Introduction
The world of sports has historically been dominated by males and much of our knowledge surrounding athletic injuries originates from our understanding of the male athlete. Data on injuries sustained by the female athlete is often underrepresented in sports medicine research, and as the number of female athletes participating in sports continues to grow, there is a significant need for an improved overall understanding of the female athlete and the associated injuries. Specifically, since 1972, the year Title IX was established, there has been a nearly 1000% increase in females playing high-school sports and a 600% increase in female participation in collegiate varsity sports. Female athletes may be predisposed to specific risk factors that increase their chance of injury, such as differences in anatomy and biomechanics compared to males, and these risk factors can be easily overlooked when they are not fully understood. In order to counter these risks, acknowledgment of gender-based differences in injury presentation is necessary for proper injury assessment and treatment to allow for successful return to play and reduced risk of reinjury. A better understanding of injury patterns and an increase in prevention programs tailored specifically to the female athlete are important to ultimately provide the most effective care for this growing population.
With increasing representation of both males and females in a variety of sports, comparative studies focusing on gender differences in injury rates and mechanisms are also increasing. However, female athletes continue to be underrepresented in the development of solutions and interventions based on such studies. For example, FIFA 11+ is a well-established prevention program developed for soccer players to decrease the risk of anterior cruciate ligament (ACL) injuries. In a systematic review, a 30% reduction rate was reported for male soccer players and a 22% reduction was seen in females, but only three of the nine studies reviewed included female athletes. Therefore there is a lack of data for females using this current prevention program, while females are two to eight times more likely to experience an ACL injury than males. Additionally, a separate systematic review reported that current prevention programs for ACL injuries have reduced risk by 85% in males compared with only 52% in females. Continuing to study the differences and applying them to the development of prevention programs and treatment methods specific to the female athlete is imperative in avoiding the physical, psychologic, and financial burdens that come with injuries. A greater understanding of the vast anatomic, physiologic, hormonal, and biomechanical gender differences is key to maintain successful female participation in athletics. This chapter will focus on the epidemiology of female athletic injuries, with the goal of providing clinicians evidence-based data to better understand just how common (or uncommon) certain injuries are, in order to be able to better guide patients, parents, and coaches.
Epidemiology Basics
Numerous studies have reported the epidemiology of injuries in female athletes compared to male athletes. In more recent years, differences in the types of injury, mechanisms of injury, and potential biological and anatomic explanations for differences in rates have also been explored. In 2018, Lin et al. performed a literature review of existing published data that discussed gender differences in epidemiology, risk factors, management, and outcomes among common sports injuries. Interestingly, there was a higher incidence in females in three main injury types: bone stress injuries, ACL injuries, and concussions. In 2019, Brant et al. studied the rates of lower extremity injuries across gender-comparable sports at the high-school level over a period of 10 years. In all the eight sports studied, females had a higher rate of injury than males (9.14 vs. 7.30 injuries per 10,000 athlete exposures (AEs), respectively). Out of all the injuries reported, 56% of them were sustained by females. Additionally, females had a higher injury rate within each individual sport, the highest seen in soccer (15.87), and females sustained a greater number of injuries within each specific injury type in every sport studied, except track and field. Among the incidences, females also experienced a higher percentage of severe injuries, characterized by over 3 weeks of time away from participation or medical disqualification, in each sport except volleyball. This study also noted that among the incidences of injuries, magnetic resonance imaging evaluation was higher in females, relating to an increased medical cost for females who sustained injuries.
