A comparison of injuries in elite male and female football players: A five- season prospective study

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1 Accepted: 13 February 2017 DOI: /sms ORIGINAL ARTICLE A comparison of injuries in elite male and female football players: A five- season prospective study J. Larruskain 1 J. A. Lekue 2,3 N. Diaz 2 A. Odriozola 1 S. M. Gil 3 1 Department of Genetics, Physical Anthropology and Animal Physiology, Faculty of Science and Technology, University of the Basque Country (UPV/EHU), Leioa, Spain 2 Medical Services, Athletic Club, Lezama, Spain 3 Department of Physiology, Faculty of Medicine and Nursing, University of the Basque Country (UPV/EHU), Leioa, Spain Correspondence Susana M. Gil, Department of Physiology, Faculty of Medicine and Nursing, University of the Basque Country (UPV/ EHU), Leioa, Spain. susana.gil@ehu.eus Funding information Vice-Chancellorship for Basque of the University of the Basque Country UPV/ EHU (Euskararen arloko Errektoreordetza) The aim was to compare the epidemiology of injuries between elite male and female football players from the same club. Injuries and individual exposure time in a male team and a female team, both playing in the Spanish first division, were prospectively recorded by the club s medical staff for five seasons ( ) following the FIFA consensus statement. Total, training, and match exposure hours per playerseason were 20% higher for men compared to women (P<.01). Total, training, and match injury incidence were 30%- 40% higher in men (P.04) mainly due to a 4.82 (95% confidence interval [CI] ) times higher incidence of contusions, as there were no differences in the incidence of muscle and joint/ligament injuries (P.44). The total number of absence days was 21% larger in women owing to a 5.36 (95% CI ) times higher incidence of severe knee and ankle ligament injuries. Hamstring strains and pubalgia cases were 1.93 (95% CI ) and (95% CI ) times more frequent in men, respectively; whereas quadriceps strains, anterior cruciate ligament ruptures, and ankle syndesmosis injuries were 2.25 (95% CI ), 4.59 (95% CI ), and 5.36 (95% CI ) times more common in women, respectively. In conclusion, prevention strategies should be tailored to the needs of male and female football players, with men more predisposed to hamstring strains and hip/groin injuries, and women to quadriceps strains and severe knee and ankle ligament injuries. KEYWORDS man, sex, soccer, woman 1 INTRODUCTION Football offers multiple physiological and psychosocial benefits, but unfortunately the risk of injury is also high. 1 Injuries negatively impact team performance 2 and economy 3 and might have long- term health consequences affecting the quality of life of players. 4 Therefore, preventing injuries is a priority in football. The first step in injury prevention is to describe the epidemiology of injuries, 5 and while the literature on male players is extensive, 6 evidence in female players is scarce 7,8 and mainly focused on anterior cruciate ligament (ACL) injuries. 9,10 Moreover, only one previous study has directly compared the injury epidemiology between male and female players at the elite European club level during the same period using the same study design, 11 concluding that Swedish male players had 20%- 40% higher injury incidence compared to their female counterparts even though the rate of moderate and severe injuries was similar. In contrast, there were no sex differences in the overall injury incidence in National Collegiate Athletic Association (NCAA) soccer, but the proportion of severe injuries was higher in women than in men. 12 Studies investigating specific injuries have reported that hamstring strains 13 and groin injuries 14 are more common in male players, whereas ACL injuries 10 and concussions 15 are more frequent in female players. Scand J Med Sci Sports. 2018;28: wileyonlinelibrary.com/journal/sms 2017 John Wiley & Sons A/S. 237 Published by John Wiley & Sons Ltd

