ABSTRACT
This narrative review scrutinizes research exploring sex-based differences in experiences of sport-related concussion. The article argues that the limitations of epidemiological studies identifying the greater incidence and severity of concussion among females require that these findings are read with caution. It secondly argues that the dominant explanations for these data are based on and extend historical tendencies to “other” female participation in sport and construct male experiences as the “norm”. Finally, the article critiques policy recommendations related to these research findings, arguing that they are likely to embed rather than challenge sex inequality in sport, and that they are both impractical and unethical. While this commentary builds on a broader body of work advocating greater sex/gender equality in sports science research, the prominence of social concerns about concussions in sport makes the broader implications of the focus on sex and sport-related concussion particularly problematic and thus in need of redress.
Introduction
A major recent development in sport’s concussion crisis has been the increased focus on how sport-related concussion (SRC) affects females. A growing body of research broadly concludes that female athletes experience SRC more frequently, experience a greater number of symptoms and symptoms of greater severity more and more severe symptoms, and take longer than males to recover. The campaign group PINK Concussions (www.pinkconcussions.com) has been established to raise awareness of how concussions specifically impact on female athletes, and the issue has been widely covered in the mainstream (Tomas, Citation2019) and scientific media (Sanderson, Citation2021).
At face value, these studies help address the historical sex imbalance in sport science and medical research. Research has shown that females constitute just 34% of all research subjects across the discipline and that 31% of studies include male only cohorts (Cowley et al., Citation2021). This bias continues despite an almost decade long awareness (Costello et al., Citation2014) and structural/organizational incentives to change. For instance, the National Institutes of Health has required sex as a biological variable to be factored into research designs since 2016. The sex imbalance of existing SRC research is demonstrated by D’Lauro et al. (Citation2022) who found that the literature informing the leading concussion consensus statements relied on study samples that were overall 80.1% male.
While others have discussed the principle of, and best practice for, advancing towards sex/gender equality in sports science research cohorts (Elliot-Sale et al., Citation2021), and SRC research in particular (D’Lauro et al., Citation2022), this review focuses on the consequences of this changing research focus. This article was stimulated by concerns about: a) how the empirical patterns evident in SRC research are interpreted through a comparison between sexes; b) how the explanations for the patterning of data compare to contiguous research areas; and c) the policy proposals that have followed from the theorization of this developing topic. What makes the explanations of the developing research on female athletes and SRC particularly significant is that concussion is widely perceived as a social issue of such significance that political interventions are warranted. No other sport-related injury has been the specific subject of state legislation or instigated so many government inquiries.
This review demonstrates that while this body of research is progressive in rejecting simple extrapolation from male-only data to female experiences, it is regressive in treating female athletes in ways which ultimately perpetuate historically rooted sex discrimination. While focused on the interpretations, explanations and recommendations contained in this research, the analysis has wider ramifications which speak to recent calls to address sex/gender bias across sports science/medicine research (Bekker et al., Citation2018; Cowley et al., Citation2021). The distinct characteristics of both the science of concussion and the social context in which these debates are played out exacerbate the likely unintended but potential negative consequences of this research trend.
Materials and methods
This review followed a four-stage process. The research was initially stimulated by a reading of popular accounts reporting on sex differences in SRC. From these articles, the scientific publications which provided the basis for this public discourse were identified and inductively explored. In March 2022, a literature search was conducted via Google Scholar combining keywords and Boolean operators (i.e., concussion and sex differences and systematic review) and without time limitations to the search period. Google Scholar was chosen for its breadth and cross-disciplinary coverage. The first 50 results (ranked according to relevance) were scrutinized, and this returned four systematic literature reviews and one scoping review directly addressing sex differences in SRC. Finally, these reviews were supplemented by non-review articles identified through the search, a snowball sample of literature identified by examining the reference lists of the retrieved review articles and a series of individual, targeted, literature searches to (e.g. incidence of concussion across sports; sex differences in sports injury rates; concussion and fertility) scrutinize the interpretations developed through the narrative review.
Published reviews were prioritized in this analysis as the most appropriate vehicle through which to gather and synthesize the existing literature. A narrative review was chosen because the aim was to highlight key areas of concern, identify gaps in the research and suggest departures for future research. The aim of the research was not to quantify the prevalence of particular themes or conduct a meta-analysis of existing data, for a number of these already exist. Rather, the objective was to provide a more exploratory and reflexive commentary using insights from the social and ethical accounts of gender and sport to illustrate what is both currently present and absent in the field. The specific literature cited to evidence the points made throughout the text has been chosen because it was deemed to be indicative of the broader trends identified within this research field.
Findings and discussion
How much can be concluded from the existing data?
