Journal Logo

APPLIED SCIENCES

Sex and Sport Differences in College Lacrosse and Soccer Head Impact Biomechanics

MIHALIK, JASON P.1,2; AMALFE, STEPHANIE A.3,4; ROBY, PATRICIA R.1,2; FORD, CASSIE B.1,5; LYNALL, ROBERT C.6; RIEGLER, KAITLIN E.3,7; TEEL, ELIZABETH F.8; WASSERMAN, ERIN B.9; PUTUKIAN, MARGOT3,10

Author Information
Medicine & Science in Sports & Exercise: November 2020 - Volume 52 - Issue 11 - p 2349-2356
doi: 10.1249/MSS.0000000000002382
  • Free

Abstract

Sport-related concussion remains a clinically challenging medical condition for health care professionals to manage. Although there have been considerable contributions to the scientific literature, many injury risk factors remain underexplored. Evaluating head impact forces is an important consideration. However, the medical community’s ability to contextualize these findings to nonmale and nonhelmeted sports is restricted, given head impact biomechanics research has traditionally favored male athletes participating in American football and ice hockey. Sport-related concussion is an important and prevalent injury for several other sports, including lacrosse (female game, 1.42/1000AE; practice, 0.29/1000AE; male game, 0.91/1000AE; practice, 0.16/1000AE) and soccer (female game, 1.83/1000AE; practice, 0.20/1000AE; male game, 0.94/1000AE; practice, 0.17/1000AE) (1–3). These sports offer a variety of head impact risk attributed to the rules and equipment requirements they encompass. For example, men’s lacrosse is a contact sport requiring participants to wear protective helmets and shoulder pads. Women’s lacrosse, played under significantly different rules, is considered a limited contact sport requiring athletes to wear protective eyewear, but not helmets. Lastly, soccer is a nonhelmeted sport whereby men and women play under identical rules. Female soccer players have historically experienced higher reported concussion injury rates than their male counterparts (4), but this is not observed in lacrosse (5).

Epidemiological studies have identified several concussion risk factors in college lacrosse and soccer. Male college lacrosse athletes sustain concussion primarily from player contact (83%), whereas female lacrosse athletes typically sustain concussions from stick contact (39%), ball contact (37%), and player contact (14%) (6). Male college soccer athletes sustain concussion primarily from player contact while heading the ball (23.6%), ball contact while heading the ball (14.6%), and player contact while goaltending (10.9%), whereas female college soccer athletes sustain head injury from player contact while heading the ball (19.9%), ball contact while heading the ball (11.0%), and player contact during general play (8.8%) (7). Irrespective of sex, it is clear player contact is a leading concussion risk factor in the lacrosse and soccer literature.

Helmet-based impact monitoring data demonstrate that, for every acute mild traumatic brain injury, there are many more impacts that do not result in incident concussions (8,9), and that impact forces in these “noninjury” impacts often exceed those observed in concussive events. Exploring general head impact biomechanics is a fundamental first step toward understanding how repetitive head impacts may be associated with short- and long-term cognitive changes (10,11). To achieve this outcome, scientists have several head impact measurement devices in the market to choose from (12). Applications range from helmet-based, headband, mouthguard, skin-fixed, and so on. Coupling external devices to athletes’ heads, collecting head impact events, and translating these sensor data to a coordinate system originating elsewhere (i.e., head center of gravity) remains a technical challenge in this area. Despite these imperfections, collecting head impact data have informed meaningful rule changes in other sports, such as the football kickoff location (13) and ice hockey body checking age (9).

Understanding head impact biomechanics in college lacrosse and soccer athletes may inform future studies and interventions designed to mitigate injury risk in these populations. For example, there remains growing concern with recent associative evidence between head impact exposure and late-life neurodegenerative disease. Studying head impact biomechanics in nonfootball populations may provide the necessary framework to contextualize these scientific debates. Although there are limited head impact data at the high school level (14,15), there are few field studies providing head impact biomechanical data for both men’s (16) and women’s lacrosse (17). Additionally, few studies have addressed head impact biomechanics in soccer (18,19). No studies, to our knowledge, compare and contrast college lacrosse and soccer nor do they compare female and male athletes. The purpose of our study was to evaluate sex, sport, and event differences by quantifying and analyzing head impact biomechanics (frequency and magnitude) in female and male college lacrosse and soccer athletes. We hypothesized that female soccer and lacrosse athletes would experience greater head impact magnitudes—but not higher frequencies—than their male sport counterparts. Head impact biomechanics would be greater in lacrosse compared with soccer.

METHODS

A prospective cohort design was implemented at two Division 1 NCAA sites (Princeton University and the University of North Carolina at Chapel Hill) to study head impact biomechanics (frequency and magnitude) sustained during men’s and women’s college lacrosse and soccer participation. The study sample is described in Table 1. Participants signed an informed consent form approved by Princeton University’s Institutional Review Board (and relied on by the University of North Carolina at Chapel Hill’s Office of Human Research Ethics) and were allowed to withdraw from the study at any time without penalty. Participants who were willing to participate were assigned head impact accelerometers and a unique participant identification number.