2 238 LARRUSKAIN et al. However, no study has compared the epidemiology of injuries in male and female football players from the same club and with medical staffs following the same procedures for diagnosing, treating, and recording injuries. This is important as there are discrepancies between medical staffs at different clubs in their injury diagnosis, return to play, and registration procedures, which might be a major source of bias in the comparison. 16 Unraveling sex differences in injury epidemiology would be valuable to tailor prevention programs, return to play strategies, and generate hypotheses on plausible risk factors. Thus, the aim of this study was to compare the incidence, distribution, and severity of injuries between elite male and female football players from the same club over several seasons. 2 MATERIALS AND METHODS 2.1 Participants and study period Athletic Club s elite male and female teams, both playing in the Spanish first division, were prospectively followed for five seasons (from July 2010 to June 2015). The female team finished all five seasons in the top 3 of the league and reached the Spanish cup finals in two occasions. The male team reached the Spanish Cup finals twice and UEFA Europa League finals once and participated in the UEFA Champions League in the last season. The study was approved by the Ethics Committee of the University of the Basque Country (CEISH/340/2015). 2.2 Injury definitions and recording procedures The club s medical staff, which remained the same for all five seasons, diagnosed, treated, and recorded all time- loss injuries, following the consensus on definitions and data collection procedures outlined by the International Federation of Association Football (FIFA). 17 Specifically, the male team was supported by two team doctors, four physiotherapists, and one pitch rehabilitator who is responsible for introducing injured players to the drills and skills that will be required when they return to full participation, whereas there were one team doctor, one physiotherapist, one masseur, and one pitch rehabilitator for the female team. Both medical staffs followed common injury diagnosis, treatment, and registration procedures, and they were present at every training session and match. The male and female teams trained at the same facilities, and medical staffs shared the same working environment, regularly interacting to discuss injury- related matters Injury definitions Injuries were recorded in the club s online database when a player was unable to participate in a future training session or match due to a physical complaint resulting from football training or match play and was considered injured until the medical staff cleared the player for full participation in training and match play. Injuries during national team duties were included. Illnesses were excluded Injury location and type Injury location was recorded as follows: head/neck, upper limbs, trunk, and lower limbs (hip/groin, thigh, knee, lower leg, ankle, foot/toe); and injury type as follows: muscle, joint/ ligament, fracture/bone stress, tendon, contusion, laceration/ skin, and concussion. 17 The specific injury diagnosis was also recorded Injury mechanism and circumstance Injuries with an acute, identifiable onset were defined as traumatic injuries, while those with a gradual onset as overuse injuries. Contact and non- contact injuries were recorded depending on whether injuries occurred due to contact with another player or object, or not Recurrent injuries Injuries at the same location and of the same type as an index injury were classified as early (0-2 months after a player s return to full participation), late (2-12 months), or delayed (>12 months) recurrences Injury severity According to the number of days lost, injury severity was recorded as minimal (1-3 days), mild (4-7 days), moderate (8-28 days), and severe (>28 days) Individual training and match exposure time Individual player exposure time in training and matches (friendly and competitive), including national team exposure, was recorded daily in minutes. 2.4 Data analysis Injury incidences are presented as the number of injuries/1000 player hours with 95% confidence intervals (CI) and were compared calculating rate ratios (RR=injury incidence of male players divided by the injury incidence of female players) with 95% CI and tested using z- statistics. 18 Proportions were compared using the χ 2 test with Yates continuity correction. Due to a non- normal distribution, differences in days lost were compared using the Mann- Whitney U test. Team differences in player characteristics, exposure time, and the percentage of player match availability were

