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.
WHAT'S KNOWN ON THIS SUBJECT:
Young hockey players sustain head impacts as severe as those observed for collegiate football players. Increasingly severe head impacts are likely to cause head injuries in athletics.
WHAT THIS STUDY ADDS:
This study adds to the body of evidence by addressing the potential importance of collision anticipation in mitigating the severity of head impacts sustained by youth hockey players.
The Centers for Disease Control and Prevention has labeled traumatic brain injury (TBI) a serious public health problem in the United States. Children <15 years of age represent up to 40% of the 1.1 million TBIs that result in emergency department visits each year.1 Perhaps more alarming is the number of young TBI victims who do not seek evaluation for their injuries.2 In addition to representing one of the sports medicine conditions most difficult to manage, TBIs were estimated to account for more than $60 billion in direct medical costs and indirect costs in 2000.3 The rates of mild TBIs in tournaments ranged from 10.7 to 23.1 cases per 1000 player-hours.4 These rates are markedly higher than regular-season rates, which were reported previously to be 0.75 concussions per 1000 player-hours.5 In contrast, as many as 10% of high school ice hockey players sustained concussions during the regular season.6
Youth athletes participating in collision sports are at particular risk for concussions.7 Skill development often is limited, because parent volunteers frequently serve as coaches and lack the expertise necessary to educate athletes adequately on proper collision techniques. Medical supervision of these young athletes also is lacking, in comparison with the medical personnel involved in collegiate and professional athletics. Second-impact syndrome, a rare but potentially fatal condition that results in immediate brain swelling when an athlete sustains a head impact (often minor) when symptoms associated with an initial TBI have not fully resolved, has been reported predominantly for adolescent athletes.8,–,10 Understanding the nature of head impacts sustained in youth athletics, with an emphasis on improving our understanding of how best to mitigate the severity of head impacts, may lead to interventions directly related to reducing the incidence of TBIs in youths, minimizing the risks for second-impact syndrome, and decreasing the financial costs incurred by our medical system.
The purpose of this investigation was to study the effect of body collision type (ie, open ice versus along the playing board) and level of player anticipation (ie, anticipated with good body position, anticipated with poor body position, or completely unanticipated) on biomechanical measures of head impact severity such as linear and rotational head accelerations and head impact severity profiles. We used the Carolina Hockey Evaluation of Children's Checking (CHECC) scale to grade player anticipation and body position from video footage. As a subset of all collisions we observed, moderately severe impacts (ie, impacts thought to be severe enough to cause injury but not severe enough to stand out to laypersons as dangerous hits) were an important group of collisions we sought to study. In particular, we examined the effect of player anticipation on moderately severe head impacts.
Study Design and Participants
This study used a prospective quantitative research design to evaluate the effects of body collision type and level of player anticipation on biomechanical measures of head impact severity in youth ice hockey players. We recruited 16 male, Bantam-level, ice hockey players (age: 14.0 ± 0.5 years; height: 171.3 ± 4.5 cm; mass: 63.7 ± 6.6 kg; playing experience: 7.8 ± 1.7 years), representing a convenience sample of participants from a single elite travel youth hockey team. Our sample included 9 forwards and 6 defensemen. One goaltender was removed from our analyses because of lack of body collisions. Data were collected in 54 games over the course of the season. Parental permission and minor assent forms approved by the University of North Carolina at Chapel Hill institutional review board were signed by each parent and player, respectively.
Head Impact Telemetry System
This study used commercially available Reebok RBK 6K and 8K helmets (Reebok-CCM Hockey, St Laurent, Canada) that were modified to accept Head Impact Telemetry (HIT) System technology (Simbex, Lebanon, NH). The helmet's foam liner was modified to accept 6 single-axis accelerometers, a battery pack, and the telemetry instrumentation (Fig 1). The custom helmets passed American Society for Testing and Materials (standard 1045-99) and Canadian Standards Association (standard Z262.1-M90) helmet standards and were approved by the Hockey Equipment Certification Council for use during competition. This instrumentation was described in detail previously.11 The head impact data were time-stamped, encoded, stored locally, and then transmitted in real time to the Sideline Response System (Riddell, Elyria, OH) through a radiofrequency telemetry link. The Sideline Response System was typically positioned along the playing surface sideboards or in the team's dressing room. The HIT System is capable of transmitting accelerometry data from as many as 100 players over a distance well in excess of the length of a standard international ice surface.
