Richard M. Greenwald

Frequency and Location of Head Impact Exposures in Individual Collegiate Football Players

CONTEXTnMeasuring 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.nnnOBJECTIVEnTo 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.nnnDESIGNnCohort study.nnnSETTINGnCollegiate football field.nnnPATIENTS OR OTHER PARTICIPANTSnOne hundred eighty-eight players from 3 National Collegiate Athletic Association football teams.nnnINTERVENTION(S)nParticipants wore football helmets instrumented with an accelerometer-based system during the 2007 fall season.nnnMAIN OUTCOME MEASURE(S)nThe number of head impacts greater than 10 g and location of the impacts on the players 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.nnnRESULTSnThe 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.nnnCONCLUSIONSnThe 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.

Rotational Head Kinematics in Football Impacts: An Injury Risk Function for Concussion

Recent research has suggested a possible link between sports-related concussions and neurodegenerative processes, highlighting the importance of developing methods to accurately quantify head impact tolerance. The use of kinematic parameters of the head to predict brain injury has been suggested because they are indicative of the inertial response of the brain. The objective of this study is to characterize the rotational kinematics of the head associated with concussive impacts using a large head acceleration dataset collected from human subjects. The helmets of 335 football players were instrumented with accelerometer arrays that measured head acceleration following head impacts sustained during play, resulting in data for 300,977 sub-concussive and 57 concussive head impacts. The average sub-concussive impact had a rotational acceleration of 1230xa0rad/s2 and a rotational velocity of 5.5xa0rad/s, while the average concussive impact had a rotational acceleration of 5022xa0rad/s2 and a rotational velocity of 22.3xa0rad/s. An injury risk curve was developed and a nominal injury value of 6383xa0rad/s2 associated with 28.3xa0rad/s represents 50% risk of concussion. These data provide an increased understanding of the biomechanics associated with concussion and they provide critical insight into injury mechanisms, human tolerance to mechanical stimuli, and injury prevention techniques.

Head impact exposure in collegiate football players

In American football, impacts to the helmet and the resulting head accelerations are the primary cause of concussion injury and potentially chronic brain injury. The purpose of this study was to quantify exposures to impacts to the head (frequency, location and magnitude) for individual collegiate football players and to investigate differences in head impact exposure by player position. A total of 314 players were enrolled at three institutions and 286,636 head impacts were recorded over three seasons. The 95th percentile peak linear and rotational acceleration and HITsp (a composite severity measure) were 62.7g, 4378rad/s(2) and 32.6, respectively. These exposure measures as well as the frequency of impacts varied significantly by player position and by helmet impact location. Running backs (RB) and quarter backs (QB) received the greatest magnitude head impacts, while defensive line (DL), offensive line (OL) and line backers (LB) received the most frequent head impacts (more than twice as many than any other position). Impacts to the top of the helmet had the lowest peak rotational acceleration (2387rad/s(2)), but the greatest peak linear acceleration (72.4g), and were the least frequent of all locations (13.7%) among all positions. OL and QB had the highest (49.2%) and the lowest (23.7%) frequency, respectively, of front impacts. QB received the greatest magnitude (70.8g and 5428rad/s(2)) and the most frequent (44% and 38.9%) impacts to the back of the helmet. This study quantified head impact exposure in collegiate football, providing data that is critical to advancing the understanding of the biomechanics of concussive injuries and sub-concussive head impacts.

