven-ways-to-reduce-risk-of-brain-trauma-in-contact-and-collision-sports
[37] https://momsteam.com/head-impact-exposure-in-youth-football-players-ages-7-8-years-effect-returning-players
[38] https://momsteam.com/health-safety/four-more-studies-find-causal-links-between-cte-contact-sports-suicide-scientifically-premature
[39] https://momsteam.com/alzheimers/cte-media-narrative-ahead-science-say-researchers
[40] https://momsteam.com/health-safety/cte-what-risk-athletes-who-stop-playing-football-after-high-school
[41] https://momsteam.com/purdue-studies-show-repetitive-head-impacts-in-high-school-football-damage-brain
[42] https://momsteam.com/repetitive-brain-trauma-and-chronic-traumatic-encephalopathy-CTE-cause-and-effect-relationship-scientifically-premature