TABLE 1 - Demographic information for study sample.
Male (n = 141) Female (n = 96) Total (N = 237)
Age, yr 19.7 ± 1.3 19.8 ± 1.2 19.8 ± 1.3
Height, cm 179.5 ± 7.5 168.2 ± 7.1 174.8 ± 9.2
Mass, kg 78.3 ± 10.8 64.0 ± 6.76 72.4 ± 11.7
Lacrosse, n (%) a
 All positions 84 (100%) 62 (100%) 146 (100%)
 Defense 24 (28.6%) 14 (22.6%) 38 (26.0%)
 Attack/forward 16 (19.0%) 10 (16.1%) 26 (17.8%)
 Midfield 36 (42.9%) 23 (37.1%) 59 (40.4%)
 Goalkeeper 7 (8.3%) 5 (8.1%) 12 (8.2%)
 Multiple 0 (0.0%) 10 (16.1%) 10 (6.8%)
 Unknown 1 (1.2%) 0 (0.0%) 1 (0.7%)
Soccer, n (%) b
 All positions 57 (100%) 34 (100%) 91 (100%)
 Defense 16 (28.1%) 6 (17.7%) 22 (24.2%)
 Attack/forward 8 (14.0%) 10 (29.4%) 18 (19.8%)
 Midfield 18 (31.6%) 14 (41.2%) 32 (35.2%)
 Goalkeeper 7 (12.3%) 3 (8.8%) 10 (11.0%)
 Multiple 8 (14.0%) 1 (2.9%) 9 (9.9%)
aTotal column percentages add up to 99.9% due to rounding.
bTotal column percentages add up to 100.1% due to rounding.
Values for age, height, and mass represent mean ± standard deviation. All other values represent frequencies (n) and percentages (%) reflecting the proportion of a given position relative to all athletes in the same sex or total category within that same sport.

Instrumentation

To ensure consistency across sites, sports, and sexes, we captured head impact biomechanics using the xPatch system (X2 Biosystems, Seattle, WA). The xPatch is a nonhelmeted lightweight device that overcomes limitations of helmeted systems by affixing over the right mastoid process of the participants’ head with an adhesive patch. It is small enough to attach directly behind the ear and avoid the hairline in most athletes. The device measures linear acceleration and rotational velocity with six degrees of freedom and provides an estimate of head impact location. The xPatch derives rotational acceleration from the measured rotational velocity. The xPatch has the capability to record data for up to 6 h once it is activated. At the beginning of the season, each study participant was assigned an xPatch labeled with a preassigned participant identification number. Head impact data were stored on the xPatch for later extraction to a cloud-based system and local laptop. There are limited studies investigating the validity of the xPatch. One study examined the core X2 technology embedded in a mouthguard (20), whereas another assessed agreement with the Hybrid III headform, reporting the maximum root mean square error was less than 53% for both linear and rotational accelerations (20,21); it performed similarly or better than other comparable head-mounted devices (22,23). Data obtained at the high school level in both girls and boys lacrosse using nonhelmet-based sensors have described limitations of these systems (14,15). We have previously published women’s college soccer head impact data in this journal using this same technology (19).

Data Collection

Trained research assistants affixed the xPatch to its corresponding study participant within 1 h before every practice or competition. The skin over the mastoid process was cleaned with an adhesive prep pad, and all loose hair was moved from the area during application. The xPatch was secured with athletic tape in cases where there was concern the xPatch adhesive patch would come loose during activity. The xPatch was secured again in instances when it fell off, and the athlete reported this to the study personnel. When the xPatch could not be located (e.g., fell off in the turf) or it was not reported to study personnel, the remainder of the session data were not collected. Because the xPatch does not communicate with a sideline system in real time, our study personnel were unable to identify these instances until the session ended. The start and end times of each practice and competition were documented. For competitions, the start and end time of each quarter or half were also documented. Participants removed their xPatch and returned it to the research assistants immediately after the event. The xPatches were regularly connected to a laptop by way of the charging station, and the data were exported and stored in a cloud database for subsequent data analysis. After data were exported, the xPatches were cleaned and charged for their subsequent deployment.

Data Reduction

We excluded data not believed to represent real on-field head impacts in men’s and women’s lacrosse and soccer. The first step applied a proprietary manufacturer-based “declacking” algorithm to the data eliminating all recorded accelerations that were not head impacts (e.g., cutting, finger-tapping the xPatch, or jumping). Second, we excluded all head impacts not exceeding a study-implemented 10g linear acceleration threshold for inclusion. We chose a 10g threshold to enhance consistency with the head impact biomechanics literature and to remain consistent with our own work in this area (9,13,19). Our final data processing step removed all data occurring outside the start and end times documented by our on-site research assistants.