Collision Type and Player Anticipation Affect Head Impact Severity Among Youth Ice Hockey Players

OBJECTIVE: The objective was to determine how body collision type and player anticipation affected the severity of head impacts sustained by young athletes. For anticipated collisions, we sought to evaluate different body position descriptors during delivery and receipt of body collisions and their effects on head impact severity. We hypothesized that head impact biomechanical features would be more severe in unanticipated collisions and open-ice collisions, compared with anticipated collisions and collisions along the playing boards, respectively. METHODS: Sixteen ice hockey players (age: 14.0 ± 0.5 years) wore instrumented helmets from which biomechanical measures (ie, linear acceleration, rotational acceleration, and severity profile) associated with head impacts were computed. Body collisions observed in video footage captured over a 54-game season were evaluated for collision type (open ice versus along the playing boards), level of anticipation (anticipated versus unanticipated), and relative body positioning by using a new tool developed for this purpose. RESULTS: Open-ice collisions resulted in greater head linear (P = .036) and rotational (P = .003) accelerations, compared with collisions along the playing boards. Anticipated collisions tended to result in less-severe head impacts than unanticipated collisions, especially for medium-intensity impacts (50th to 75th percentiles of severity scores). CONCLUSION: Our data underscore the need to provide players with the necessary technical skills to heighten their awareness of imminent collisions and to mitigate the severity of head impacts in this sport.

Maximum Principal Strain and Strain Rate Associated with Concussion Diagnosis Correlates with Changes in Corpus Callosum White Matter Indices

On-field monitoring of head impacts, combined with finite element (FE) biomechanical simulation, allow for predictions of regional strain associated with a diagnosed concussion. However, attempts to correlate these predictions with in vivo measures of brain injury have not been published. This article reports an approach to and preliminary results from the correlation of subject-specific FE model-predicted regions of high strain associated with diagnosed concussion and diffusion tensor imaging to assess changes in white matter integrity in the corpus callosum (CC). Ten football and ice hockey players who wore instrumented helmets to record head impacts sustained during play completed high field magnetic resonance imaging preseason and within 10xa0days of a diagnosed concussion. The Dartmouth Subject-Specific FE Head model was used to generate regional predictions of strain and strain rate following each impact associated with concussion. Maps of change in fractional anisotropy (FA) and median diffusivity (MD) were generated for the CC of each athlete to correlate strain with change in FA and MD. Mean and maximum strain rate correlated with change in FA (Spearman ρxa0=xa00.77, pxa0=xa00.01; 0.70, pxa0=xa00.031), and there was a similar trend for mean and maximum strain (0.56, pxa0=xa00.10; 0.6, pxa0=xa00.07), as well as for maximum strain with change in MD (−0.63, pxa0=xa00.07). Change in MD correlated with injury-to-imaging interval (ρxa0=xa0−0.80, pxa0=xa00.006) but change in FA did not (ρxa0=xa00.18, pxa0=xa00.62). These results provide preliminary confirmation that model-predicted strain and strain rate in the CC correlate with changes in indices of white matter integrity.

Spectrum of acute clinical characteristics of diagnosed concussions in college athletes wearing instrumented helmets

OBJECTnConcussive head injuries have received much attention in the medical and public arenas, as concerns have been raised about the potential short- and long-term consequences of injuries sustained in sports and other activities. While many student athletes have required evaluation after concussion, the exact definition of concussion has varied among disciplines and over time. The authors used data gathered as part of a multiinstitutional longitudinal study of the biomechanics of head impacts in helmeted collegiate athletes to characterize what signs, symptoms, and clinical histories were used to designate players as having sustained concussions.nnnMETHODSnPlayers on 3 college football teams and 4 ice hockey teams (male and female) wore helmets instrumented with Head Impact Telemetry (HIT) technology during practices and games over 2-4 seasons of play. Preseason clinical screening batteries assessed baseline cognition and reported symptoms. If a concussion was diagnosed by the team medical staff, basic descriptive information was collected at presentation, and concussed players were reevaluated serially. The specific symptoms or findings associated with the diagnosis of acute concussion, relation to specific impact events, timing of symptom onset and diagnosis, and recorded biomechanical parameters were analyzed.nnnRESULTSnData were collected from 450 athletes with 486,594 recorded head impacts. Forty-eight separate concussions were diagnosed in 44 individual players. Mental clouding, headache, and dizziness were the most common presenting symptoms. Thirty-one diagnosed cases were associated with an identified impact event; in 17 cases no specific impact event was identified. Onset of symptoms was immediate in 24 players, delayed in 11, and unspecified in 13. In 8 cases the diagnosis was made immediately after a head impact, but in most cases the diagnosis was delayed (median 17 hours). One diagnosed concussion involved a 30-second loss of consciousness; all other players retained alertness. Most diagnoses were based on self-reported symptoms. The mean peak angular and rotational acceleration values for those cases associated with a specific identified impact were 86.1 ± 42.6g (range 16.5-177.9 g) and 3620 ± 2166 rad/sec( 2 ) (range 183-7589 rad/sec( 2 )), respectively.nnnCONCLUSIONSnApproximately two-thirds of diagnosed concussions were associated with a specific contact event. Half of all players diagnosed with concussions had delayed or unclear timing of onset of symptoms. Most had no externally observed findings. Diagnosis was usually based on a range of self-reported symptoms after a variable delay. Accelerations clustered in the higher percentiles for all impact events, but encompassed a wide range. These data highlight the heterogeneity of criteria for concussion diagnosis, and in this sports context, its heavy reliance on self-reported symptoms. More specific and standardized definitions of clinical and objective correlates of a concussion spectrum may be needed in future research efforts, as well as in the clinical diagnostic arena.