3 LARRUSKAIN et al. compared using Student s t test. Each season was divided into three periods: preseason (July and August), competitive phase I (from September to January), and competitive phase II (from February to June). Sample size calculations showed that with 80% power and a two- sided significance of 0.05, to detect significant differences between overall injury incidences of 7.7 injuries/1000 h in men and 5.5 injuries/1000 h in women, h of exposure per team would be required. 18 The significance level was set at P<.05, and statistical analyses were performed using Microsoft Excel 2011 (Microsoft, Redmond, WA, USA) and GraphPad Prism v.6.0c (GraphPad Software, La Jolla, CA, USA). 3 RESULTS Player and team characteristics are presented in Table 1. Total, training, and match exposure hours per player- season were 17%- 20% higher for men compared to women (P<.05). Furthermore, the male team had 35% more training sessions (P<.01) and 30% more matches (P=.02) per week, but the match hours/total hours of exposure ratio was similar (P=.94). 3.1 Injury incidence Total, training, and match injury incidence were 30%- 40% higher in men (P.04, Table 2). The proportion of contact injuries was higher for men (P=.03), but there were no sex TABLE 1 Player and team characteristics and exposure time Men Women Total nº of players Player seasons Age (years) 25±4 25±5 Height (cm) 182±6* 167±6 Body mass (kg) 76±6* 59±7 Team size 25±3* 20±1 Exposure Total h Training h Match h Total h/player/season 306±80* 257±82 Training h/player/season 264±70* 221±71 Match h/player/season 42±25* 36±16 Training sessions/week 5.3±0.4* 3.9±0.2 Matches/week 1.3±0.2* 1.0±0.1 Match exposure ratio # 0.136± ±0.043 h: hours. *P<.05. # Match hours/total hours of exposure. Values are mean±sd. 239 differences in the proportion of traumatic and overuse injuries (P=.47), or in the percentage of recurrent injuries (P.07). Total injury incidence was significantly higher in men during the first and second competitive phases (Figure 1). The mean percentage of player match availability per season was similar for men and women (88±4% vs 88±8%, respectively, P=.94). 3.2 Injury location and type The incidence of muscle and joint/ligament injuries was similar between men and women, but contusions were five times more frequent in men (Table 3). Regarding muscle injuries, men had almost a two times higher incidence of hamstring strains, while women had a two times higher incidence of quadriceps strains, with these being the most frequent injuries for each sex, respectively. Moreover, men had an 11 times higher incidence of pubalgia cases; and there were statistical trends for hip/groin injuries to be 1.58 times more frequent in men, and for ACL ruptures to be almost five times more common in women. Even though there were no sex differences in the incidence of ankle ligament sprains, the incidence of ankle injuries affecting the syndesmosis was higher for women (men [n=2] 0.05 vs women [n=7] 0.28/1000 h, RR=0.19 [95% CI ], P=.04). No concussions were recorded in men or in women. 3.3 Injury severity Women had a 21% larger number of total days lost (Table 4), and almost a two times higher total injury burden (men: 116 days lost/1000 h vs women: 216 days lost/1000 h, P<.01). In addition, while the incidence of minimal and mild injuries was significantly higher in men, women suffered a higher proportion of severe injuries (Table 2) and had an almost five times higher incidence of severe joint/ligament injuries (men [n=7] 0.2 vs women [n=21] 0.8/1000 h, RR=0.22 [95% CI ], P<.01). Specifically, severe injuries affecting the ligaments of the knee and ankle were more frequent in women (for both injuries: men [n=2] 0.05 vs women [n=7] 0.28/1000 h, RR=0.19 [95% CI ], P=.04). Among severe knee ligament injuries, there were two ACL ruptures in men, and six ACL ruptures and one medial collateral ligament sprain in women. Regarding severe ankle ligament injuries, men sustained two lateral sprains, and women suffered two medial sprains, two syndesmosis injuries, two sprains affecting both the lateral ligament and the syndesmosis and one sprain affecting both the medial ligament and the syndesmosis. ACL ruptures were responsible for more than 40% of all absence in women (Table 4), and joint/ligament injuries caused a higher percentage of all absence in women in comparison with men. Conversely, lay- off times were more distributed across injury types in men, with hamstring strains being the predominant