We developed the CHECC List to enable standardized evaluations of video footage of body collisions. The CHECC List is scored on the basis of 11 readily observable relative body positions (Table 1) a player may be in when involved in a body collision. In addition to these positions, whether the collision occurred on the open ice or along the playing boards and the level of body collision anticipation were variables of interest included on the CHECC List. The level of anticipation for each body collision was characterized into one of the following overall impressions: anticipated with a good body position (“good anticipation”), anticipated with a poor body position (“poor anticipation”), or unanticipated. A collision with good anticipation was one in which the athlete, at a minimum, was looking in the direction of the impending collision (ie, saw the hit coming) and was in a general athletic readiness position, including knee and trunk flexion with feet shoulder-width apart, and used his legs to drive his shoulders through the collision. A collision with poor anticipation was a collision in which the athlete saw the collision coming but was not in a ready position. Unanticipated collisions were collisions the athlete did not see coming (ie, was not looking in the direction of the impending collision), regardless of whether the athlete was in a ready position. A subsample of body collisions observable on the video footage were reevaluated by the principal investigator ≥3 months after the end of the initial video analyses. Intrarater κ agreements ranged from 0.40 to 0.92 for the 15-item CHECC List. Interrater agreements suggested moderate to strong agreement between hockey coaches who reviewed a subsample of collisions independent of one other.
Before each game, we synchronized the date and time on our single video camera with the date and time on the Sideline Response System. This allowed us to align accurately body collisions observable on the video footage with the biomechanical measures of head impact severity recorded by our instrumented helmets. We recorded video footage for all 54 games over the course of the season. In an attempt to maximize the number of collisions captured on our footage, we followed the movements of the puck carrier. Impacts that occurred outside the view of the camera were excluded from analyses.
Before the start of the season, players were measured for helmet and facemask size. Players were properly fitted with Reebok RBK 6K/8K helmets (and their personal facemasks) by a certified athletic trainer. The athletic trainer instructed each participant to wet his hair to simulate sweating and ensured that the chinstrap was fastened securely to the helmet and fit tightly under the chin. Helmet and facemask fits were verified on a biweekly basis, to ensure proper fitting throughout the course of the season.
Impacts that occurred outside team-sanctioned events were omitted from our analyses. Because impacts with <10 g of linear acceleration (measured in terms of gravity force) are considered negligible with respect to impact biomechanical features and their relationships to head trauma, only impacts with linear acceleration of >10 g were included in our analyses. Our biomechanical measures of head impact severity consisted of linear acceleration, rotational head acceleration, and Head Impact Technology severity profile (HITsp). Resultant linear acceleration was defined as the change in velocity of the estimated center of gravity of the head attributable to an impact and the associated direction of motion of the head. Resultant rotational acceleration was defined as the change in angular velocity of the head attributable to an impact and its direction in a coordinate system with the origin at the estimated center of gravity of the head. The methods used to compute linear and rotational accelerations from the 6 single-axis accelerometers were described in detail previously.12 The HITsp is a weighted composite score including linear and rotational accelerations, impact duration, and impact location.13
A linear regression model was used to analyze the data. Initially, descriptive statistics (means and 95% confidence intervals [CIs]) were calculated for resultant linear acceleration, resultant rotational acceleration, and HITsp. Subsequently, 3 separate linear regression models were used, with resultant linear acceleration, resultant rotational acceleration, and HITsp as the dependent variables. The data were skewed rightward, with numerous low-magnitude head impacts and relatively few high-magnitude impacts. Therefore, the data were logarithmically transformed to satisfy the normality assumption of the linear regression model; all results estimated with the model were back-transformed to their original scale for ease of presentation and interpretation.
Each player sustained multiple impacts over the course of the study. Therefore, the data represent repeated measures for the same group of youth players over time. To account for the lack of statistical independence because of repeated measures, we used a variation of linear regression known as random intercepts, general mixed linear models.14 This model is similar to regular linear regression but allows each player to have his own value for the model intercept. A player variable was included in every model.