Gender Differences in Head Impacts Sustained by Collegiate Ice Hockey Players

PURPOSEnThis study aimed to quantify the frequency, magnitude, and location of head impacts sustained by male and female collegiate ice hockey players during two seasons of play.nnnMETHODSnDuring two seasons, 88 collegiate athletes (51 females, 37 males) on two female and male National Collegiate Athletic Association varsity ice hockey teams wore instrumented helmets. Each helmet was equipped with six single-axis accelerometers and a miniature data acquisition system to capture and record head impacts sustained during play. Data collected from the helmets were postprocessed to compute linear and rotational accelerations of the head as well as impact location. The head impact exposure data (frequency, location, and magnitude) were then compared between genders.nnnRESULTSnFemale hockey players experienced a significantly lower (P < 0.001) number of impacts per athlete exposure than males (females = 1.7 ± 0.7, males = 2.9 ± 1.2). The frequency of impacts by location was the same between genders (P > 0.278) for all locations except the right side of the head, where males received fewer impacts than females (P = 0.031). Female hockey players were 1.1 times more likely than males to sustain an impact less than 50 g, whereas males were 1.3 times more likely to sustain an impact greater than 100 g. Similarly, males were 1.9 times more likely to sustain an impact with peak rotational acceleration greater than 5000 rad·s(-2) and 3.5 times more likely to sustain an impact greater than 10,000 rad·s(-2).nnnCONCLUSIONSnAlthough the incidence of concussion has typically been higher for female hockey players than male hockey players, female players sustain fewer impacts and impacts resulting in lower head acceleration than males. Further study is required to better understand the intrinsic and extrinsic risk factors that lead to higher rates of concussion for females that have been previously reported.

Head impact exposure in male and female collegiate ice hockey players

The purpose of this study was to quantify head impact exposure (frequency, location and magnitude of head impacts) for individual male and female collegiate ice hockey players and to investigate differences in exposure by sex, player position, session type, and team. Ninety-nine (41 male, 58 female) players were enrolled and 37,411 impacts were recorded over three seasons. Frequency of impacts varied significantly by sex (males: 287 per season, females: 170, p<0.001) and helmet impact location (p<0.001), but not by player position (p=0.088). Head impact frequency also varied by session type; both male and female players sustained more impacts in games than in practices (p<0.001), however the magnitude of impacts did not differ between session types. There was no difference in 95th percentile peak linear acceleration between sexes (males: 41.6 g, females: 40.8 g), but 95th percentile peak rotational acceleration and HITsp (a composite severity measure) were greater for males than females (4424, 3409 rad/s(2), and 25.6, 22.3, respectively). Impacts to the back of the helmet resulted in the greatest 95th percentile peak linear accelerations for males (45.2 g) and females (50.4 g), while impacts to the side and back of the head were associated with the greatest 95th percentile peak rotational accelerations (males: 4719, 4256 rad/sec(2), females: 3567, 3784 rad/sec(2) respectively). It has been proposed that reducing an individuals head impact exposure is a practical approach for reducing the risk of brain injuries. Strategies to decrease an individual athletes exposure need to be sport and gender specific, with considerations for team and session type.