While the consistency of data regarding the increased frequency of SRC among female athletes has been evidenced through scoping (Blyth et al., Citation2021) and systematic reviews (McGroarty et al., Citation2020) and meta-analyses (e.g. Dave et al., Citation2022; Van Pelt et al., Citation2021), the studies are consistently silent on the limitations of SRC epidemiology. More generally, the problems of accurately charting the incidence of all sport-related injuries are well-rehearsed (e.g. Zech et al., Citation2022), but these problems are particularly pronounced in relation to concussion, where self-diagnosis rather than clinical-diagnosis is common, and where clinical diagnosis cannot be determined by objective measures (e.g. scans or biomarkers). Indicatively, one systematic review of concussion amongst female athletes draws on nine studies, only one of which utilized a clinically diagnosed cohort (McGroarty et al., Citation2020). Compounding these questions of reporting reliability is the longstanding concern that a distinct issue in relation to concussion is the widespread phenomenon of athlete “under-reporting” (e.g. Kerr et al., Citation2018). SRC epidemiology is thus relatively heavily reliant on relatively subjective data.
Because the incidence of concussion is relatively hard to accurately quantify, the data revealing male-female differences need to be read with particular caution. The oft-cited Centers for Disease Control estimate of 1.6–3.8 million sport and recreation concussions per year in the US (e.g. D’Lauro et al., Citation2022) is indicative of the lack of precision possible. Moreover, the rate of reporting concussions has markedly changed over time, for instance, doubling in the NCAA over 15 years (Daneshvar et al., Citation2011) and increasing five-fold across 5 FIFA (Men’s) World Cups (Junge & Dvorak, Citation2015). These changes partly reflect the prominence of concussion education as a prevention strategy. Additionally, it is believed that “psychological and sociocultural factors in sport play a significant role in the uptake of any injury-prevention strategy” (McCrory et al., Citation2017, p. 8), and it is has been demonstrated (especially in responses to Covid) that sex can significantly influence reception to public health messaging. In general, female cohorts are most likely to be compliant and follow proscribed guidance (Galasso et al., Citation2020). As the reported rates of SRC are relatively subjective, temporally variable, and increasingly shaped by public health interventions, comparing the emerging data on female SRCs with the male-dominated studies of the past is inherently problematic.
Despite the limitations, most epidemiologists of sport-related injuries would recognize that rates of concussion are strongly influenced by the type of sport, the rules under which those sports are played, and the level of performance (Van Pelt et al., Citation2021). Moreover, most researchers would likely agree that the rules of a sport are just one aspect of the broader context or environment contributing to injury incidence. Other factors might include team culture, playing resources, coaching and other forms of medical and performance support. Consequently, even studies that explore the impact of biological sex on concussion by controlling for sport rules – which explains why female concussion research frequently focusses on soccer (Blyth et al., Citation2021; Dave et al., Citation2022) – continue to prioritize a single variable; namely biological sex. Correlatively, these studies largely disregard the broader environmental factors which influence rates of sports injury.
How have the findings been explained?
The risk factors commonly identified to explain the greater incidence of SRC among females fall within three broad domains: social, physical and physiological. Proposed social differences primarily relate to the impact of sex/gender on rates of reporting concussion (Zech et al., Citation2022), and differences in training and technique (e.g. heading in football) (Blyth et al., Citation2021). Potential physical factors include differences in visual awareness and relative neck strength (Blyth et al., Citation2021). Hypotheses of physiological difference focus on the structure and composition of men’s and women’s brains (Hannah et al., Citation2021; Van Pelt et al., Citation2021), neck musculature (supporting the skull), and hormonal differences which are, in turn, linked to menstruation (Dave et al., Citation2022; McGroarty et al., Citation2020). What these hypotheses have in common is that they reflect historical tendencies to “other” (de Beauvior, Citation1997) females by treating evidence from research on males as the “norm”.
This “othering” can firstly be illustrated in the disconnection of causal hypotheses contained in female SRC research and the aetiology of SRC more generally. In relation to the broader sporting population, there is no consensus over the relationship between relative (neck) strength and SRC, or consistency between measured force of impact and concussion incidence (as noted, e.g., by Blyth et al., Citation2021 in relation to head acceleration when heading a football). Indeed, what is notable about research on SRC incidence and/or severity is the relative uncertainty in relation to individual risk factors. The current understanding of SRC (based on the historic male research bias) is that there is significant heterogeneity in the way in which individuals experience concussion and symptom manifestation. Conversely, in the development of SRC research on female subjects, hypotheses emphasize female homogeneity.
