Is better detection the answer?
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 noted, the problem with sideline assessment tools in current use, such as the Standardized Assessment of Concussion (SAC) and the Sports Concussion Assessment Tool 2 (SCAT2), is that they test verbal memory, not the visual memory that he and the Purdue researchers found impaired in the functionally, but not clinically impaired, players who experienced at least short-term neurologic trauma from repeated, sub-concussive hits to the head.
All too often, even hits hard enough to cause an athlete to experience 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 simply missed by sideline personnel or because the athlete fails to report them (a 2010 study 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 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.
Are reducing full-contact limits or hit counts a solution?
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, some 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 that athletes sustain during contact and collision sports.
Limiting or eliminating contact practices in football, for example, would, according to a 2013 study 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] and ice hockey,  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. 
"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 (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" will remain "educated guesses, at best."
Broglio and his colleagues thus recommended that policymakers proceed with caution in imposing such limits.
Indeed, the 2014 University of Rochester study  suggests that, "rather than monitor total head hits, as has been suggested [by Sports Legacy Institute in its much ballyhooed 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 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."
Question of balance
Finding a way to reconcile two competing demands - minimizing contact practice in order to reduce the number of concussions sustained in practice and the number of sub-concussive impacts that emerging science suggests may have a deleterious cumulative effect 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 - will be difficult.
"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 the study, however, that 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. A March 2013 review of current risk-reduction strategies in the British Journal of Sports Medicine  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 challenge," said Bazarian, the author of the Rochester studies[8,23] "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."
The science has not yet come close to determining that number.
1. Talavage T, Nauman E, Breedlove E, et. al. Functionally-Detected Cognitive Impairment in High School Football Players Without Clinically-Diagnosed Concussion. J Neurotrauma. 2010; DOI: 10.1089/neu.2010.1512.
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 E, Breedlove EL, et al. Functionally-detected cognitive impairment in high school football players without clinically diagnosed concussion. J Neurotrauma. 2013;doi:10.1089/neu.2010.1512 (e-publ April 11, 2013)
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/).
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.
Originally posted April 6, 2012; most recently revised July 7, 2014