Effect of Infraction Type on Head Impact Severity in Youth Ice Hockey

PURPOSEnTo identify the effects of infractions sustained during participation in youth ice hockey on biomechanical measures of head impact severity.nnnMETHODSnSixteen adolescent Bantam-aged male ice hockey players (age = 14.0 +/- 0.5 yr, height = 171.3 +/- 4.5 cm, mass = 63.7 +/- 6.6 kg) were equipped with accelerometer-instrumented helmets to collect biomechanical measures relating to head impacts (linear acceleration, rotational acceleration, and Head Impact Technology severity profile (HITsp)) sustained while participating in ice hockey. Single-camera video footage from 54 games was synchronized with the head impact data, and all viewable collisions (n = 665) were evaluated as resulting from a legal collision or an infraction. Infractions were further categorized into boarding or charging, checking from behind, and elbowing or intentional head contact. Statistical analyses included random-intercepts general linear mixed models.nnnRESULTSnInfractions were observed in 17.3% (115/665) of all body collisions. Overall, collisions involving infractions had higher linear accelerations (P = 0.012) and HITsp (P = 0.021) than collisions with no infraction. Specifically, elbowing, head contact, and high sticking infractions resulted in greater linear acceleration (P = 0.005) and HITsp (P = 0.010) than collisions with no infraction. A strong trend for higher rotational accelerations in this infraction type compared with legal collisions was also present (P = 0.059).nnnCONCLUSIONSnInfractions result in higher measures of head impact severity than noninfraction collisions. Athletes and coaches should conform to playing rules, and officials should enforce more stringently existing rules and assess more severe penalties to participants who purposefully attempt to foul an opponent at the youth ice hockey level.

Does Cervical Muscle Strength in Youth Ice Hockey Players Affect Head Impact Biomechanics

Objective:To evaluate the effect of cervical muscle strength on head impact biomechanics. Design:Prospective cohort. Setting:Field setting. Participants:Thirty-seven volunteer ice hockey players (age = 15.0 ± 1.0 years, height = 173.5 ± 6.2 cm, mass = 66.6 ± 9.0 kg, playing experience = 2.9 ± 3.7 years). Interventions:Participants were equipped with accelerometer-instrumented helmets to collect head impact biomechanics (linear and rotational acceleration) throughout an entire playing season. Before the season, isometric cervical muscle strength was measured for the anterior neck flexors, anterolateral neck flexors, cervical rotators, posterolateral neck extensors, and upper trapezius. Data were analyzed using random intercept general mixed linear models, with each individual player as a repeating factor/cluster. Main Outcome Measures:Dependent variables included linear and rotational head accelerations. Cervical strength data were categorized into tertiles, creating groups with high, moderate, and low strength. Strength measures were averaged and normalized to body mass. Results:Significant differences in cervical muscle strength existed across our strength groups (P < 0.05). No differences were observed in linear or rotational acceleration across strength groups for the anterior neck flexors (PLin = 0.399; PRot = 0.060), anterolateral neck flexors (PLin = 0.987; PRot = 0.579), cervical rotators (PLin = 0.136; PRot = 0.238), posterolateral neck extensors (PLin = 0.883; PRot = 0.101), or upper trapezius (PLin = 0.892; PRot = 0.689). Conclusions:Our hypothesis that players with greater static neck strength would experience lower resultant head accelerations was not supported. This contradicts the notion that cervical muscle strength mitigates head impact acceleration. Because we evaluated cervical strength isometrically, future studies should consider dynamic (ie, isokinetic) methods in the context of head impact biomechanics.