In a separate study, Rechel et al. reported the epidemiology of injuries at the high-school level that required surgery over a 5-year period and found an overall rate of 1.45 per 10,000 AEs and 6.3% of all high-school sport-related injuries nationally. Among gender-comparable sports, including soccer, basketball, and baseball/softball, females experienced a higher overall injury rate of 1.20 compared with males at 0.94 per 10,000 AEs ( P = .004) and a significantly greater injury rate within soccer and basketball. Baseball and softball were the only gender-comparable sports where males had a higher rate of injury incidences, but it was not found to be significant. Of note, over half of soccer and basketball injuries were related to the knee and females made up a significantly greater proportion of those knee injuries in both sports (74.4% in soccer and 68.8% in basketball). Overall, 68.7% of injuries sustained by females were of the knee and 1.6% were of the shoulder when compared to males in whom 41.5% of injuries were o the knee and 11.1% were of the shoulder ( P < .001). In addition, females experienced complete ligament sprains at a significantly higher rate than males (54.1% vs. 23.2%; P < .001), whereas males faced more fractures (30.0% vs. 17.5%; P < .001) and dislocations (9.1% vs. 1.1%; P < .001). Notably, 90.8% of complete ligament sprains occurred at the knee. Interestingly, of the 48% of injuries that required medical disqualification for the season, 45.3% were complete ligament sprains while 18.6% of injuries resulting in time loss of less than 1 week from participation were fractures. Furthermore, injuries resulting in medical disqualifications from sports activities for the season occurred in 55.6% of females compared to 45.0% of males. In a separate study, Chan et al. reported the epidemiology of Achilles tendon injuries at the collegiate level. Although the injury rate was comparable between females and males, it is worth noting that females had greater time loss, higher rates of season-ending injuries, and higher operative rates, as well as poorer postoperative performance. Additionally, the recurrence rate among female athletes was almost twice as high compared to male athletes.
In 2018, Baugh et al. described the epidemiology of injuries specific to volleyball teams in the National Collegiate Athletic Association (NCAA) using data from 2013 to 2014 and 2014–15 seasons. Consistent with the previous epidemiologic studies mentioned, female athletes experienced injury at a significantly higher rate overall (7.07 per 10,000 AEs) compared with males (4.69) as well as a higher rate of time-loss injuries, indicated by greater than 24 h of participation restriction, compared with males (2.62 vs. 1.75). The lower extremities were found to be the most frequently injured location in both time-loss injuries and non-time-loss injuries, and females had a higher proportion of lower extremity injuries within both groups. Specific to time-loss injuries, ankle and knee injuries were the most frequent type sustained by female athletes, whereas concussions due to ball contact and hand and wrist sprains occurred most frequently in male athletes. It was noted that the higher injury rate seen in females could be attributed to the higher rates of injury during practices and in preseason compared with males.
With respect to the mechanism of injury, each of the abovementioned studies that analyzed gender differences in this area reported that males were more likely to experience an injury after player-to-player contact, whereas females experienced primarily noncontact injuries. , In a similar sense, a higher rate of overuse-related injuries was found in females compared to greater ball contact-related injuries found in males. Additionally, significantly higher rates of injury were sustained during competition compared to during practice among all athletes. , After providing possible explanations as to why this trend may exist (e.g., increased aggressiveness, exposure to high-risk activities and physical contact, and illegal activity), Rechel et al. recommended developing new rules, strict officiating, and implementing drills in practice that teach high-risk skills to improve these rates. It is worth mentioning that this study also discussed areas to focus on when implementing prevention efforts for male-dominated sports, such as football and wrestling. However, prevention techniques for female athletes and even for gender-comparable sports were not described. For example, the authors provided explanations for understanding the mechanism of shoulder injuries and recommendations for potential solutions for prevention of these injuries, which occurred at a higher rate in male athletes, but at a lower overall rate compared with other injuries. No such suggestions for the prevention of knee injuries were mentioned, which made up the greatest proportion of total injuries requiring surgery and also had significantly higher rates in females. The authors merely suggested that efforts should be made and only cited other studies that have provided explanations as to why the gender differences may exist in complete ligament sprains of the knee. On the contrary, Brant et al. mentioned the importance of improving preventative care particularly for females, given their high risk of lower extremity sports injuries, in order to reduce rates of injury and the impact injuries can have on athletes. Similarly, Baugh et al. discussed possible explanations and specific prevention methods that alluded to the gender differences found in the mechanisms of injury, type of injury, and the time during the season that the injury occurred. The authors also suggested that future research should address these differences.
Concussion Epidemiology: Gender-Based Differences
Studies on concussions have become increasingly popular, particularly in contact sports such as football and hockey, and mainly focus on male athletes involved in these sports. However, female athletes have been shown to have arguably higher rates of concussion incidences, especially in gender-comparable sports. Specifically, one literature review exploring gender differences in concussion incidence found that 9 out of the 10 studies reviewed had reports of higher concussion rates in females. As an entire chapter is dedicated to understand concussions in the female athlete, this section will focus primarily on epidemiologic differences in concussions between males and females.