4 240 LARRUSKAIN et al. TABLE 2 Injury incidence in elite male and female football players Injuries Men Women Number (%) Incidence (95% CI) Number (%) Incidence (95% CI) Rate ratio (95% CI) P value a Total ( ) ( ) 1.32 ( ) <.01 Training 160 (50) 4.78 ( ) 75 (47) 3.43 ( ) 1.39 ( ).02 Match 161 (50) ( ) 80 (50) ( ) 1.32 ( ).04 Mechanism Traumatic 156 (48) 4.01 ( ) 81 (51) 3.19 ( ) 1.26 ( ).09 Overuse 163 (50) 4.19 ( ) 72 (45) 2.84 ( ) 1.48 ( ).01 Circumstance Contact 92 (28) # 2.37 ( ) 30 (19) 1.18 ( ) 2.00 ( ) <.01 Non-contact 226 (70) # 5.81 ( ) 125 (78) 4.92 ( ) 1.18 ( ).14 Recurrence No 224 (69) 5.76 ( ) 124 (78) 4.88 ( ) 1.18 ( ).14 Yes 99 (31) 2.55 ( ) 36 (23) 1.42 ( ) 1.80 ( ) <.01 Early 40 (12) 1.03 ( ) 11 (7) 0.43 ( ) 2.38 ( ).01 Late 41 (13) 1.05 ( ) 13 (8) 0.51 ( ) 2.06 ( ).02 Delayed 18 (6) 0.46 ( ) 12 (8) 0.47 ( ) 0.98 ( ).96 Severity Minimal 100 (31) # 2.57 ( ) 25 (16) 0.98 ( ) 2.61 ( ) <.01 Mild 95 (29) 2.44 ( ) 35 (22) 1.38 ( ) 1.77 ( ) <.01 Moderate 91 (28) 2.34 ( ) 64 (40) * 2.52 ( ) 0.93 ( ).65 Severe 37 (11) 0.95 ( ) 36 (23) * 1.42 ( ) 0.67 ( ).09 CI: confidence interval. a P value for the comparison of injury incidence between men and women. # Higher injury proportion in men vs women (P<.05). *Higher injury proportion in women vs men (P<.05). Missing data, men: training/match=2, mechanism=4, circumstance=5; women: training/match=5, mechanism=7, circumstance=5. source of absence and muscle, tendon and contusion injuries accounting for a higher proportion of days lost compared to women. Finally, the median absence caused by muscle and joint/ligament injuries was larger in women. FIGURE 1 Distribution of total injury incidence across the season in elite male and female football players. PS: preseason, CP: competitive phase. Higher injury incidence in male players compared to female players during CP I (* RR=1.35, 95% CI , P=.04) and CP II ( # RR=1.33, 95% CI , P=.05) 4 DISCUSSION %- 40% higher injury incidence in men The higher overall injury incidence in men is in agreement with the only previous study prospectively comparing injury incidence between men and women at the elite European club level in football, where Hagglund et al. 11 reported a 20%- 40% higher injury incidence in men, but only in the incidence of minimal and mild injuries. A higher injury incidence was also observed in the United States Major League Soccer 19 in comparison with the Women s United Soccer Association league. 8 Junge & Dvorak 20 investigated sex differences in

5 LARRUSKAIN et al. 241 TABLE 3 Injury incidence according to location and type in elite male and female football players Men Women Injuries Number (%) Incidence (95% CI) Number (%) Incidence (95% CI) Rate ratio (95% CI) P value a Location/Diagnosis Head/neck 5 (2) 0.13 ( ) 3 (2) 0.12 ( ) 1.09 ( ).91 Upper limbs 8 (2) 0.21 ( ) 2 (1) 0.08 ( ) 2.61 ( ).22 Trunk 25 (8) 0.64 ( ) 8 (5) 0.32 ( ) 2.04 ( ).08 Low back pain 12 (4) 0.31 ( ) 6 (4) 0.24 ( ) 1.31 ( ).59 Lower limbs 285 (88) 7.33 ( ) 147 (92) 5.79 ( ) 1.27 ( ).02 Hip/groin 63 (20) 1.62 ( ) 26 (16) 1.02 ( ) 1.58 ( ).05 Adductor strain 35 (11) 0.90 ( ) 19 (12) 0.75 ( ) 1.20 ( ).52 Pubalgia 17 (5) # 0.44 ( ) 1 (1) 0.04 ( ) ( ).02 Thigh 94 (29) 2.42 ( ) 48 (30) 1.89 ( ) 1.28 ( ).17 Hamstring strain 59 (18) 1.52 ( ) 20 (13) 0.79 ( ) 1.93 ( ).01 Quadriceps strain 17 (5) 0.44 ( ) 25 (16)* 0.98 ( ) 0.44 ( ).01 Knee 37 (11) 0.95 ( ) 26 (16) 1.02 ( ) 0.93 ( ).78 Ligament injury 11 (3) 0.28 ( ) 14 (9)* 0.55 ( ) 0.51 ( ).10 ACL rupture 2 (1) 0.05 ( ) 6 (4)* 0.24 ( ) 0.22 ( ).06 Meniscus/cartilage 7 (2) 0.18 ( ) 8 (5) 0.32 ( ) 0.57 ( ).28 Lower leg 34 (11) 0.87 ( ) 18 (11) 0.71 ( ) 1.23 ( ).47 Calf strain 19 (6) 0.49 ( ) 13 (8) 0.51 ( ) 0.95 ( ).90 Ankle 43 (13) 1.11 ( ) 24 (15) 0.95 ( ) 1.17 ( ).54 Ligament injury 37 (11) 0.95 ( ) 20 (13) 0.79 ( ) 1.21 ( ).50 Lateral sprain 21 (7) 0.54 ( ) 10 (6) 0.39 ( ) 1.37 ( ).41 Foot/toe 14 (4) 0.36 ( ) 5 (3) 0.20 ( ) 1.83 ( ).25 Type Muscle 143 (44) 3.68 ( ) 84 (53) 3.31 ( ) 1.11 ( ).44 Joint/ligament 83 (26) 2.13 ( ) 58 (36)* 2.28 ( ) 0.93 ( ).69 Contusion 59 (18) # 1.52 ( ) 8 (5) 0.32 ( ) 4.82 ( ) <.01 Tendon 16 (5) 0.41 ( ) 6 (4) 0.24 ( ) 1.74 ( ).25 Fracture/bone stress 4 (1) 0.10 ( ) 3 (2) 0.12 ( ) 0.87 ( ).86 Laceration/skin 1 (0.3) 0.03 ( ) ACL: anterior cruciate ligament, CI: confidence interval. a P value for the comparison of injury incidence rates between men and women. # Higher injury proportion in men vs women (P<.05). *Higher injury proportion in women vs men (P<.05). injury epidemiology in national team tournaments, and the injury rate was higher for men in FIFA World Cups, but not in other tournaments. In NCAA soccer, a recent study by Roos et al. 12 showed no sex differences in injury incidence, whereas earlier studies separately reporting data on male 21 and female 7 players showed a higher incidence of match injuries in men and a higher incidence of training injuries in women. As the incidence of muscle and joint/ligament injuries was similar between men and women, differences in the overall injury rate appear to be explained by the five times higher incidence of contusions in men. The higher incidence of contusions is also likely the reason why men had a higher incidence of minimal and mild injuries, as 83% of contusions were minimal or mild in severity (data not shown). The higher rate of contusions in men might be due to higher intensity and more contact situations in male football, 11 together with the fact that men appear to have a two times higher risk of suffering an injury in tackling situations. 