Secondly, the problems of “othering” female sport participants become more pronounced when we compare the role ascribed to social factors in female SRC literature relative to the role ascribed to social factors in the aetiology of sports injury in general and in the specific field of SRC injury. In both of these fields, the significance of risk factors based on social differences is much more centrally recognized. For instance, many sports injury models propose essentially individual behavioural prevention initiatives, while more advanced multifactorial models (e.g. Meeuwisse, Citation1994) recognize that “sports injuries do not fit a traditional cause-and-effect model” (Parsons et al., Citation2021, p. 986). Conversely, explanations of sporting injuries to females, and only sporting injuries to females, predominantly rely on physiological explanations. The difference has historically characterized the study of ACL injuries (Theberge, Citation2006) but is exemplified in the SRC field. For instance, Hannah et al. (Citation2021) firstly identify sex, player position, competitive level, aggression, and environment as possible risk factors for concussion (among soccer players) but prioritize the role of physiological factors (anatomic, hormonal and demyelination) in explaining the sex-based differences in concussion experiences. Ultimately, comparison of the paradigms forwarded to explain female experiences of SRC with the dominant understandings of both SRC and sport injury more generally reveals the disposition to understand male experience both as the “norm” and socially determined and female experience as being “other” and essentially biological. Typically, this “othering” leads to physiological explanations of female experiences which are at variance with the more broadly biopsychosocial accounts of the aetiology of sports injury per se.
The use of sex-specific explanatory paradigms must be seen against a broader backdrop of sex/gender bias in (sports) medical research (Sims & Heather, Citation2018). Sport medicine research has a long history of problematizing female participation due to concerns in relation to reduced reproductive capacity (Vertinsky, Citation1989). Interestingly, some research linking SRC and female fertility has begun to emerge (e.g. Snook et al., Citation2017), but no such research on SRC and male fertility has yet been published. This is not to say that research highlighting the impact of SRC on females or indeed the role of hormonal variations be avoided. Rather, it is important that this does not obscure other avenues of enquiry. Researchers need to be aware of and embrace the potential unconscious bias drawn from the historical predominance of sex-based explanations of experiential difference in sport.
What recommendations have been made?
Existing recommendations fall into three main categories: a) targeted risk mitigation (Hannah et al., Citation2021) such as training interventions (Master et al., Citation2021) and sex-specific rule changes (Blyth et al., Citation2021); b) targeted responses such as sex-differentiated surveillance, injury management and treatment (Alsalaheen et al., Citation2021; Hannah et al., Citation2021; McGroarty et al., Citation2020); and c) calls for further research.
Targeted risk mitigation
Historically, the rules governing sports have rarely been the target of sport injury prevention strategies (Vriend et al., Citation2017). Their prominence in both public debates about, and policy responses to, SRC is indicative of the unique level of social concern this particular sport injury generates. However, while recognizing that such policy recommendations are likely motivated by a desire to safeguard the health of female athletes, there is a danger that the heightened concern over SRC may be used to justify approaches which in other contexts would be regarded as regressive. Specifically, implementing different training regulations for and/or rule changes which substantiate and formalize sex differences in sports participation run counter to recent developments towards greater equality. When such sex-differentiated interventions have historically been deployed, they have reproduced and reinforced sex/gender inequalities (Lenskyj, Citation1986). Targeted risk mitigation is therefore problematic from ethical and potentially legal viewpoints.
Conversely, there are two practical benefits to targeted risk mitigation which is not sex-segregated. First, where rule changes (e.g. to tackling) or differently regulated forms of training (e.g. restricting contact) can be shown to reduce the risks of SRC (e.g. prohibiting certain forms of tackling), non-sex-segregated measures would reduce concussion overall and thus benefit all sports participants. Second, non-sex-segregated risk mitigation would better align with the broader principles of occupational health and safety in prioritizing the adaptation of work practices to be safe for all participants rather than seeking to preserve the practice of traditional male domains of activity (Theberge, Citation2012). Non-sex-segregated mitigation measures, therefore, would likely be more effective in addressing broader social concerns about SRC.
Targeted responses
Recommendations for the greater surveillance of female athletes are illustrative of the problematic explanatory biases of this field. Specifically, these recommendations logically follow from the (problematic) belief that sex-differentiated experiences stem primarily from physical and physiological risk factors discussed above. Conversely, a belief that the greater incidence and severity of SRC among females (in part) stems from social factors (i.e. greater propensity to report injury) would logically lead to recommendations for male athletes to undergo greater surveillance to ensure compliance. Nowhere in the academic literature or media coverage has it been recommended that the autonomy of male athletes should be subject to greater restrictions than exist for female athletes.
Calls for sex-based differences in approaches to the medical management of concussion contradict existing medical guidance that emphasizes case-by-case treatment (McCrory et al., Citation2017). This recommendation stems from the diverse manifestation and nonlinear trajectory of concussion signs and symptoms. While this knowledge has been largely established through male-dominated SRC research, it aligns with the broader principle that clinical judgement should take relative precedence in areas of greater medical uncertainty (Kienle & Keine, Citation2011).
