Gender inequities in sporting environment and resources may distort estimates of sex differences in ACL injury rates

This week, the GenderSci Lab has a new paper out in the British Journal of Sports Medicine critically analyzing the measurement of sex disparities in ACL injury rates in sports. 

Reported to be 2 to 10 times higher among women compared to men, women’s ACL injuries have recently been in the spotlight. In light of several high profile injuries ahead of the Women’s World Cup, media outlets such as the Guardian and the New York Times reported a “crisis of ACL injuries” and characterized ACL injuries as a “curse stalking women’s soccer.” 

The GenderSci Lab’s study, however, reveals systematic biases in a key metric used in estimates of sex disparities in ACL injury rates in sports. The article demonstrates that gendered factors (e.g., availability and quality of resources, compensation structures, ability to train amidst competing responsibilities outside of sports) may undermine the comparability of injury rates between women and men. As a result, recent headlines claiming much higher rates of ACL injury among women and girls may be misleading.

“The case of ACL knee injury shows how gender biases in the construct of exposure time may obscure potential social drivers of gender/sex disparities in ACL and other injuries and lead to inaccurate inferences regarding the causes of these health inequities”

A key metric used to calculate disparities between men and women in ACL injury rates may be biased

Sports scientists calculate rates of injury among women by counting numbers of ACL injuries and dividing that number by an estimate of playing and training time, known as “athlete-exposures.” The study, led by Annika Gompers and Ann Caroline Danielsen, is the first to examine the limitations of the construct of “athlete-exposures” through a gender lens.

The GenderSci Lab team identified two key factors that may skew comparisons of athlete-exposures (AEs) by gender/sex and artificially inflate measures of injury rates among women athletes: training to match ratio and team size (Figure 1). 

Systematic gender differences in both of these factors affect the gender-specific denominator that is used to compute injury rates, with women’s denominator not only being systematically smaller than that of men but also including a higher proportion of time at high risk of injury (i.e., matches).

Key takeaways

• The gendered context of women’s and men’s sports likely contributes to the disparities observed in ACL injury rates.

• Failure to account for gendered structural inequities such as differences in training to match ratio (T:M ratio) and team size between women and men jeopardizes the comparability of gender/sex-specific injury rates.

• Not accounting for these factors may lead to an inflation of injury disparities by gender/sex; incorporating these factors may reveal that ACL injury rates among women are more similar to men than previously thought.  

• Gendered inequities in T:M ratio and team size likely also put women at higher individual risk of injury - but these factors cannot be studied with how most data are currently collected.

• Training and competition exposures should be recorded separately in data collection and researchers using exposure estimates to investigate gender/sex disparities in injury should control for potential gender differences in T:M ratio and team size at the analytical stage. 

• There are multiple gendered factors that can’t be captured by a quantitative measure like AEs (e.g. inequities in compensation structure and quality of equipment and training). These factors should nonetheless be qualitatively considered to contextualize observed gender/sex disparities in injury rates.  

Figure 1: Illustration of how T:M ratio and team size can influence reported injury rates by gender/sex over the course of a hypothetical season, with example AE calculations. The illustration shows that the smaller number of training sessions and smaller roster size for the women’s team compared to the men’s team makes the total number of AEs smaller among women. This means that the same number of injuries in a season appears to be a higher rate of injuries (almost double) among women than men athletes. The individual risk of injury is higher among women (1/20) compared to men (1/28), due in part to a higher proportion of exposure time being at higher risk for injury (matches), and in part to smaller team size. However, this difference is artificially inflated by the aggregate measure of AEs (figure adapted from Danielsen, Gompers et al. 2024). 

In addition to these two factors that may systematically bias comparisons of AEs — and therefore injury rates — by gender, there are many gendered differences in sporting environments that are salient for injury risk and that are unlikely to be captured in quantitative measurements of injury disparities. For example, there are established gender inequities in compensation structures, availability and quality of facilities and equipment, and caliber of training: these factors cannot be neatly incorporated into a measure such as AEs, and require qualitative consideration to make accurate inferences about gender/sex disparities in ACL injury rate. These gendered factors are shaped by the historical underinvestment in women’s sports and likely have a substantial impact on injury risk. 

Advancing our understanding of ACL injury rates among women

The case of ACL knee injury shows how gender biases in the construct of exposure time may obscure potential social drivers of gender/sex disparities in ACL and other injuries and lead to inaccurate inferences regarding the causes of these health inequities.

Public discourse and research aimed at trying to understand the observed higher rates of ACL injuries among women is marked strongly by the biological sex essentialism that so often frames hypotheses around gender/sex disparities in health outcomes — i.e., the assumption that innate sex-linked biological factors alone explain differential health outcomes between women and men. In the case of ACL injuries, biomedical researchers have primarily focused on sex differences in anatomy (such as women’s supposedly wider “Q angle,” measured as the angle between the pelvis and the kneecap, despite evidence that height largely accounts for this disparity) or the influence of the menstrual cycle. 

ACL injuries are debilitating, expensive, and take time away from sport for professional athletes. Women athletes deserve solid statistics and science to understand the contributors to knee injury. The GenderSci Lab’s research contributes to this aim by showing that current research on ACL injury rates among women fails to account for biases in the data produced by gendered factors, which undermines the comparability of injury rates between women and men. This is important because when investigating why rates of ACL injuries are observed to be higher among women, if measures used to quantify them obscure key socially-mediated drivers of gender/sex disparities, promising opportunities for addressing such disparities and preventing injuries may be overlooked. In order to increase the robustness and validity of gender/sex comparisons of injury rates using measures like AEs, we suggest that researchers collect data on and control for potential differences in training to match ratio and team size in their analyses. 

Stay tuned for more from the GenderSci Lab on gender/sex, biology, and musculoskeletal health

The GenderSci Lab’s work on gender/sex disparities and ACL knee injury in women is part of our larger research program, “Sex in Motion,” on gender/sex, biology, and musculoskeletal health. The work will be a focus of a Radcliffe Exploratory Seminar in Spring 2025, “Sexed Bodies in Sport: Accelerating interdisciplinary research on the social dimensions of female athlete health and injury.”  

Our work on gender and ACL knee injury connects two core areas of the GenderSci Lab’s mission: shedding light on gendered, structural determinants of observed sex differences and advancing methods and standards of evidence for measuring gender/sex disparities. 

Author Contributions:

Annika Gompers and Ann Caroline Danielsen drafted the blog post. Members of the GenderSci Lab provided feedback and edits, particularly Sheree Bekker, Madeleine Pape, Trish Homan, Katharine Lee, Abigail Higgins, and Marion Boulicault. Sarah Richardson provided several substantive reviews and revisions. 

Suggested Citation:

Gompers, A., Danielsen, A., and Richardson, S.S, (2024). Gender inequities in sporting environment and resources may distort estimates of sex differences in ACL injury rates. GenderSci Lab Blog. https://www.genderscilab.org/blog/gender-inequities-acl-injury