Statistical Analysis

We analyzed our data to quantify the sex and sport differences in head impact biomechanics (frequency and magnitude) sustained by female and male lacrosse and soccer athletes. Separate random intercepts general linear mixed models were used for each dependent variable (linear acceleration and rotational acceleration), which were log transformed to correct for positive skewness. Independent variables included sex, sport, and event type (competition versus practice). Post hoc analyses were conducted to examine significant interactions using a Tukey–Kramer adjustment to correct for multiple comparisons. To further explore our head impact biomechanics data, we categorized the linear and rotational accelerations for each impact as either mild, moderate, or severe based on cutpoints empirically generated through univariate analyses and visual examination of the distribution of impacts. Linear acceleration thresholds were empirically derived from the data we collected as mild (10g–20g), moderate (20g–40g), and severe (>40g). For rotational acceleration, the empirically derived thresholds were mild (<4000 rad·s−2), moderate (4000–7500 rad·s−2), and severe (>7500 rad·s−2). We felt these thresholds would be consistent with previous work presenting head impact frequency across a range of linear and rotational severities and across the large head impact frequencies reported in this study (19). We used separate repeated-measures negative binomial Generalized Estimation Equations models to test the sex, sport, and event type effects on the number of impacts falling within each of the severity categories.

RESULTS

A total of 91 soccer (34 female athletes; 57 male athletes) and 146 lacrosse (62 female athletes; 84 male athletes) athletes were equipped with xPatch sensors over the course of this 2-yr study. The majority of athletes in both sports played the midfield position (n = 91), followed by defense (n = 60) and attack/forward (n = 44). Complete demographic characteristics of the study sample can be found in Table 1. A total of 35,036 valid head impacts were recorded over the data collection period. There were 19,607 impacts among male athletes (median, 246 per player per season; 6 per player-session) and 15,429 among female athletes (median, 302 per player per season; 8 per player-session). There were 20,587 impacts among soccer athletes (median, 342 per player per season; 10 per player-session) and 14,449 among lacrosse athletes (median, 158 per player per season; 4 per player-session). The descriptive and statistical data for these impacts across sex, sport, and event type are provided in Tables 2 and 3. Most impacts for both linear and rotational accelerations were observed in the lowest severity bin (linear acceleration, n = 24,319, 69.4%; rotational acceleration, n = 25,313, 72.3%). The thresholds used to categorize head impacts as mild, moderate, or severe, and the resulting head impact frequencies are described in Table 4.

TABLE 2 - Means ± standard deviations of head impact outcomes by sport, sex, and event type.
Soccer Lacrosse
Male Female Male Female
Linear acceleration (g)
 Competition 21.70 ± 13.81 19.05 ± 12.06 21.21 ± 14.69 16.46 ± 10.75
 Practice 18.91 ± 13.21 18.06 ± 12.09 21.96 ± 14.70 20.39 ± 14.90
Rotational acceleration (rad·s−2)
 Competition 4040 ± 3446 3457 ± 2979 3509 ± 2902 2426 ± 2297
 Practice 3069 ± 3031 3010 ± 2741 3606 ± 3035 3273 ± 3042

TABLE 3 - Means ± standard deviations for main effects of sex and sport, and their interactions with event type (competition, practice) on head impact linear and rotational accelerations.
Sex Effect Sport Effect
Male Female Soccer Lacrosse
Linear acceleration (g)
 Competition 21.52 ± 14.15 18.68 ± 11.92 20.05 ± 12.81 19.73 ± 13.77
 Practice 20.88 ± 14.27 18.60 ± 12.84 18.41 ± 12.57 21.66 ± 14.75
 Combined across event 21.09 ± 14.28 18.71 ± 12.60 19.11 ± 12.78 21.36 ± 14.62
Rotational acceleration (rad·s−2)
 Competition 3838 ± 2359 3310 ± 2914 3676 ± 3175 3173 ± 2774
 Practice 3416 ± 3044 3071 ± 2816 3034 ± 2863 3542 ± 3039
 Combined across event 3519 ± 3104 3167 ± 2863 3280 ± 3006 3484 ± 2999
Main effect means in bold.

TABLE 4 - Frequencies (n) and percentages (%) of head impacts categorized as mild, moderate, or severe head impacts.
Mild Moderate Severe
Peak linear acceleration (g)
 Thresholds (g) <20 20–40 >40
 Impact frequency (n) 24,319 7769 2948
 Percent of head impacts (%) 69.41 22.17 8.41
Peak rotational acceleration (rad·s−2)
 Thresholds (rad·s−2) <4000 4000–7500 >7500
 Impact frequency (n) 25,313 6270 3453
 Percent of head impacts (%) 72.25 17.90 9.86