Body collision type and level of anticipation were included as separate independent variables (in addition to player) in the statistical models used in this study. In addition, we used the same technique (random intercepts, general mixed linear models) to analyze the effect of player anticipation on medium-intensity and high-intensity collisions, defined as collisions representing the upper-middle (50th to 75th percentiles) and upper (≥75th percentile) HITsp quartiles, respectively. We also sought to identify how relative body positioning affected head impact measures. Therefore, we performed separate analyses that included each relative body position descriptor (Table 1) as an independent variable in separate random intercepts, general mixed linear models. The level of significance was set at P < .05 a priori.
Body Collisions Analyzed
We observed a total of 666 body collisions for which we were able to complete a CHECC List and evaluate whether the collision took place along the boards or on the open ice, judge whether the collision was anticipated, and determine relative body positioning. Of these collisions, 63.2% (421 of 666 collisions) took place along the playing boards, and the remaining 36.8% (245 of 666 collisions) occurred on the open ice. With respect to level of anticipation, 47.3% (315 of 666 collisions) were anticipated with good relative body position, 37.4% (249 of 666 collisions) were anticipated with poor relative body position, and the remaining 15.3% (102 of 666 collisions) were deemed to be unanticipated.
Body Collision Type
We observed statistically significant increases in head linear (F1,14 = 5.40; P = .036) and rotational (F1,14 = 12.75; P = .003) accelerations associated with impacts sustained on the open ice, compared with those sustained along the playing boards (Table 2). The data suggested a strong trend toward a significant increase in HITsp for open-ice collisions, compared with those sustained along the playing boards (F1,14 = 4.38; P = .055).
Level of Anticipation
Linear acceleration tended to be highest for unanticipated collisions, followed by anticipated impacts with poor positioning and anticipated collisions with good positioning, but these differences were not statistically significant (F2,28 = 1.46; P = .249). There were no differences in rotational head acceleration (F2,28 = 1.24; P = .304) or HITsp (F2,28 = 0.70; P = .503) according to anticipation type. In our analyses of the most-severe head impacts (top 25% of HITsp values), we found no significant differences in head impact measures according to the level of anticipation (P > .05). However, in our analyses of medium-intensity head impacts (50th to 75th percentiles of HITsp), we observed a significant difference in rotational acceleration (F2,19 = 6.83; P = .006), such that values for collisions with good anticipation (1215.11 rad/s2 [95% CI: 1112.6–1327.1 rad/s2]) or poor anticipation (1218.9 rad/s2 [95% CI: 1107.2–1341.9 rad/s2]) were significantly lower than those for unanticipated collisions (1465.7 rad/s2 [95% CI: 1240.7–1731.4 rad/s2]). We also observed a significant difference in HITsp (F2,19 = 4.35; P = .028), such that values for collisions with good anticipation (15.2 [95% CI: 15.0–15.5]) or poor anticipation (15.3 [95% CI: 15.1–15.5]) were significantly lower than those for unanticipated collisions (15.6 [95% CI: 15.3–15.9]). All other analyses did not yield statistically significant findings (P > .05).
Relative Body Positioning
Analyses of body positioning during collisions revealed only 1 association among the 15 items, that is, athletes who drove into or through a body collision with their legs experienced lower linear acceleration (20.5 g [95% CI: 19.2–21.9 g]) than did athletes who did not use their legs during a collision (21.7 g [95% CI: 20.1–23.5 g]; F1,13 = 4.67; P = .049). No differences were noted for rotational acceleration (F1,13 = 0.62; P = .446). A moderate trend observed for HITsp (F1,13 = 3.47; P = .085) suggested that athletes who used their legs to drive through a body collision experienced lower values (15.3 [95% CI: 14.6–16.1]) than did athletes who did not use their legs to drive through a collision (16.0 [95% CI: 15.1–16.9]). All other comparisons evaluating the effect of relative body positioning did not reveal any statistically significant differences (P > .05).
