Past, present, and future of head injury research.

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