22 In addition, the higher match congestion and larger training and match exposure per player- season observed in male players have been previously associated with a higher injury incidence. 23

6 242 LARRUSKAIN et al. TABLE 4 Consequences of injuries according to location and type in elite male and female football players Men Women Injuries Total days lost % total days lost Days lost per injury a Total days lost % total days lost Days lost per injury a Location/Diagnosis Head/neck (1-4) (1-10) Upper limbs 97 2 # 6 (2-48) (13-74) Trunk # 4 (1-18) (1-22) Low back pain 74 2 # 5 (1-18) (1-22) Lower limbs (1-253) * 13 (1-836)* Hip/groin # 8 (1-72) (2-48) Adductor strain # 9 (1-42) (2-48) Pubalgia # 8 (1-72) Thigh # 7 (1-86) (2-74)* Hamstring strain # 9 (1-86) (3-74) Quadriceps strain (2-34) * 18 (2-61) Knee (1-253) * 20 (1-836)* Ligament injury (1-190) * 33 (2-836) ACL rupture ( ) * 256 ( ) Meniscus/cartilage (1-118) 454 8* 42 (4-208) Lower leg (1-107) (2-131)* Calf strain (1-107) 419 8* 19 (6-131)* Ankle (1-237) (2-106)* Ligament injury # 5 (1-237) (2-106)* Lateral sprain # 6 (1-31) (2-52) Foot/toe # 5 (1-57) (4-66) Type Muscle # 7 (1-107) (1-131)* Joint/ligament (1-237) * 15 (1-836)* Contusion # 3 (1-57) (1-13) Tendon # 14 (1-253) (2-19) Fracture/bone stress # 25 (5-99) (4-74) Laceration/skin Total (1-253) (1-836) ACL: anterior cruciate ligament, CI: confidence interval. a Days lost per injury are presented as median (range). # Higher proportion of days lost or larger absence per injury in men vs women (P<.05). *Higher proportion of days lost or larger absence per injury in women vs men (P<.05). 4.2 Higher incidence of severe injuries and longer absences in women Despite sustaining half the amount of injuries and having a squad with five fewer players per season compared to men, the total number of days lost was 21% larger in women. Moreover, the proportion of severe injuries was significantly higher in women in agreement with data from NCAA soccer, 12 and injury burden, expressed as the numbers of days lost per 1000 hours of exposure, was almost two times larger in women. Such long lay- off times in women are explained by the higher incidence of severe joint/ligament injuries, with the almost five times higher risk of ACL rupture being the main reason. In line with these findings, previous studies have reported a 2-3 times higher risk of sustaining ACL injuries in female football players, 10 potentially due to sex differences in biomechanics and neuromuscular control of the trunk, hip, and knee. 9 Considering that ACL ruptures

7 LARRUSKAIN et al. accounted for 43% of all absence in women, there is no question that prevention strategies should focus on this injury, not only to avoid lengthy player absences, but also to prevent knee osteoarthritis. 4 Severe ankle ligament injuries were also more frequent in women. In particular, women sustained a five times higher incidence of ankle injuries affecting the syndesmosis, which are known to induce much longer lay- off times compared to other ankle ligament injuries in elite male players. 24 In contrast, Waterman et al. 25 investigated risk factors for syndesmotic and medial ankle sprains in the United States Military Academy and reported a higher rate of syndesmotic sprains in male cadets playing intercollegiate soccer compared to their female counterparts. Considering the methodological discrepancies between the present study and that of Waterman et al. (eg, population of military cadets, athlete exposure as unit of exposure, inclusion of first- time injuries only), and the small number of syndesmosis injuries in both studies, more evidence is needed to draw any decisive conclusion. Furthermore, sex differences in recovery time from injury are likely to exist, 26 and understanding them is necessary to adapt return to play strategies. In the present study, return to play times from thigh muscle strains, ACL ruptures, and calf strains was longer in women than in men, but with the exception of calf strains, there were no statistically significant differences. Recovery time was similar between sexes for adductor strains and ankle lateral sprains. Previous studies in football players have not found sex differences in return to play time from ACL injury 27 or hamstring strain, 28 even though there is evidence suggesting that women recover slower from concussions. 