Impact Severity

Across both sports and event types, male athletes sustained higher linear accelerations on average than female athletes (P = 0.04; for all group means, see Table 2). On average, lacrosse athletes sustained higher linear accelerations than soccer athletes (P = 0.023). There was no difference in linear acceleration comparing competitions and practices, collapsing across sex and sport (P = 0.10). However, there was a significant interaction between event and sport (P < 0.001) linear acceleration. Comparing impacts across event for each sport separately, soccer athletes sustained significantly higher linear accelerations during competitions compared with practices (P < 0.001), whereas lacrosse athletes received significantly lower linear accelerations during competition as compared with practice (P < 0.001). Comparing across sport for each event separately, lacrosse athletes sustained significantly higher linear accelerations than soccer athletes during practices (P < 0.001), but not during competitions (P = 0.90). Differences in linear accelerations across event type also differed by sex (P < 0.001). Comparing impacts across events separately by sex, male athletes had significantly higher linear accelerations during competitions compared with practices (P = 0.004), whereas female athletes showed the opposite pattern of slightly lower linear accelerations during competition (P < 0.001). Comparing impacts across sex separately across event, male athletes sustained significantly higher linear accelerations than female athletes during competitions (P = 0.001), but not during practices (P = 0.71). We did not find a significant interaction between sex and sport (P = 0.45) nor a three-way interaction between sex, sport, and event (P = 0.08) on linear acceleration.

For the rotational accelerations model, there were no main effects of sex (P = 0.09), sport (P = 0.41), or event (P = 0.13). There were significant interactions between event and (a) sport (P < 0.001) and (b) sex (P < 0.001) on rotational acceleration, reflecting the same pattern of effects as was found on linear accelerations (Table 2). Again, soccer athletes had significantly greater rotational acceleration impacts during competitions compared with practice (P < 0.001), whereas lacrosse athletes showed lower rotational acceleration outcomes during competition (P < 0.001). During competitions, soccer athletes received higher rotational accelerations than lacrosse athletes (P < 0.001), but the reverse was seen during practice (P = 0.04). Male athletes sustained significantly higher rotational accelerations during competitions versus practices (P < 0.001), whereas female athletes sustained significantly lower rotational accelerations during competition (P = 0.02). Finally, male athletes sustained significantly higher rotational accelerations than female athletes during competitions (P < 0.001), but not during practices (P = 0.29). There were no significant interactions between sex and sport (P = 0.17), nor between sex, sport, and event (P = 0.18).

Impact Frequency

Male athletes sustained significantly more impacts with linear accelerations that qualified as either moderate or severe than female athletes (P < 0.002), but there was no such difference for the frequency of impacts across categories defined by rotational acceleration (P = 0.163). The impact frequencies across rotational acceleration severity categories significantly differed by sport (P < 0.001) and event type (P = 0.004). Soccer players experienced more impacts with rotational accelerations categorized as mild or severe than did lacrosse players, whereas moderate impact frequency is roughly equivalent across the two sport groups. Finally, there was a difference in the distribution of impacts across rotational acceleration categories between competition and practice, but the large difference seems to be in the mild category, with many more mild impacts sustained during practices. We did not find main effects for sport (P = 0.12) or event type (P = 0.30) for the distribution of peak linear acceleration across the severity categories (Table 5). Due to power issues, we were unable to fully interrogate interactions for these categorical analyses.

TABLE 5 - Comparing categorized linear acceleration and rotational acceleration frequency distributions by sex, sport, and event type.
Peak Linear Acceleration (g) Peak Rotational Acceleration (rad·s−2)
Mild Moderate Severe P Mild Moderate Severe P
Male 12,856 (65.6) 4855 (24.8) 1896 (9.7) < 0.002* 11,647 (75.5) 2453 (15.9) 1329 (8.6) 0.163
Female 11,463 (74.3) 2914 (18.9) 1052 (6.8) 13,666 (69.7) 3817 (19.5) 2124 (10.8)
Soccer 14,966 (72.7) 4096 (19.9) 1525 (7.4) 0.116 15,215 (73.9) 3379 (16.4) 1993 (9.7) < 0.001*
Lacrosse 9353 (64.7) 3673 (25.4) 1423 (9.8) 10,098 (69.9) 2891 (20.0) 1460 (10.1)
Practice 17,456 (70.1) 5369 (21.6) 2080 (8.4) 0.299 18,301 (73.5) 4236 (17.0) 2368 (9.5) 0.004*
Game 6488 (68.5) 2207 (23.3) 771 (8.1) 6600 (69.7) 1869 (19.7) 997 (10.5)
*Denotes significance.
Values reported in parentheses represent the within-category percentages to facilitate relative comparisons.

DISCUSSION

Cumulative head impact exposure has been raised as a potential factor linked to neurodegeneration and late-life neurological deficits in American football athletes (24,25). It is important to better understand head impact frequency and magnitude experienced by athletes in all sports to better enhance equipment standards where equipment is worn. It may also inform standards in sports where equipment is being considered and establish behavior modification/rule changes and policy changes designed to decrease head injury risk and mitigate potential long-term neurological effects. Overall, our data demonstrate that the majority of head impacts in men’s and women’s lacrosse and soccer are mild for both linear acceleration and rotational acceleration. We assert this based on our mean values for linear and rotational acceleration and by the highly right skewed distribution consistent with extant head impact biomechanics literature among noninjured athletes (8,26,27). We observed that male lacrosse and soccer student-athletes experienced a greater relative frequency of moderate and severe linear head impacts than their female counterparts, and that soccer athletes experience a greater relative frequency of severe rotational accelerations than lacrosse athletes irrespective of sex.

