The article authored by Drs. Kevin M. Guskiewicz and Jason P. Mihalik, presents a very effective summary of current head injury research that will serve as an excellent resource for scientists interested in sport-related concussions (4). We agree with many of their statements, and wish to expand on three specific topics: the history of the Head Impact Telemetry (HIT) System, biomechanically based injury threshold, and accurate head impact exposure data. In 2003, the HIT System was an exciting new technology that we first implemented at Virginia Tech to collect head acceleration data for true exposure numbers for our players (2). Since then, the technology and methods have been well validated and published in leading peer-reviewed biomechanics journals (1,2,5). Many researchers began to see the value in the HIT System, and adopted similar data collection protocols. During the past 8 yr, more than 115,000 head impacts have been recorded at Virginia Tech, and more than 1,500,000 head impacts through the implementation of the HIT System at other universities and high schools. Collectively, across all institutions, these data represent the most comprehensive biomechanical data set characterizing head impact and concussion in humans (Figure).
As the authors have discussed, defining a biomechanical threshold for the onset of concussion from head acceleration data has proven challenging. Concussion prediction based upon linear acceleration magnitudes, alone, is likely not to be specific enough to be accepted. This is because concussion is the result of the combination of linear head acceleration, rotational head acceleration, impact duration, and impact location and direction. One study made progress by creating a weighted metric composed of several biomechanical parameters that had improved predictive capabilities when compared with single biomechanical parameters (3). Furthermore, adaptations of the HIT System technology have been used to build upon existing results to get an idea of the tissue level response of the brain caused by head impact, which may be more predictive than head kinematics (7,8). More work geared toward understanding these interactions is currently needed.
It is critical that we understand the importance of head impact exposure, in other words, capturing all player head impacts in practice and games. As Drs. Guskiewicz and Mihalik noted, this is a primary limitation of the National Football League studies that examined only a few severe impacts. It is effectively impossible to evaluate player injury risk from 31 reconstructions when a single player will see more than 1000 head impacts per season and likely more than 10,000 in their career. The first step is to quantify player exposure by measuring all head impacts in practice and games. Finally, underreporting of concussion among athletes has been reported to be as high as 53% of concussions (6). The issue of underreporting seems to be widespread and creates substantial problems for both the treatment and prevention of concussion. Recently, states have been adopting laws that restrict athletes who sustain concussions from returning to play until medical clearance is granted. Ironically, although these laws are well-intended, they may further escalate the issue of underreporting of concussion symptoms by athletes who are unwilling to participate. Underreporting of concussion is an issue that must be addressed for positive progress.
During the last 8 yr, data have been collected from many researchers using the HIT System, resulting in a valuable data set characterizing head impacts and concussions in humans. A biomechanical threshold may be possible, but more research is required to overcome the challenges associated with investigating concussions in football players. Such studies are encouraged and can have direct applications toward the treatment and prevention of concussions.
Stefan M. Duma
Wake Forest University Center for Injury Biomechanics
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2. Duma SM, Manoogian SJ, Bussone WR, et al
. Analysis of real-time head accelerations in collegiate football players. Clin. J. Sport Med.
3. Greenwald RM, Gwin JT, Chu JJ, Crisco JJ. Head impact severity measures for evaluating mild traumatic brain injury risk exposure. Neurosurgery
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5. Manoogian S, McNeely D, Duma S, Brolinson G, Greenwald R. Head acceleration is less than 10 percent of helmet acceleration in football impacts. Biomed. Sci. Instrum.
6. McCrea M, Hammeke T, Olsen G, Leo P, Guskiewicz K. Unreported concussion in high school football players: Implications for prevention. Clin. J. Sport Med.
7. Rowson S, Brolinson G, Goforth M, Dietter D, Duma S. Linear and angular head acceleration measurements in collegiate football. J. Biomech. Eng.
8. Takhounts EG, Ridella SA, Hasija V, et al
. Investigation of traumatic brain injuries using the next generation of simulated injury monitor (SIMon) finite element head model. Stapp. Car Crash J.