29 However, this question remains to be answered under more strict experimental conditions, controlling for initial injury severity and standardizing return to play protocols. A final point to consider is that the female team had five fewer players per season. A lower number of available players might have led to some female players playing hurt or not fully recovered from minor injuries, potentially increasing the risk of sustaining severe injuries. 4.3 Hamstring strains and pubalgia cases were more frequent in men In contrast to women, absence time was more distributed across injury types in men, with muscle, tendon, and contusion injuries accounting for a higher percentage of all absence. In particular, hamstring strains were the most common injury and the major source of absence in men. The 93% higher rate of hamstrings strains observed in male players is in accordance with a study by Cross et al. 13 where NCAA male soccer players were 64% more likely to sustain a hamstring strain compared to their female counterparts. Conversely, Hagglund et al. 11 did not find significant differences. As postulated by Cross et al., 13 male players perform 243 more high intensity runs and at higher absolute intensities, 30 showing a slower recovery from them. 31 Greater fatigue in men might lead to a decreased eccentric hamstring strength 32 and changes in sprinting mechanics, 33 which are thought to increase the risk of hamstring strain. 34 Regarding injuries to the hip/groin area, a review on the epidemiology of groin injuries in senior football revealed a more than twofold higher rate in male compared to female footballers. 14 In the present study, hip/groin injuries were 1.58 times more frequent and accounted for a significantly larger proportion of days lost in men than in women. Specifically, the incidence of adductor strains was similar between men and women, but the incidence of pubalgia cases was 11 times higher in men. The predominance of athletic pubalgia in men has been well described and attributed primarily to sex differences in pelvic anatomy with women having a lighter and wider pelvis and larger subpubic angle (the angle between the inferior pubic rami) helping in the transference of destabilizing forces away from the pubic region to the lower extremities. 35 Diagnosing groin injuries is complex, and the heterogeneous taxonomy used by clinicians and researchers is confusing. The use of the term pubalgia was not recommended in the recent Doha agreement meeting on terminology and definitions in groin pain in athletes. In this regard, the term pubalgia used in the present study included all four clinical entities for groin pain defined in the Doha agreement meeting (adductor- related, iliopsoas- related, inguinal- related, and pubic- related groin pain) Higher incidence of quadriceps strains in women Quadriceps strain was the most common injury in this female team, being two times more frequent compared to men. In contrast, previous studies in Scandinavian female football reported a four times lower incidence of quadriceps strains, and hamstring strains were 3-4 times more common than quadriceps strains. 11,37 The reason for this regional discrepancy is unknown, but could be due to extrinsic factors, such as playing style, climate, or training content. Kicking has been pointed out as the primary mechanism of quadriceps strain, and the activation of hip flexors seems important to protect the quadriceps during the swing phase. 38 In this sense, women show a lower iliacus activation compared to men during ball kicking, 39 which might predispose women to quadriceps strains and men to hip/groin injuries. The fact that men and women play with a ball of the same weight also needs to be considered, as women kick a relatively heavier ball. Sex differences in training content may be another important factor (eg, the type and amount of kicking, strength, or preventive training), but training content data were not collected and it is a limitation of the study.