Soccer competition-related head impacts were more severe than those sustained during practices. This finding contradicts current literature looking at both male and female collegiate soccer head impact biomechanics. Lynall et al. (19) found that female soccer players experience higher-magnitude impacts during practice compared with competition, whereas Reynolds et al. (28) found no differences between competition and practice impact severities in male soccer players. We found that boys had higher-magnitude impacts during competitions, but there was no significant difference observed during practice. Zuckerman et al. (29) report that ball contact was mechanism for 25% of diagnosed concussions in their soccer sample. Sakamoto et al. (30) also report that male soccer players were able to generate greater ball speed than female soccer players. In combination, these reports may help to explain our sex differences in soccer. Notwithstanding, the sex effects for our findings warrant further study.

Lacrosse athletes experience higher-magnitude impacts during practice compared with competition, and this difference appears to be driven by female athletes for whom the mean linear acceleration during competition was approximately 4g lower than those sustained during practices. Previous investigations (31) have found no differences in head impact magnitude between events in lacrosse athletes; however, our study demonstrates that female lacrosse athletes experienced a lower linear and rotational acceleration than male lacrosse athletes. The true clinical implication of a 4g difference in linear acceleration is a very complicated matter for which the scientific literature does not yet inform a meaningful response. On the one hand, a 4g difference on any given head impact may be sufficient to exceed an athlete’s tolerance to injury for that particular event. Additionally, a 4g difference multiplied by the number of head impacts sustained in a season, year, and/or career may accumulate to be a worthy consideration when studying the effects of subconcussive head impact exposure on long-term neurological outcomes in athletes participating in these college sports. Despite these potential clinical implications, we caution the reader from overinterpreting a 4g difference given the technical limitations with measurement errors produced by head impact monitoring devices, including the one we employed in this study. Although our study did not focus on injury, we did observe nine incident concussions during the study period. The head impact biomechanics for these injuries are provided in Table 6.

TABLE 6 - Recorded linear and rotational accelerations (ranked in descending order of linear acceleration) for diagnosed concussions in nine college lacrosse and soccer players.
Athlete Sex Sport Position Linear Acceleration (g) Rotational Acceleration (rad·s−2)
A Female Lacrosse Defense 115.0 2860.5
B Male Lacrosse Midfield 105.0 13,011.5
C Female Soccer Midfield 86.1 5340.9
D Male Lacrosse Attack 84.8 7375.0
E Female Soccer Goalkeeper 76.7 7434.9
F Male Lacrosse Midfield 71.4 12,973.1
G Male Lacrosse Midfield 47.4 5367.2
H Male Soccer Defense 39.9 5665.4
I Female Lacrosse Defense 24.5 4152.9

It is important to note that we studied soccer and lacrosse programs from only two universities. It should be noted that our institutions may not be representative of all collegiate lacrosse and soccer programs, and certainly do not represent high school or youth lacrosse and soccer programs. Any discrepancies between our results and those presented by previous research may also be due to coaching styles at our institutions, which may also be influenced by the comprehensive concussion research programs at our respective organizations. Additionally, the data presented in Table 2 suggest that female lacrosse players have lower severity impacts compared with all other groups. However, it is important to note that these represent raw means and not statistical model implied means. Model implied means demonstrate the means are closer together, which is an important consideration when calculating the means controlling for sport and event. This is something to consider in the context of data previously published in this space, whereby analyses may have been incorrectly performed resulting in difficulty making cross-study comparisons. It is possible that player aggression may differ between female and male athletes, and that this aggression may contribute to the data we are observing. Studies in youth ice hockey do not necessarily agree with this hypothesis given reported differences in head impact biomechanics between female and male athletes, despite no differences in self-reported aggression (32). We also submit that characterizing coaching style and relating the influence of coaches on player behavior and head impact biomechanics is a difficult construct to measure. Despite this, attempts have been made to study this in football. For example, Martini et al. (33) report that different offensive strategies (e.g., run vs pass offense) result in differences in head impact frequency and severity. It will be helpful to explore how these, and other possible factors, may influence lacrosse and soccer player safety in future studies.