A study published in 2019 by Kerr et al. described the epidemiology of sport-related concussions across 20 sports at the high-school level during the 2013–14 to 2017–18 school years. Overall, 9542 concussions were reported for an overall injury rate of 4.17 per 10,000 AEs, with football having the highest rate of 10.40. However, among gender-comparable sports (i.e., soccer, basketball, swimming, baseball/softball, cross-country, and track and field), females experienced higher concussion rates than males (3.35 vs. 1.51), as well as a larger proportion of recurrent concussions (9.3% vs. 6.4%). Interestingly, the rate of recurrent concussions decreased over the course of the 5-year study period. This trend is most likely due to legislations implementing mandatory concussion protocols in youth and high-school athletics, which have been passed in all 50 American states and the District of Columbia beginning in 2009. ,
Since the passing of these traumatic brain injury laws, studies have demonstrated an increase in concussion rates, possibly due to increased awareness and reporting, and a decrease in recurrent concussion rates, likely accredited to the improved management of removal from play and return-to-play requirements. It is worth mentioning that studies consistently show higher rates of concussion incidences in females than males over time among gender-comparable sports, both before and an even greater increase after the passing of concussion policies ( P < .001). Similar results were found in a previous study looking at 20 sports among high-school players between 2008 and 2010. Over the course of the study, 1936 concussions were reported for an overall rate of 2.5 per 10,000 AEs and accounted for 13.2% of the total injuries. Comparing all sports, football had the highest percentage of concussions (47.1%) and the highest rate (6.4). Within gender-comparable sports, females had a significantly higher concussion incidence rate than males (1.7 vs. 1.0), and in 18 out of the 20 sports studied, females experienced more recurrent concussions than males.
O’Connor et al. studied high-school concussion trends between 2011 and 2014 and reported a 56% higher incidence rate in females (1.56) than males (1.00) in gender-comparable sports. The researchers also found that although there was no gender difference in the number of symptoms, 44.4% of males reported symptom resolution within less than 7 days versus only 32.2% of females. The authors noted that a possible explanation for this trend could be based off of social norms and their influence on likeliness to report symptoms. For example, male athletes may believe that they are expected to have a “tough attitude” about injury, whereas females may be less likely to be influenced by their peers’ opinions on concussions. , At the collegiate level, Davis-Hayes et al. studied sport-related concussions among the Columbia University varsity athletes in a 15-year retrospective cohort consisting of 68.5% males and 31.5% females. Despite the difference in representation, the prevalence of having at least one concussion was greater in female athletes than in male athletes (23.3% and 17.0%, respectively; P = .01). Notably, in contrast to the previously mentioned studies, no gender differences were found in the return-to-play duration, the number and types of symptoms, or the neurophysiologic test performance. A study by Zuckerman et al. reported that the relative risk of sustaining a concussion among female athletes at the collegiate level was 53% higher in basketball, 83% higher in soccer, and 265% higher in baseball/softball when compared with male athletes. Notably, this same study reported that the risk in women’s lacrosse, a noncontact sport, was 64% higher than that in men’s lacrosse, a contact sport, despite the rule changes implemented in women’s lacrosse to reduce contact. Generally speaking, reports consistently showed player-to-player contact as the most frequent mechanism of concussion injuries in male athletes, with incidence rates between 55% and 77% compared to 40%–53% in females. Player-to-playing surface contact or player-to-ball contact tend to be the most common cause of concussions in female athletes, with incidence rates reported between 34% and 45% compared to 18%–24% in males. , , ,
Interestingly, a retrospective, self-report-based study in collegiate athletes observed association patterns between concussions and musculoskeletal injuries. Overall, those who reported a history of concussion were more likely to also report a history of ankle sprains and knee injuries compared to those with no concussion history ( P = .004). Females were found to have increased reports of concussions, ankle sprains, and knee injuries at rates of 29.5%, 48.9%, and 41.0%, respectively, compared with males with rates of 18.0%, 43.0%, and 23.0%, respectively. Specific to females, those who reported multiple concussions had significantly greater odds of also reporting ankle sprains and knee injuries compared to those without a concussion history (73.5% vs. 39.0%). However, this trend was not seen in females who reported a history of only one concussion and it was not found to be statistically significant in male athletes with a history of either one or multiple concussions. An important limitation of this study is that the type of relationship between concussions and musculoskeletal injuries was unclear with regard to which injury came first, as the study was conducted based on self-reported injury histories without a timeline, but the researchers did determine that a relationship existed in this population.