To our knowledge, this is the first study focused on determinants of biomechanical measures associated with head impact severity among youth ice hockey players. The findings suggest that head impact severity decreases with heightened player anticipation of impending body contact, especially with respect to head impacts of moderate severity. This agrees with our anecdotal impression that athletes who are more aware of their surroundings and better at anticipating and preparing for impending body collisions are able to mitigate the severity of the impact. Surprisingly, very little is known about the biomechanical variables that cause mild TBI in sports and, perhaps alarmingly, there are very few suggested methods to reduce head impact forces that would not significantly increase helmet size or drastically change the nature of the game of hockey.
Differences in rotational acceleration between unanticipated and anticipated collisions in the medium-intensity range (50th to 75th percentiles of HITsp) were greater than differences we observed in the high- and low-intensity ranges. This suggests that severe impacts (top 25% of linear acceleration or HITsp values) may be equally dangerous regardless of anticipation. More importantly, for impacts that may not seem severe to coaches, parents, and other players, there is evidence suggesting that player anticipation may mitigate severity. Therefore, anticipating collisions for these medium-intensity impacts may play a role in reducing the risk of injury.
Unanticipated collisions yielded greater rotational acceleration and HITsp, compared with anticipated, moderately severe collisions. According to the basic tenet of the role of cervical musculature in mitigating the severity of head impacts, the effective mass of the head-neck-trunk segment is thought to increase significantly when the cervical musculature is contracted, resulting in less acceleration of the head. When an impact is unanticipated and the cervical musculature is not tensed and prepared for a collision, the anecdotal tenet suggests that the effective mass is reduced to approximately that of the head. With a simple Newtonian approach and given an equal force from a body collision, the head would experience substantially greater acceleration and, therefore would be more likely to sustain an injury when the neck is not tensed. Previous work used an external force applicator device to identify aspects of event anticipation in a physically active population15 and among collegiate soccer athletes16 and reported mixed findings. Future studies should continue to investigate anticipation as a potential intervention for preventing mild TBI. In our sample, we were able to capture a concussive event. In this injury, the athlete was struck from behind. We measured relatively low linear acceleration (31.8 g) and disproportionately high rotational acceleration (2911.0 rad/s2). In careful review of the video footage, it was noted that the player did not anticipate the check from behind. Immediately after the collision, the player's head experienced a notable whiplash-type mechanism. The linear acceleration associated with the injury represented the lowest recorded measure we observed for concussion in youth ice hockey and was one-half the lowest linear acceleration causing concussion reported previously for collegiate football players.17
TBI attributable to linear accelerative forces is thought to result in more-focal lesions, whereas rotational mechanisms of injury result in diffuse cerebral injuries.18,–,20 The greater rotational forces in collisions on the open ice are likely the result of movement of the player's head, because no contact with the rigid playing boards takes place after the body collision. The mean difference in rotational acceleration between open-ice collisions and collisions along the playing boards was of the magnitude of ∼200 rad/s2. This mean difference exceeds the absolute rotational acceleration reported previously to result in injury (163.35 rad/s2),17 which suggests that this seemingly minimal difference has some clinical relevance to TBI. At this time, the clinical relevance of small differences in repeated impacts on the developing brain, as we report in our results, is not known. Potential negative effects of cumulative impacts over time, with respect to susceptibility to concussion, longer recovery times after concussion, impaired brain function after concussion, and potential long-term effects on cognition, have been postulated by others. When an athlete experiences a rotational mechanism, it is thought that rotation of the cerebrum about the brainstem produces shearing and tensile strains. Because activity in the midbrain and upper brainstem is responsible for alertness and responsiveness, it is perhaps not surprising that rotational mechanisms contributing to TBI are thought to be more likely to result in loss of consciousness, compared with predominantly linear types of impacts or impulses.20–24 We did not track whether the ice rink was a traditional or Olympic-sized surface. Although we can only speculate, we hypothesize that we would see relatively more open-ice collisions on Olympic-sized surfaces and increased numbers of collisions along the playing boards in traditional arenas. This would be an interesting independent variable, in terms of youth ice hockey injury prevention, to be included in future studies.