The event differences we observed in soccer athletes appears to be driven largely by male athletes, which agrees with current literature stating that female soccer athletes tend to experience higher-magnitude head impacts in practices rather than competition (19). The differences between practice and competition rotational acceleration in lacrosse athletes is being driven largely by female lacrosse athletes who experience fewer high-magnitude impacts during games, likely due to the women’s lacrosse rules prohibiting contact. Previous work has identified differences in linear and rotational accelerations (for games and practices) between college soccer athletes (18.5g; 95% confidence interval [CI], 17.2–19.9) compared with college football (26.8g; 95% CI, 25.0–28.7), high school football (25.2g; 95% CI, 23.4–27.2), and college lacrosse (21.3g; 95% CI, 19.9–22.8) (34). The values provided here are for practice linear accelerations; game linear and game/practice rotational accelerations all followed the same order. However, when including all athletes in our analyses, there is a sport main effect (differences between lacrosse and soccer without regard to sex). Our finding is that college soccer athletes experience higher rotational accelerations than college lacrosse athletes, and it appears largely driven by male soccer athletes and female lacrosse athletes, although there was not a significant interaction between sport and sex. This is likely due to the fundamental differences in men’s and women’s lacrosse gameplay including the extent to which body contact and checking are permitted in the men’s game, and possibly the equipment used. This underscores the importance of considering men’s and women’s lacrosse as different sports. However, for men’s and women’s soccer, the playing rules and equipment are identical for both, implying that the differences seen in biomechanical forces are due to effects of sex or potentially play style, but not body contact rules or equipment.

Given the preponderance of head impact biomechanics research in the literature focuses on American football and to a considerably lesser extent ice hockey, a discussion of the contrasting nature of our data in relation to football data in the literature is warranted. Our observed average linear accelerations for all soccer and lacrosse athletes were slightly lower than those observed in American football (8,27,35). This supports the hypothesis that football collisions generate considerably higher energies and, thus, result in greater linear accelerations and more frequent severe head impacts than the sports we studied which do not emphasize body collisions (all except men’s lacrosse). Interestingly, the observed rotational accelerations for our lacrosse and soccer athletes were up to three times larger than those typically observed in football athletes (27). Soccer athletes often intentionally rotate their head to redirect the incoming ball to a specific target. Thus, the rotational accelerations they are sustaining may simply be due to the manner in which they are performing this sport-specific task. Head impact mechanisms sustained in lacrosse differ from those observed in soccer, and both differ from football head impacts. We do not yet understand how intentionally striking a ball with one’s head changes the outcomes we measure compared with being struck by a player or stick. Our research team feels these are important factors to consider because this area of research continues to evolve. Another possible explanation for these sport differences may be due to technological differences between the head impact sensor we used and those used in previous football studies. Typically, football studies have used the Head Impact Telemetry (HIT) System. The HIT System uses data collected from six single-axis accelerometers to compute linear acceleration, and then uses that information to estimate rotational acceleration about the frontal and sagittal axes. It is unable to estimate rotational acceleration about the vertical—and arguably most important—axis. The xPatch independently measures rotational velocity, and uses these data to compute rotational acceleration, which may overestimate the rotational acceleration sustained by our athletes. It is our opinion that true measures of rotational acceleration likely fall in between these two devices: the HIT System underestimates, whereas the xPatch overestimates. Future research should validate different head impact technologies, particularly given the growing interest in developing helmet standards inclusive of rotational acceleration, and to adopt head protection in sports previously without (e.g., helmets in women’s lacrosse, headbands/headgear for soccer).

Limitations

The xPatch is one of many technologies able to assess nonhelmeted, in vivo head impact biomechanics; however, there are currently limited xPatch validity studies. The importance of validating impact sensor technology with video analysis in male and female lacrosse players has been demonstrated by other researchers (14,15). Cortes et al. (15) used two different impact sensors to measure forces in boy’s and girl’s lacrosse, including the xPatch for girls, and concluded that these sensor technologies may overestimate head impact events. Only 48% of the impacts were a result of direct contact to the head as characterized by video, and for the girls, only 32% of impacts were verified via video analysis. Given the resources available to complete this study, we do not have video data for all events. There are considerable challenges to capturing these data, particularly in nonhelmeted sports, and limited technologies available to accomplish this technical task. Although every effort was made to ensure strong adhesion between the xPatch and the athlete’s skin, there are instances when the xPatch fell off due to adhesion failure, contact with device, or sweat. Because the xPatch does not have real-time data transmission, our personnel were unable to identify all instances of “dropped” xPatch devices. In some instances, data collection was not permitted for some of our participants in some of the sessions. These device failures (e.g., dropped, lost, dead battery, etc.) are not unique to the xPatch. Although we did not formally record these instances, our study personnel did not report this as an issue during regular study meetings. We believe it should still be considered when interpreting our exposure outcomes, and that future studies should consider documenting these instances in more detail. Despite this, our study has presented our findings without bias while factoring in these technical limitations.

