Although sports-related concussions may occur in contact and non-contact sports, the highest incidence is in contact sports that involve frequent collisions, such as soccer and basketball [17, 18]. The mechanisms by which concussions occur may differ by sport and position, but the most common mechanism of injury is player-to-player contact . In a soccer study, Chandran et al.  reported a 2.5 times greater risk of head injuries from player-to-player contact than from other mechanisms, such as contact with the ball, goalpost, or soccer field. In non-contact sports, there is greater variation in the mechanism of concussion by sport. Many recent studies have reported on the incidence of concussions in various sports [3, 29, 34], and there is sufficient evidence suggesting that sex-based differences affect concussion rates. We previously performed a meta-analysis comparing the incidence of concussion between male and female athletes in different sports .
In addition to the incidence of concussion, injury mechanisms have been shown to differ between male and female athletes . Female athletes are more likely to incur a head injury because of contact with a playing surface or an apparatus than men, while male athletes are more likely to be injured by contact with another player . In the context of concussions, it is important to address biological sex and societal gender because certain behaviors may contribute to health disparities. Differences in injury mechanism between male and female athletes could be due to both differences in physiology and/or differences in the way they play the sport. Our first meta-analysis included 38 studies that reported sex-based differences in the incidence of concussions. Although there is ample evidence describing the problem, there has been no meta-analysis on sex or gender differences for the concussion mechanism or activity across different types of sports. This evidence would help provide reasons behind the growing health problem of sports-related concussions. An understanding of the underlying etiology by pooling data from primary studies across different settings and sport types will inform the development of interventions that can actually reduce concussion rates.
We therefore conducted a systematic review and meta-analysis in which we asked: (1) In which sports are female athletes less likely to experience concussions from player contact? (2) In which sports are female athletes more likely to experience concussions because of ball or equipment contact?
Materials and Methods
Search Strategy and Criteria
A medical librarian (BJ) performed a comprehensive search of the PubMed, EMBASE, and Cochrane Library databases according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines . Search terms were “sex,” “gender,” “sex differences,” “gender identity,” “brain concussion,” “concussion,” “traumatic brain injury,” “sports,” “athletes,” “incidence,” “epidemiology,” “symptoms,” and “injury rate” (see Supplemental Digital Content 1, http://links.lww.com/CORR/A240).
Studies published between January 2000 and December 2018 were included if they reported the incidence of concussion and the sport mechanism or activity for both male and female athletes so that we could perform head-to-head sex and gender comparisons. Other potential studies were identified by manually searching the reference lists of included studies and relevant review articles. Studies were excluded if they were not published in English, reported data on non-sports-related concussions (for example, emergency room settings), or were conference abstracts (grey literature). Two co-authors (JC, KS) independently screened titles and abstracts and reviewed the full text of the article. Any disagreements were resolved through consensus or by a third reviewer (DL). We assessed the quality of studies using the Newcastle-Ottawa Scale for cohort studies . A score was given to each of the following categories: selection, comparability, and outcome. Scores per category were then summed to determine quality.
The following data were independently extracted from full-text articles: type of sport, incidence of concussion, mechanism of concussion (contact with player, surface, ball, or equipment), activity when the concussion occurred (for example, heading, fielding, shooting), population (university or high school), and database used. Each study in the meta-analysis contributed at least two pairs of numbers: the number of concussions in male and female athletes and the total number of male and female athletes. The rate ratio was calculated as the rate of concussions in female athletes/rate of concussions in male athletes. A value greater than 1 indicated that women experienced more concussions than men, while a value less than 1 indicated that men had a higher incidence of concussion. For studies that reported the incidence of concussion for multiple sports or for a different mechanism or activity, data were extracted separately. Thus, some studies contributed multiple sex and gender comparisons to the meta-analysis.
Ten studies were included in the meta-analysis [1, 8, 16, 22, 24, 27, 28, 30, 36, 37]. The PRISMA flowchart includes the search strategy and reasons for exclusion (Fig. 1). All studies were retrospective and used a high school or collegiate national injury surveillance database such as the National Collegiate Athletic Association-Injury Surveillance Program, High School Reporting Information Online, or National Athletic Treatment, Injury, and Outcomes Network (Table 1). The highest number of sex comparisons were found for soccer. Four studies reported concussion data for multiple sports and thus contributed multiple sex comparisons. Track and field, volleyball, and swimming and diving were excluded from the analysis because of a low incidence of concussion (five or fewer). Meta-analyses were conducted for lacrosse, soccer, baseball and softball, basketball, and ice hockey.
