Biomechanics of Sport Concussion: Quest for the Elusive Injury Threshold

Kevin M. Guskiewicz; Jason P. Mihalik

Disclosures

Exerc Sport Sci Rev. 2011;39(1):4-11. 

In This Article

In-helmet Accelerometer Research

Real-time accelerometer data collection is a novel method available to researchers who are attempting to better understand the biomechanics of mTBI, but the earlier study designs were limited and unable to provide a realistic and meaningful interpretation of the data. For example, in a multisport study, Naunheim et al.[23] attempted to investigate the linear accelerations sustained by high school student-athletes - specifically, an ice hockey defenseman, football offensive lineman, football defensive lineman, and a soccer player. A triaxial accelerometer was inserted within a football and ice hockey helmet, and linear acceleration values were recorded during actual play. The data obtained from the soccer player allowed for limited interpretation firstly because there was no method of affixing the accelerometer to the player's head so the soccer player wore an instrumented football helmet. Secondly, game data were not captured; instead, the soccer player was asked to head 23 balls kicked to him or her at a standardized velocity. The mean linear acceleration measured in the football and ice hockey players was 29.2g and 35.0g, respectively.

Duma et al.[5] were the first to use acceleration-measuring technology in helmets for large numbers of athletes during normal practice and game situations. His group used the Head Impact Telemetry (HIT) System technology (Simbex, Lebanon, NH) incorporated within the Sideline Response System (Riddell Corp.; Elyria, OH). A major component of the HIT System is a unit composed of six spring-loaded single-axis accelerometers that are inserted into football helmets (Fig. 1). Duma et al. reported the magnitude of head impacts to be 32 ± 25g. This contrasts the range of 20g to 23g in a similar sample of Division I collegiate football players studied by our University of North Carolina research group using the HIT System technology.[19] The football-related data reported by Naunheim et al.[23] also are much higher than our observed football impact data, which averaged 22g of linear acceleration.[19] A statistically significant difference was then observed by comparing the linear acceleration of head impacts of football players across three different event types. Head impacts sustained in helmets-only and full-contact practices were significantly higher than those sustained in games or scrimmages. This finding was somewhat surprising, given that our earlier investigations found the incidence of concussion to be 6–8 times higher in games than practices.[10,13] Furthermore, our preliminary data in youth ice hockey players suggest that mean linear accelerations average about 19g.[22] Although the earlier study by Naunheim et al.[23] represented an important advance toward real-time data collection, her group was limited by a very small sample and did not transform the data to render it to be normally distributed; this tends to overestimate the actual linear acceleration values measured and leads us to believe that the actual values were probably closer to those captured in the more recent studies involving the HIT System.

Figure 1.

The HIT System technology with six spring-loaded single-axis accelerometers inserted into the football helmet, the Sideline Response System, and computer screenshot showing real-time impact in a football player.

A number of explanations exist to account for these differences. Linear acceleration is a highly skewed measure, with most of all head impacts yielding low linear acceleration outcomes. Duma et al.[5] calculated the mean and SD of their impacts without first controlling for the highly skewed distribution of their data. Furthermore, they alternated eight accelerometer units among their sample, selectively targeting players throughout the course of the season. The methodology used by our research team involved measuring all head impacts sustained by each study subject (~50 per season) in every practice and game throughout the season and alternated between players only to replace an individual who no longer remained in our study because of a season-ending injury. Using this methodology, we established that college football players experience approximately 950 subconcussive (without injury) head impacts in a given collegiate season, suggesting that the brain can withstand many impacts in a given season. This is in agreement with work by Schnebel et al.,[29] whereby 54,154 head impacts were sustained by 56 football players (40 college and 16 high school; mean of 967 per athlete), with only two reported concussions.

Others have joined in the effort of implementing the HIT System in the realm of high school and college football. It was not until recently, however, that more extensive study of youth ice hockey started. We have been studying head impact biomechanics in Bantam- and Midget-aged youth ice hockey players for two complete hockey seasons. Preliminary data in this sample suggest that 13- and 14-yr-old ice hockey players sustain head impacts nearing the magnitude of those sustained by college football players.[22] Although this technology is still considered novel, it is gaining popularity within the research community to better understand the nature of head impacts sustained by athletes. Future research in this regard should focus on helping players to better protect themselves and their opponents from sustaining high-magnitude impacts, which should result in lower incidences of mTBI in amateur, college, and professional sports.

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