Many researchers have attempted to provide reasonable explanations for gender-based differences observed in concussion rates, mechanisms, severity, and the associated number of symptoms. A range of potential factors that have been described include neck musculature and strength, cerebral blood flow and cerebrovascular organization, and differences in hormonal environments. , However, a physiologic basis for explaining the gender differences in the risk of sustaining a concussion is controversial because of the very limited evidence of supporting data and minimal understanding of the role these factors may play. Additional studies directly assessing a variety of factors and their relation to observed gender differences in concussion rates are necessary in order to ensure a proper approach toward prevention and treatment methods on an individual basis.
Epidemiology of Upper Extremity Injuries: Gender-Based Differences
Shoulder Instability
While the topic of shoulder instability is described in detail in another chapter, it is important to highlight epidemiologic differences between males and females specific to this common shoulder pathology. Historical studies have typically reported anterior shoulder instability to be a male-dominated injury pattern, and most studies have few (or no) female patients, making it difficult to extrapolate findings to the female patient population. A study by Patzkowski et al. analyzed shoulder instability in 36 collegiate female athletes who underwent surgical intervention. The authors noted that previous reports have shown higher instability rates in males, but they also had an underrepresentation of females in their respective studies. When comparing gender-comparable sports, the rates of shoulder instability were actually equal between males and females. Additionally, compared with previous studies that included both males and females, the authors of this study found a higher rate of labral tears and humeral avulsion of the glenohumeral ligament lesions in this female cohort. When compared with an equivalent population of male athletes from a separate study, females experienced greater injuries while participating in noncontact sports and had more combined (anterior and posterior) labral tears. The authors reported that females were two times more likely than males to experience shoulder instability after contact with an object or contact with the ground, whereas males were more frequently injured due to player-to-player collision.
Researchers have attempted to identify potential underlying neuromuscular differences that may explain gender-based differences in the rates of shoulder instability seen in athletes. Often studied is the condition of general joint hypermobility, which has been suggested as a possible cause for increased incidences of joint instability, but the exact relationship is inconclusive. It is important to note that joint laxity, defined as the physiologic and asymptomatic motion of the glenohumeral joint that allows for normal range of motion, is not the same as joint instability, defined as symptomatic and abnormal motion of the glenohumeral joint that causes pain, subluxation, or dislocation of the shoulder. Furthermore, hypermobility is considered when generalized joint laxity (four or more laxity test results are positive) is associated with musculoskeletal complaints, which appears to be more common in females. One study found an association between general joint hypermobility, measured using the Beighton scale, and glenohumeral joint instability, reporting that patients with hypermobility (a Beighton scale score of 2 or more) were almost 2.5 times more likely to have a history of glenohumeral joint instability. In this study, females were more likely to have general joint hypermobility and higher overall Beighton scale scores compared with males, but interestingly, patient gender was found to be unrelated to having a history of instability. Notably, Reuter and Fichthorn reported that rates of general joint hypermobility were not significantly different between females (16.2%) and males (8.7%), but females did have a significantly higher rate of localized joint hypermobility than males. Additionally, fewer females had a Beighton scale score of 0 compared to males (22.6% vs. 44.2%). However, contrary to the findings by Cameron et al. there was no increased risk for musculoskeletal injury in young female adults with general joint hypermobility. Johnson and Robinson reviewed shoulder instability in patients with joint hyperlaxity and stated that male and female athletes were equally affected by acquired joint hyperlaxity.
Other studies have reported that females undergoing surgery for shoulder instability have lower postoperative scores, greater functional deficits, and poorer outcomes compared with males. , The reason for this is unclear, and certainly, further research is needed to better delineate why female gender is a risk factor for worse outcomes.