Additionally, proposals for sex-differentiated concussion management (e.g. extended return to play protocols) may be impractical. More generally there are significant moves towards the standardization of concussion protocols across sports, because cross-sport variations have been deemed to have confused the public and led to higher levels of non-compliance. Institutionalizing different rules, training methods, or medical surveillance for males and females would run counter to recent attempts to generate greater sex equality in sport.
Further research
Calls for further research logically flow from what has already been described. For instance, Blyth et al. (Citation2021) advocate further research to establish whether disallowing heading in training will mitigate the consequences of concussion in female athletes, Van Pelt et al. (Citation2021) note the need to better understand the mechanism of concussion, and Dave et al. (Citation2022) identify the need to explore different physiological responses to biomechanical forces. Where such studies are based on the existing bias towards sex-segregated cohorts, they are likely to continue to treat female SRC as an anomaly or “other”. Consequently, a more radical solution is required to advance SRC science, involving a paradigmatic shift on two fronts.
The paradigmatic shift required firstly entails a move away from single-sex study cohorts. Some evidence of this awareness can be seen in D’Lauro et al.'s (Citation2022) call for the “intentional recruitment and funding of gender diverse participants in concussion studies” (p. 1), but this approach is not yet widespread. It would now be recognized that sex-segregated sport and sports science can only be justified if and where it facilitates greater equality between the sexes (for instance, a potential argument against sex de-segregation could be that it would, initially at least, marginalize females from the socially most significant sporting competitions.). Moreover, it is only through study designs which support sex-disaggregation (Cowley et al., Citation2021) that existing hypotheses in relation to physiological, physical and social factors can be rigorously tested.
Secondly, the paradigmatic shift requires a different conceptualization of the relationship between biological and social factors. Calls for a move away from studies in which the so-called intrinsic (e.g. brain structure and composition, hormones) and extrinsic (rules, equipment, reporting behaviour) risk factors are compartmentalized (Parsons et al., Citation2021), and the advocacy of studies in which the bio-, psycho- and social determinants of behaviour are (a) understood to be interdependent and (b) treated (relatively) equally, are particularly relevant to the study of SRC.
Conclusion
Attempts to correct the historical sex-bias of SRC research are both welcome and accord with broader trends in scientific practice and social development. The central concerns raised here stem from a desire to maximize the gains of this progressive development. To do so, we must recognize the constraints of the broader research culture in which such sex segregation has historically been the norm. Challenging current approaches enables us to more clearly see that “strictly biological explanations [of injury] may be insufficient to improve outcomes for girls and women” (Parsons et al., Citation2021, p. 984).
The social context in which SRC research exists presents additional challenges. It has been widely argued that the depth of public and political concerns about SRC has meant that the translation of scientific findings for mass consumption has been particularly problematic (Ahmed & Hall, Citation2017). While media outlets must shoulder much of the responsibility for reporting accuracy, the scientific community’s duty of care necessitates cognizance of how research might be represented, and the potential consequences once findings are publicly disseminated. While the emerging research on SRCs amongst females may have been inspired by concerns to address female health and/or sex-bias in sports science, paradoxically in its current form it has the potential to restrict female freedoms, perpetuating and extending inequalities.
Equally, the public consumption of this research may also have unintended consequences for the medical management of SRC in male athletes. Specifically, by “othering” female athletes and emphasizing the relative frequency and/or worse outcomes of SRC, this research could be interpreted as suggesting that male concussions are within a “normal” range, and thus of lesser (or no) concern. A potential consequence of recent sex-based comparisons has been to direct attention away from a particular cohort of sports participants who, in absolute terms, experience more concussions and about whom the existing data suggest a more compelling link to longer term neurodegenerative conditions (Mackay et al., Citation2019; Mez et al., Citation2017; Ueda et al., Citation2023).
Finally, applying an ethical lens to these findings leads us to compare the calls to protect women from brain injury in (certain) sports, with a broader agenda to regulate more closely those sports which epidemiological data shows to be most dangerous. The ethical choices implicitly made in the recommendations that have followed this former body of research are that the risks of SRC are so much more pronounced (and reliably documented) that it is justifiable to implement measures which will restrict female sport participation. The libertarian principles, which ultimately continue to legitimize participation in boxing and other relatively injurious sports, should not be applied differently to males and females. While “history compels us to ensure that decisions about genetic superiority are supported by objective, rigorous, and reproducible data” (Tannenbaum & Bekker, Citation2019, p. 1120), we are equally compelled to shape the future of sport participation in the light of ethical principles regarding equality and social justice.
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