During the 2007, 2008 and 2009 fall football seasons, a total of 314 players from three National Collegiate Athletic Association (NCAA) football programs (Brown University, Dartmouth College, and Virginia Tech) participated in this observational study after informed consent was obtained with institutional review board approval. Of these players, 146, 106 and 62 were monitored during one, two and three seasons, respectively. This participant turnover was expected, and due primarily to typical roster fluctuations on a collegiate team. Each player was assigned a unique identification number and categorized into one of eight position units defined by the team staff as the player’s primary position: defensive line (DL, n = 49), linebacker (LB, n = 47), defensive back (DB, n = 55), offensive line including tight ends (OL, n = 75), offensive running back (RB, n = 37), wide receiver (WR, n = 30), quarterback (QB, n = 14), and Special Teams (ST, n = 7), which were not included in this analysis because of the relatively low number of players.
All players wore Riddell VSR-4, Revolution, or Speed (Riddell, Chicago IL) football helmet models that were instrumented with the HIT System. The HIT System is an accelerometer-based device that computes linear and rotational acceleration of the center of gravity (CG) of the head, as well as impact location on the helmet (Beckwith, et al., 2007
; Crisco, et al., 2004
; Manoogian, et al., 2006
). The HIT System is specifically designed to measure head accelerations by elastically coupling the accelerometers to the head, isolating them from the helmet shell. Data were reduced in post-processing to exclude any acceleration event with peak resultant linear head acceleration less than 10g in order to eliminate head accelerations from non-impact events (e.g. running, jumping, etc.) (Ng, et al., 2006
). Data reduction methods are described in detail elsewhere (Brolinson, et al., 2006
; Duma, et al., 2005
; Funk, et al., 2007
; Manoogian, et al., 2006
), as was the accuracy of the HIT algorithm (Crisco, et al., 2004
). Laboratory impact tests of a Hybrid III dummy fitted with all helmets instrumented with the HIT System determined that the linear and rotational accelerations measured by the HIT System were within ± 4% of those measured concurrently by the internally instrumented Hybrid III headform (Duma, et al., 2005
Head impact exposure was defined for each individual player using measures of impact frequency, location and magnitude. A team session (session) was defined as either a formal team practice (players wore protective equipment with the potential of head contact) or a game (competitions and scrimmages). An individual player was defined to have participated in a session when at least one head impact was recorded for that given player within the specified time of the team session. Five measures of impact frequency were computed: practice impacts was the total number of head impacts for a player during all practices; game impacts was the total number of head impacts for a player during all games; impacts per season was the total number of head impacts for a player during all team sessions in a single season; impacts per practice was the average number of head impacts for a player during practices; and impacts per game was the average of the number of head impacts for a player during games.
Impact locations to the helmet and facemask were computed as azimuth and elevation angles in an anatomical coordinate system relative to the center of gravity of the head (Crisco, et al., 2004
) and then categorized as front, side (left and right), back, and top. Front, left, right and back impact locations were four equally spaced regions centered on the mid-sagittal plane All impacts above an elevation angle of 65° from a horizontal plane through the CG of the head were defined as impacts to the top of the helmet (Greenwald, et al., 2008
Impact magnitude was quantified by peak linear acceleration (g) and peak rotational acceleration (rad/s2
) (Crisco, et al., 2004
). Peak rotational acceleration was calculated as the vector product of peak linear acceleration and a point of rotation 10 cm inferior to the CG of the head. Laboratory testing has confirmed that this location is consistent with the impact response of the Hybrid III dummy (Duma, et al., 2005
). Additionally, a non-dimensional measure of head impact severity, HITsp (Greenwald, et al., 2008
) was computed. HITsp transforms the computed head impact measures of peak linear and peak angular acceleration into a single latent variable using Principal Component Analysis, and applies a weighting factor based on impact location (Greenwald, et al., 2008
). It thus serves as a measure of impact severity, with weight given to factors shown in previous head injury research (linear and rotational acceleration, impact duration and location (Gadd, 1966
; Gennarelli, et al., 1972
; Hodgson, 1970
; Pellman, et al., 2003a
)) to predict increased likelihood of clinical or structural injury. Impacts were further reduced for analysis by computing the 95th
percentile value of all seasonal impacts for each individual player.
Results were expressed as median values and [25–75% interquartile range], because each study variable was not normally distributed (Shapiro-Wilk test; P < 0.001). The significance of the differences among player positions in impact frequency (impacts per season) and in severity measures (95th percentile peak linear acceleration, 95th percentile rotational acceleration, and 95th HITsp) were examined separately using a Kruskal-Wallis one-way ANOVA on ranks with a Dunn’s post-hoc test for all pairwise comparisons. Statistical significance was set at α = 0.05 and the reported P values are those for the post hoc test. An identical approach was used to examine the significance of the differences among player positions in frequency and the 95th percentile peak linear and rotational acceleration at each location. Statistical comparison among impact location were performed with a Friedman repeated measures ANOVA on ranks. All statistical analyses were performed using SigmaPlot (Systat Software, Chicago, IL).