Lower Extremity Epidemiology: Gender-Based Differences
Ankle Injuries
Several studies have analyzed the epidemiology of ankle injuries in various sports and reported notable differences within multiple aspects of injury between female and male athletes. Hunt et al. collected prospective data over a 2-year period to evaluate the epidemiology of foot and ankle injuries of NCAA Division I athletes participating in 37 sports. Foot and ankle injuries accounted for 27% of all injuries, and women’s gymnastics, women’s cross-country, women’s soccer, and men’s cross-country were the four sports with the highest incidence rates. Overall, female athletes had a greater incidence of foot and ankle injuries than males (4.07 vs. 3.94 per 1000 AEs; P < .05) and sustained these injuries at a significantly higher rate (53% vs. 47%; P < .05). In addition, females experienced a significantly higher proportion of injuries resulting in at least 1 day of missed participation in comparison to males. Furthermore, the average number of days missed was 71.2 days for females and 43.8 days for males.
Hosea et al. explored gender-based differences in the epidemiology of ankle injuries in basketball players, including at both the collegiate and high-school levels. The authors found that females had a significantly greater risk than males of sustaining an ankle injury (a ratio of 1.25:1) both overall and specifically for Grade I ankle sprains, which accounted for 72% of the total ankle injuries documented ( P = .0001). There was no significant difference in risk between males and females for Grade II and Grade III ankle sprains, fractures, or syndesmosis injuries. Additionally, as the level of competition increased from high school to college, the risk of suffering an ankle injury doubled for both genders. Similar trends were observed in a separate study that investigated the effects of gender, level of competition, and sport on first-time ankle ligament trauma. In this study, a total of 901 high-school and collegiate athletes (544 female and 347 male athletes) participating in basketball, soccer, lacrosse, and field hockey were included for analysis. Ankle ligament sprains due to an inversion injury occurred in 43 athletes (29 female and 14 male athletes), resulting in an incidence rate of 0.85 sprains per 1000 AEs. Overall, the risk of sustaining an ankle sprain was higher in females than in males, but it was not found to be statistically significant. Notably, female basketball players had a significantly greater risk than male basketball players. Contrary to the study by Hosea et al. the relative risk of suffering a first-time ankle injury was similar between high-school and collegiate athletes in this population. On the other hand, it was noted by Beynnon et al. that the complimentary data between these two studies suggests that there could be a gender bias for ankle ligament sprains that may depend on sport. Variations in anatomic alignment and neuromuscular control between males and females in combination with different demands in their respective sports could explain why this trend was observed. Additionally, differences in mechanisms of muscle recruitment that have been studied for ACL injuries have increased awareness that these same differences could also exist in ankle instability.
Anterior Cruciate Ligament Injuries
Perhaps the most common acute injury that has been consistently reported to have a significantly higher incidence in females than males is ACL injuries. Over 50% of annual ACL injuries occur in high-school and collegiate athletes and females are two to eight times more likely than males to sustain an ACL injury. In a systematic review, Montalvo et al. reported rates of ACL injury risk across sports subclassified by the level of contact. In every category, females sustained ACL injuries at a higher rate than males. In contact sports, the total rate of injury was 1.51 per 10,000 AEs (females, 1.88; males, 0.87; P < .0001). For fixed-object high-impact rational landing activities (i.e., gymnastics, obstacle courses in military training and races, etc.), the overall injury rate was 2.62, with significantly higher rates in females than males (4.80 vs. 1.75, respectively, P < .001). In collision sports, limited-contact sports, and noncontact sports, females had higher ACL injury rates than males, but these results were not found to be statistically significant.
A review of the difference between male and female ACL tears reported that the risk of ACL injury in collegiate soccer and basketball players was 4.4%–5% in females versus 1.7% in males. The same review noted gender-based trends of surgical outcomes after ACL reconstruction and found that initial outcomes following surgery were poorer in females, but at 2 year after operation the results appeared to be equal.