We sought to identify whether a relative body position could be identified that would best mitigate impact forces associated with body collisions sustained in youth ice hockey. Only 1 of the relative body position descriptors yielded significant results. Teaching players to “skate through the body check” has been encouraged by USA Hockey and has long been taught by coaches to young players in the United States and abroad. It is possible that, because of the large number of low-magnitude, noninjurious impacts we observed, we would not expect any differences between athletes who exhibited a positive relative body position descriptor and those who did not. In our opinion, the ready position taught by USA Hockey25 should continue to be taught to young hockey players until further research suggests better interventions targeted at reducing mild TBI in youth athletes. Our study included elite male youth hockey players from a single team. Therefore, it may be difficult to generalize our findings to all hockey players, including less-skilled youth players and players with less-skilled coaches. Future work should include these other cohorts and also should seek to understand the effects of body collisions on female hockey players.
Coaches across all player age levels, competition divisions, and player genders can play a significant role in teaching young hockey players to anticipate collisions more effectively. Coaches responsible for all athletes playing collision sports should possess the necessary training and certification to ensure safe player development. In practices, game-related drills with full contact would pattern players to adapt to constantly changing scenarios during play. A relatively recent trend in coaching practices is the implementation of “small games” drills. These drills emphasize high speed, quick movements, and game-related tasks (eg, passing, shooting, and checking) in small, confined spaces (ie, corner of the rink). These drills are excellent at forcing athletes to play with a heightened sense of awareness that allows them to anticipate incoming body collisions. Officials are charged with the task of immediately and severely penalizing any player who takes advantage of opponents in susceptible, vulnerable positions to deliver unsuspected collisions. Player education is important in promoting safe hockey practices at all levels. This can include playing within the confines of the regulations, teaching players how to anticipate collisions safely and how to deliver and to receive body collisions effectively, and providing young athletes with an environment that supports the reporting of concussions and the safe management of these injuries. Coaches, officials, parents, and medical professionals should all be tasked with this objective.
We used an athletic research model with the goals of better understanding the nature of head impacts sustained by youth ice hockey players and describing how body collision type and level of anticipation may mitigate the severity of these head impacts. The idea that heightened player anticipation can mitigate the severity of head impacts is supported by our data, especially for medium-intensity collisions. Our finding of increasing head impact severity with decreasing anticipation suggests that coaches should target this aspect of ice hockey in their technical development of players during practices, to promote the skills necessary to keep the safety of participants at the forefront. We recommend that hockey coaches spend time during practices educating players on how to deliver and to receive body collisions safely in all areas of the ice, including along the playing boards and on the open ice. Prospective studies evaluating the effects of player education and technical training in reducing injury rates at the youth level should be undertaken. Although continued research in this area is necessary, we must use the information we have and implement interventions designed to increase the safety of youth ice hockey players, to prevent the onset of TBI in young athletes.
Funding for this research was obtained from the Ontario Neurotrauma Foundation, the National Operating Committee on Standards for Athletic Equipment, and the USA Hockey Foundation.
- Accepted February 24, 2010.
- Address correspondence to Jason Mihalik, PhD, CAT(C), ATC, University of North Carolina at Chapel Hill, Department of Exercise and Sport Science, 209 Fetzer Gymnasium, South Road, Chapel Hill, NC 27599. E-mail:
FINANCIAL DISCLOSURE: Dr Greenwald has a financial interest in the HIT System technology used to collect data in this study.
- CHECC =
- Carolina Hockey Evaluation of Children's Checking •
- TBI =
- traumatic brain injury •
- HIT =
- Head Impact Telemetry •
- HITsp =
- Head Impact Telemetryseverity profile •
- CI =
- confidence interval
- Langlois JA,
- Rutland-Brown W,
- Thomas KE
- Finkelstein EA,
- Corso PS,
- Miller TR
- Mihalik JP,
- Guskiewicz KM,
- Jeffries JA,
- Greenwald RM,
- Marshall SW
- Demidenko E
- Holbourn AHS
- Ommaya AK,
- Gennarelli TA
- Ommaya AK,
- Hirsch AE,
- Flamm ES,
- Mahone RH
- 25.↵USA Hockey. Heads Up Hockey Program Guide: Safer Hockey, Smarter Hockey, Better Hockey. Colorado Springs, CO: USA Hockey; 2009. Available at: www.usahockey.com/uploadedFiles/USAHockey/Menu_Education_and_Training/heads%20up%20hockey%20program%20guide%202004.pdf. Accessed September 7, 2009
- Copyright © 2010 by the American Academy of Pediatrics