All 10 included studies were rated as having fair quality, due to the retrospective design of the studies and because the outcome of interest (the incidence of concussion) was present at the start of the study, which can lead to misclassification of the outcome or exposure (sports participation) (see Supplemental Digital Content 2, http://links.lww.com/CORR/A241).
Data from each study were pooled using the DerSimonian-Laird random-effects model in StataSE version 14 (Stata Corp, College Station, TX, USA). In the presence of heterogeneity, the random-effects model gives more conservative estimates with wider confidence intervals than fixed-effects models because it assumes that the meta-analysis includes only a sample of all possible studies . In addition, the random-effects model accounts for within-study variability (random error) and between-study variability (heterogeneity). Separate meta-analyses were performed for each sport. Summary estimates of the rate ratio comparing women and men were calculated for each of the three concussion mechanisms: contact with another player, playing surface, or ball or equipment. When data were available, the pooled rate ratio was also reported for the sport-specific activity resulting in the concussion. The rate ratio for each comparison and the 95% CI are displayed in forest plots.
Sports with a Lower Risk of Player Contact-induced Concussions in Female Athletes
Female athletes had a lower risk of concussions due to player contact in the sports of basketball, lacrosse, soccer, and ice hockey. Based on a small number of studies [26, 37], the pooled rate ratio estimates indicated that there were no differences between male baseball players and female softball players for concussions due to player contact (rate ratio 0.90 [95% CI 0.51 to 1.60]; p = 0.73). Male basketball players had a higher relative risk of concussion because of contact with another player (rate ratio 0.86 [95% CI 0.76 to 0.97]; p = 0.01). For basketball activity, no differences were found for defending, general play, or shooting (Fig. 2). Male basketball players were more likely to experience a concussion while rebounding than female players were (rate ratio 0.55 [95% CI 0.52 to 0.58]; p < 0.001). However, this estimate was based on only two studies [16, 37] and largely driven by Gessel et al. . Male athletes had a higher relative risk of concussion due to player contact than female athletes in lacrosse (rate ratio 0.33 [95% CI 0.25 to 0.43]; p < 0.001) (Fig. 3), soccer (rate ratio 0.70 [95% CI 0.66 to 0.75]; p < 0.001) (Fig. 4), and ice hockey (rate ratio 0.64 [95% CI 0.56 to 0.73]; p < 0.001) (Fig. 5). Based on two studies [28, 30], the relative risk of concussions sustained while body-checking was higher in male hockey players than in female players (rate ratio 0.30 [95% CI 0.18 to 0.50]; p < 0.001). A few studies had data to specifically evaluate soccer activity, including the concussion mechanism while heading the soccer ball (see Supplemental Digital Content 3, http://links.lww.com/CORR/A0). Male soccer players were more likely than female players to experience concussions because of player contact (rate ratio 0.80 [95% CI 0.72 to 0.90]; p < 0.001) and surface contact (rate ratio 0.54 [95% CI 0.31 to 0.96]; p = 0.04) when heading the ball, although the latter estimate was based on only two studies [8, 24] (Fig. 6).
Sports with a Higher Risk of Ball/Equipment-induced Concussions in Female Athletes
There were no differences between male baseball and female softball players for ball/equipment-induced concussions (rate ratio 0.78 [95% CI 0.57 to 1.05]; p = 0.10) (Fig. 7). Male baseball players were more likely to experience a concussion from being hit by a pitched baseball (rate ratio 0.14 [95% CI 0.12 to 0.16]; p < 0.001), although this finding was mostly based on one large study using injury surveillance databases . On the other hand, female softball players were almost twice as likely to experience a concussion while fielding the ball (rate ratio 1.77 [95% CI 1.17 to 2.70]; p = 0.007). There was no difference between male and female basketball players for surface (rate ratio 0.80 [95% CI 0.57 to 1.12]; p = 0.18) or ball contact (rate ratio 19.34 [95% CI 0.01 to 52085.47]; p = 0.46). Female lacrosse players were more likely to experience a concussion because of contact with the ball or equipment (rate ratio 3.24 [95% CI 2.10 to 4.99]; p < 0.001), as were female soccer players (rate ratio 2.04 [95% CI 1.67 to 2.49]; p < 0.001). When heading the soccer ball, female players were more likely than male soccer players to experience concussions due to ball contact (rate ratio 2.63 [95% CI 1.84 to 3.77]; p < 0.001). No studies reported concussion data on contact with the hockey puck or other equipment, but female hockey players were more likely than male hockey players to experience concussions because of contact with the ice surface (rate ratio 3.00 [95% CI 2.00 to 4.50]; p < 0.001).