We chose to categorize magnitude outcomes in addition to reporting mean values to provide the reader with clinically applicable indications of head impact magnitudes. The cutoff for linear acceleration was chosen at 10g for the purposes of our study given the data that exist in American football and the data regarding soccer and heading impacts which are often in the 10g to 20g range. However, other studies in soccer and lacrosse (14,15,21) have chosen a higher cutoff of 20g to avoid artifact and measuring the impacts that occur with other play behaviors, such as cutting, jumping, and hard stops. It is very likely that had we adopted a higher threshold (e.g., 20g), our data would have grossly overestimated head impact biomechanics in our cohort owing to the data we present demonstrating that most head impacts (69%) were mild with mean linear acceleration values less than 20g. Our study also did not include head impact location. We believe our data provide an important foundational framework for subsequent studies in lacrosse and soccer. These two sports have not been studied in great detail. Any intervention studies designed to mitigate head injury risk with playing technique (lacrosse checking, soccer heading) or protective equipment (lacrosse helmets, soccer headgear) will benefit from including head impact location.

CONCLUSIONS

Our data suggest that most head impacts are relatively mild, and that male athletes experience higher-magnitude head impacts. Head impact biomechanics are an important factor when considering injury risk; however, there is a dearth of literature investigating head impact biomechanics in female athletes and in nonhelmeted sports. Future research should continue to characterize head impact biomechanics in women’s and in nonhelmeted sports as well as validate nonhelmeted head impact technologies.

The authorship team would like to acknowledge sports medicine colleagues and staff at the University of North Carolina at Chapel Hill [Alain Aguilar, Kyra Busque, Kody Campbell, Kim Chase, Nicole Fava, Justin Fegley, Missy Fraser, Chris Hirth, Yuri Jean-Baptiste, Grace Jungclas, Jacob Mir, Jacob Powell, Emily Quatromoni, Kim (Chase) Stevens, Jackie Stucker, Catherine (Lenhardt) Van Eeden, and Nina Walker] and Princeton University [Jasper Chang, Casey Maxwell, George O’Neil, and Russell Steves] for assistance with team logistics and data collection. We also want to acknowledge the consultation provided by Ruben Echemendia, PhD, related to study design and methodology. This project was funded by an investigator-initiated sponsored research grant awarded by the National Operating Committee on Standards for Athletic Equipment (NOCSAE) to Princeton University and UNC-Chapel Hill. The opinions expressed herein are those of the authors and do not necessarily reflect the opinions of the NOCSAE.

