Frequency and location of head impact exposures in individual collegiate football players

Abstract

Context: Measuring head impact exposure is a critical step toward understanding the mechanism and prevention of sport-related mild traumatic brain (concussion) injury, as well as the possible effects of repeated subconcussive impacts.

Objective: To quantify the frequency and location of head impacts that individual players received in 1 season among 3 collegiate teams, between practice and game sessions, and among player positions.

Design: Cohort study.

Setting: Collegiate football field.

Patients or other participants: One hundred eighty-eight players from 3 National Collegiate Athletic Association football teams.

Intervention(s): Participants wore football helmets instrumented with an accelerometer-based system during the 2007 fall season.

Main outcome measure(s): The number of head impacts greater than 10 g and location of the impacts on the player's helmet were recorded and analyzed for trends and interactions among teams (A, B, or C), session types, and player positions using Kaplan-Meier survival curves.

Results: The total number of impacts players received was nonnormally distributed and varied by team, session type, and player position. The maximum number of head impacts for a single player on each team was 1022 (team A), 1412 (team B), and 1444 (team C). The median number of head impacts on each team was 4.8 (team A), 7.5 (team B), and 6.6 (team C) impacts per practice and 12.1 (team A), 14.6 (team B), and 16.3 (team C) impacts per game. Linemen and linebackers had the largest number of impacts per practice and per game. Offensive linemen had a higher percentage of impacts to the front than to the back of the helmet, whereas quarterbacks had a higher percentage to the back than to the front of the helmet.

Conclusions: The frequency of head impacts and the location on the helmet where the impacts occur are functions of player position and session type. These data provide a basis for quantifying specific head impact exposure for studies related to understanding the biomechanics and clinical aspects of concussion injury, as well as the possible effects of repeated subconcussive impacts in football.

Figure 1

A, Football players wore instrumented helmets during practices and competitions to record the frequency, magnitude, and location of head impacts. These helmets were instrumented with 6 accelerometers (a), telemetry electronics (e), and a battery (b). B, The HIT System (developed by Simbex, Lebanon, NH, and marketed commercially as the Sideline Response System by Riddell Inc, Elyria, OH) comprises an instrumented helmet, a sideline receiver, and a laptop computer. C, The regions that defined the front, right, back, and top impact locations on the helmet and face mask are shown.

Figure 2

The total number of head impacts for individual players during the season differed with team. A, The complete distribution of the number of season head impacts is plotted as a cumulative histogram for all players of each team. The x-axis shows the number of season impacts, and the y-axis gives the number of players, as a percentage of the team, with the given number of season impacts or greater. B, An ordinary histogram of the same data.

Figure 3

The number of season head impacts increased with athlete-exposures. An athlete-exposure was defined as 1 player in 1 session in which he or she is exposed to the possibility of athletic injury. However, athlete-exposure was a poor predictor of the number of season head impacts for any given player.

Figure 4

The number of head impacts for individual players of each team during A and B, all practices, and C and D, all games. The data are plotted as a cumulative histogram with the number of A, practice and C, game impacts plotted on the x-axis and the percentage of players on each team with the number of impacts, or greater, plotted on the y-axis. B and D, Ordinary histograms of the same data.

Figure 5

The number of head impacts for individual players A and B, per practice, and C and D, per game for players on each team. The data are plotted as a cumulative histogram with the number of impacts A, per practice, and C, per game plotted on the x-axis and the percentage of players on each team with the number of impacts or greater plotted on the y-axis. B and D, Ordinary histograms of the same data.

Figure 6

The mean (±1 SD) numbers of impacts A, per practice, and B, per game across player positions did not differ with team and were grouped together. Impacts per game were typically 2.4 times greater than the impacts per practice across these various positions.

Figure 7

A–H. The mean (±1 SD) percentage of season head impacts at each helmet location (front, left, right, back, top). Most players had the highest percentage of impacts to the front of the helmet. Offensive linemen had the greatest percentage of impacts to the front of the helmet, whereas quarterbacks had the greatest percentage of impacts to the back of the helmet.