The increase in participation of girls and women in sports has led to the publication of two review articles focusing on the possibility of sex or gender differences in concussion incidence [13, 14]. Although there is a large body of evidence on sex and gender differences in the incidence of concussions [3, 29, 34], fewer studies have compared the mechanisms or activities that result in concussions. Knowing the underlying etiology will enable targeted interventions that can reduce this growing health problem. To address the potential increase in injury risk among female athletes, competition rules have been altered to reduce player contact in sports such as women’s lacrosse and ice hockey. The overall findings of this meta-analysis—that concussions in male athletes tend to occur through player contact and concussions in female athletes tend to occur through ball/equipment or surface contact—remain true even for sports that have the same rules of play for men and women (for example, soccer and basketball).
Our meta-analysis has several limitations. We did not include conference abstracts or non-English articles in our search strategy, which may lead to missing unpublished data or important articles. Only a few studies contributed to the pooled estimates in some comparisons, and in these cases they were driven by one or two larger studies. Although the level of consistency was high across studies for concussion mechanisms in soccer and lacrosse, we used a random-effects model to account for any heterogeneity. This approach will lead to wider conference intervals and can reduce the precision around pooled estimates. Because of the paucity of data, we were unable to distinguish between concussions that occurred during competition and practice or at the high school and collegiate levels, which may have varying concussion rates. In addition, all of the studies were conducted in the United States and were rated as having fair quality because of their retrospective design. For lacrosse and ice hockey, the incidence of concussions in male and female players may not be directly comparable given differences in the body contact allowed and the protective equipment worn.
This meta-analysis found that the risk of concussion in male athletes was more likely because of impact with another player in contact sports such as basketball, lacrosse, soccer, and ice hockey. Limited evidence also indicated that male hockey players had a higher relative risk because of body-checking. A few studies presented data that allowed comparing specific differences between male and female soccer player when heading the ball [8, 16, 24, 37]. Male soccer players were more likely than female players to have a heading-related concussion because of player or ground contact. This difference in the mechanism of concussion may be attributed to the fact that men are more inclined to rough play or to adopt an aggressive playing style, which implicates the role of gender, not just sex, and behaviors that contribute to concussions.
In female athletes, most concussions occurred because of contact with the ball or equipment, in lacrosse, soccer, and when heading the soccer ball. Additionally, female hockey players were more likely than male players to experience concussions after hitting the ice surface. There may be biological factors or training differences that result in reduced neck girth and strength relative to head size and head-neck length in female athletes . This decreased neck strength may reduce the athlete’s ability to withstand external forces, such as when heading a soccer ball, and increase acceleration and displacement during impact [4, 15, 18, 32, 33]. One study in high school athletes has shown that neck strength protects against concussions, with each additional pound of neck strength resulting in a 5% reduction in the odds of experiencing a concussion . A similar suggestion has been made for increased neck-torso strength in male athletes, who can better absorb impact throughout the upper body rather than just the head area alone [2, 12, 19]. Sex hormones have also been suggested to play a role in concussions because estrogen and progesterone can alter neuroexcitability, but few studies have been performed in humans [11, 25, 35]. In addition, cerebral blood flow and glucose metabolism have been proposed to explain the sex difference [4, 9, 10]. With no clear factors to possibly explain this difference, future concussion research should focus on how male and female athletes differ physiologically in response to biomechanical forces .
Although rule changes may have contributed to reducing the sex or gender disparity in concussion incidence for certain sports, this intervention alone is not enough to reduce concussion rates. The pattern of the underlying concussion-causing mechanism is the same regardless of whether there are differences in the men’s and women’s game. This finding indicates that further interventions are needed to truly reduce concussion rates, such as the adoption of neck-strengthening exercises or other prevention training, greater enforcement of rules to decrease levels of contact in men’s sports, or the required use of protective head equipment in women’s sports. More research is needed to determine the etiology of concussions to implement injury prevention measures and enact policies that will have a sustained impact on reducing sports-related concussions in athletes. Furthermore, reporting sex/gender differences across sports will encourage sex/gender-specific analyses and possible trials to alter concussion risk in the future. Male and female athletes may require different modes of training or protection to continue safe play.
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