REFERENCES

1. Pierpoint LA, Caswell SV, Walker N, et al. The first decade of web-based sports injury surveillance: descriptive epidemiology of injuries in US high school Girls' lacrosse (2008–2009 through 2013–2014) and National Collegiate Athletic Association Women's Lacrosse (2004–2005 through 2013–2014). J Athl Train. 2019;54(1):42–54.
2. Roos KG, Wasserman EB, Dalton SL, et al. Epidemiology of 3825 injuries sustained in six seasons of National Collegiate Athletic Association men's and women's soccer (2009/2010–2014/2015). Br J Sports Med. 2017;51(13):1029–34.
3. Kerr ZY, Quigley A, Yeargin SW, et al. The epidemiology of NCAA men's lacrosse injuries, 2009/10–2014/15 academic years. Inj Epidemiol. 2017;4(1):6.
4. Gessel LM, Fields SK, Collins CL, Dick RW, Comstock RD. Concussions among United States high school and collegiate athletes. J Athl Train. 2007;42(4):495–503.
5. Covassin T, Swanik CB, Sachs ML. Sex differences and the incidence of concussions among collegiate athletes. J Athl Train. 2003;38(3):238–44.
6. Lincoln AE, Hinton RY, Almquist JL, Lager SL, Dick RW. Head, face, and eye injuries in scholastic and collegiate lacrosse: a 4-year prospective study. Am J Sports Med. 2007;35(2):207–15.
7. Zuckerman SL, Kerr ZY, Yengo-Kahn A, Wasserman E, Covassin T, Solomon GS. Epidemiology of sports-related concussion in NCAA athletes from 2009-2010 to 2013-2014: incidence, recurrence, and mechanisms. Am J Sports Med. 2015;43(11):2654–62.
8. Mihalik JP, Bell DR, Marshall SW, Guskiewicz KM. Measurement of head impacts in collegiate football players: an investigation of positional and event-type differences. Neurosurgery. 2007;61(6):1229–35.
9. Mihalik JP, Guskiewicz KM, Marshall SW, Blackburn JT, Cantu RC, Greenwald RM. Head impact biomechanics in youth hockey: comparisons across playing position, event types, and impact locations. Ann Biomed Eng. 2012;40(1):141–9.
10. Moore RD, Lepine J, Ellemberg D. The independent influence of concussive and sub-concussive impacts on soccer players' neurophysiological and neuropsychological function. Int J Psychophysiol. 2017;112:22–30.
11. Stewart WF, Kim N, Ifrah CS, et al. Symptoms from repeated intentional and unintentional head impact in soccer players. Neurology. 2017;88(9):901–8.
12. O'Connor KL, Rowson S, Duma SM, Broglio SP. Head-impact-measurement devices: a systematic review. J Athl Train. 2017;52(3):206–27.
13. Ocwieja KE, Mihalik JP, Marshall SW, Schmidt JD, Trulock SC, Guskiewicz KM. The effect of play type and collision closing distance on head impact biomechanics. Ann Biomed Eng. 2012;40(1):90–6.
14. Caswell SV, Lincoln AE, Stone H, et al. Characterizing verified head impacts in high school girls' lacrosse. Am J Sports Med. 2017;363546517724754.
15. Cortes N, Lincoln AE, Myer GD, et al. Video analysis verification of head impact events measured by wearable sensors. Am J Sports Med. 2017;45(10):2379–87.
16. Marchesseault ER, Nguyen D, Spahr L, Beals C, Razak B, Rosene JM. Head impacts and cognitive performance in men's lacrosse. Phys Sportsmed. 2018;46(3):324–30.
17. O'Day KM, Koehling EM, Vollavanh LR, et al. Comparison of head impact location during games and practices in Division III men's lacrosse players. Clin Biomech (Bristol, Avon). 2017;43:23–7.
18. Lamond LC, Caccese JB, Buckley TA, Glutting J, Kaminski TW. Linear acceleration in direct head contact across impact type, player position, and playing scenario in collegiate women's soccer players. J Athl Train. 2018;53(2):115–21.
19. Lynall RC, Clark MD, Grand EE, et al. Head impact biomechanics in women's college soccer. Med Sci Sports Exerc. 2016;48(9):1772–8.
20. Camarillo DB, Shull PB, Mattson J, Shultz R, Garza D. An instrumented mouthguard for measuring linear and angular head impact kinematics in American football. Ann Biomed Eng. 2013;41(9):1939–49.
21. McCuen E, Svaldi D, Breedlove K, et al. Collegiate women's soccer players suffer greater cumulative head impacts than their high school counterparts. J Biomech. 2015;48(13):3720–3.
22. Cummiskey B, Schiffmiller D, Talavage TM, et al. Reliability and accuracy of helmet-mounted and head-mounted devices used to measure head accelerations. Proc Inst Mech Eng P J Sports Eng Technol. 2017;231(2):144–53.
23. Tyson AM, Duma SM, Rowson S. Laboratory evaluation of low-cost wearable sensors for measuring head impacts in sports. J Appl Biomech. 2018;34(4):320–6.
24. Guskiewicz KM, McCrea M, Marshall SW, et al. Cumulative effects associated with recurrent concussion in collegiate football players: the NCAA Concussion Study. JAMA. 2003;290(19):2549–55.
25. McKee AC, Cantu RC, Nowinski CJ, et al. Chronic traumatic encephalopathy in athletes: progressive tauopathy after repetitive head injury. J Neuropathol Exp Neurol. 2009;68(7):709–35.
26. Broglio SP, Sosnoff JJ, Shin S, He X, Alcaraz C, Zimmerman J. Head impacts during high school football: a biomechanical assessment. J Athl Train. 2009;44(4):342–9.
27. Rowson S, Brolinson G, Goforth M, Dietter D, Duma S. Linear and angular head acceleration measurements in collegiate football. J Biomech Eng. 2009;131(6):061016.
28. Reynolds BB, Patrie J, Henry EJ, et al. Effects of sex and event type on head impact in collegiate soccer. Orthop J Sports Med. 2017;5(4):2325967117701708.
29. Zuckerman SL, Totten DJ, Rubel KE, Kuhn AW, Yengo-Kahn AM, Solomon GS. Mechanisms of injury as a diagnostic predictor of sport-related concussion severity in football, basketball, and soccer: results from a regional concussion registry. Neurosurgery. 2016;63(Suppl 1):102–12.
30. Sakamoto K, Sasaki R, Hong S, Matsukura K, Asai T. Comparison of kicking speed between female and male soccer players. Procedia Eng. 2014;72:50–5.
31. Reynolds BB, Patrie J, Henry EJ, et al. Quantifying head impacts in collegiate lacrosse. Am J Sports Med. 2016;44(11):2947–56.
32. Schmidt JD, Pierce AF, Guskiewicz KM, Register-Mihalik JK, Pamukoff DN, Mihalik JP. Safe-play knowledge, aggression, and head-impact biomechanics in adolescent ice hockey players. J Athl Train. 2016;51(5):366–72.
33. Martini D, Eckner J, Kutcher J, Broglio SP. Subconcussive head impact biomechanics: comparing differing offensive schemes. Med Sci Sports Exerc. 2013;45(4):755–61.
34. Reynolds BB, Patrie J, Henry EJ, et al. Comparative analysis of head impact in contact and collision sports. J Neurotrauma. 2017;34(1):38–49.
35. Urban JE, Flood WC, Zimmerman BJ, et al. Evaluation of head impact exposure measured from youth football game plays. J Neurosurg Pediatr. 2019;1–10.
Keywords:

CONCUSSION; FOOTBALL; HEAD IMPACT FORCES; INJURY PREVENTION; SPORTS INJURY; SPORTS SAFETY

Copyright © 2020 by the American College of Sports Medicine