Ryman Augustsson et al. conducted a study in a youth athlete population to understand the role played by lower extremity muscle strength in traumatic knee injuries. In their study cohort, the authors found that 17% of females sustained an ACL injury compared to 2% of males ( P = .001), and a significant majority of injured females were among the weak muscle strength group compared to the strong group. There was no difference observed in injury rates between the two weak and strong muscle strength groups within the male cohort. Within the weak muscle group, the rate of traumatic knee injury in females was significantly higher at 0.66 injuries per athlete compared with males who had a rate of 0.19 ( P < .0001); within the strong muscle group, no gender difference was observed. With this data, the authors suggested that weaker lower extremity muscle strength could be an important risk factor for ACL injuries in female athletes and that it should be included in screening for injury prevention training. Additional anatomic and biomechanical risk factors that have been studied include gender differences in quadriceps angles, width of the intercondylar notch, ligament size, knee abduction during landing, and muscle activation patterns, among many others. , ,
Overuse Injury Epidemiology: Gender-Based Differences
Overuse injuries are incredibly common among all levels of female athletes. In a review describing stress fractures in female athletes, Abbott and colleagues assessed risk factors and reported that overall, stress fractures account for up to 10% of orthopedic injuries and 20% of sports medicine injuries. Notably, up to 13% of stress fractures occur in females and between 80% and 95% are in the lower extremities. Several other studies have consistently reported higher rates of stress fracture injuries in females compared with males.
In 2019, Valasek et al. performed a retrospective chart review in pediatric patients to explore age and gender differences of overuse injuries. Males had higher proportions of apophysis, physis, and articular cartilage injuries particularly of the upper extremities, mainly attributable to overhead throwing in baseball, as this was the sport that resulted in the most injuries among males. On the other hand, females had higher rates of injuries to bones, injuries to tendons, and “other” problems, such as patellofemoral pain syndrome, medial tibial stress syndrome, and exertional compartment syndrome. Females additionally had greater lower extremity and pelvis overuse injuries, the majority of which were sustained during track and field and cross-country.
Among some of the most common overuse injuries are stress injuries, including stress fractures and stress reactions, which are caused by cumulative and repetitive impact resulting in abnormal bone remodeling. , Rizzone et al. researched the epidemiology of stress fractures in collegiate athletes over a 10-year period and reported an overall injury rate of 5.70 per 100,000 AEs and a total of 671 incidences of stress fractures during 11,778,145 AEs. Women’s cross-country had the highest injury rate (28.59), followed by women’s gymnastics (25.58) and women’s outdoor track (22.26). Among gender-comparable sports, female athletes had a higher overall rate of stress fractures compared with male athletes (9.13 vs. 4.44), as well as higher rates within basketball, cross-country, soccer, indoor track, and outdoor track. Stress fractures sustained by female athletes occurred at the highest rate during preseason and more often in the foot and lower leg, while male athletes experienced greater stress fractures in the lower back, lumbar spine, and pelvis. Season-ending stress fractures and recurrent stress fractures did not differ between males and females, and they accounted for 20% and 22% of total stress fractures, respectively. Additionally, the researchers observed higher rates of stress fracture incidence in collegiate athletes (5.70 per 100,000 AEs) compared to recent data on stress fracture incidence in high-school athletes (1.54 per 100,000 AEs). ,
Frequently associated with stress fractures is the female athlete triad/RED-S (relative energy deficiency in sport), which consists of three components, namely, low energy availability with or without disordered eating, menstrual dysfunction, and low bone mineral density (BMD). The presence of the components alone or in combination presents a significant health risk to physically active females, and having just one or two of the components can lead to a diagnosis of the female athlete triad. , Furthermore, as the number of female athlete triad-related risk factors an athlete has increases, the cumulative risk for bone stress injury also increases. A study by Nose-Ogura et al. assessed the risk of stress fractures due to the female athlete triad in elite female athletes and found that amenorrhea was present in 39.0% of female athletes, low energy availability was present in 14.0%, and low BMD was present in 22.7% of female athletes. A combined 19.4% of participants had a variation of two out of the three components of the female athlete triad and 5.3% had all three components. The female athlete triad/RED-S and its impact on the female athlete is discussed in detail in a separate chapter. Overall, studies continue to show that all female athletes in general, not just endurance athletes, are at high risk for stress fracture development and various intrinsic and extrinsic risk factors are at play. Awareness of the epidemiology among female athletes and the knowledge of the risks and how these injuries present themselves can aid in an accurate, detailed diagnosis. Additionally, early screening for all contributing factors can facilitate quicker return to play while still allowing efficient bone healing. A multidisciplinary approach toward managing all potential causes of overuse injuries is necessary in creating effective individualized treatment strategies for the female athlete. A compilation of recommendations for evidence-based care can be found in Table 20.1 .
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