8 244 LARRUSKAIN et al. 4.5 Methodological considerations The present study provides a robust sex comparison of injury epidemiology in elite football players by having medical staff from the same club diagnose, treat, and record injuries, avoiding potentially unreliable data that might come from medical staffs at different clubs. 16 However, the study also has limitations. First, the sample size of players and injuries is small, and results should be cautiously interpreted. The analysis of only one team per sex limits the external validity of the results. The study had adequate power to investigate differences in the overall injury incidence, but longer follow- up periods are necessary for specific, less frequent injuries. As an example, to detect a significant difference between rates of 1.5 and 1.0 injuries/1000 h, with a power of 80% and a two- sided significance of 0.05, hours of exposure per sex would have been required, 18 but the present study only had hours in men and hours in women. Second, in addition to the sex differences in exposure time, other differences might exist in training content, preventive strategies or everyday lives of the players, and these were not accounted for. For instance, even though all female players had professional status in the last two seasons, some of them had jobs besides football that could alter their risk of injury and recovery time, for example, by preventing them from training or taking full advantage of medical treatment. Furthermore, there were differences in the medical staff- toplayer ratio between teams, which might have been a major factor in the observed differences, as a higher availability of medical staff might increase the time spent on injury prevention, treatment, and rehabilitation and the quality of such interventions. Third, the lack of concussions is likely an underestimation as previous studies have reported that female football teams can expect one concussion per season, being twice as frequent in women as in men. 12,15 Players might have underreported or not recognized their symptoms, or the medical staff may have failed to identify them. Diagnosing and managing concussions is challenging, and as a history of concussions might lead to neurocognitive deficits, adequate procedures are needed. 40 Finally, this study is useful to uncover possible sex differences in injury risk, but can only hypothesize with regard to their cause. Therefore, future research is required, first, to replicate these findings in larger cohorts, and second, to explore the reasons for the differences in injury incidence and recovery time between men and women. In addition to biological differences, male and female football differ in many other aspects (eg, training load and content, fixture congestion, staffing, dedication level), and their investigation in relation to the observed injury patterns may help identify risk factors and develop sex- specific preventive programs. These are necessary steps toward safer football participation in men s and women s football. 4.6 Perspective With only one previous study directly comparing the incidence of injuries in elite male and female football players at the elite European club level, 11 the present study provides a robust sex comparison by having medical staff from the same club diagnose, treat, and record all injuries. Injury incidence was higher in men, which might be attributed to a higher incidence of contusions. In addition, men appear to be more predisposed to hamstring strains and hip/groin injuries, with hamstring strains being the primary source of absence. On the other hand, female players had longer absences due to a higher incidence of severe knee and ankle ligament injuries. Finally, and for the first time, a higher incidence of quadriceps strains and ankle syndesmosis injuries was observed in women. In conclusion, the present study presents findings on the differences between men and women in injury incidence and may help clinicians tailor preventive strategies to the specific needs of male and female football players. ACKNOWLEDGEMENTS JL was supported by a PhD Studentship from the Vice- Chancellorship for Basque of the University of the Basque Country UPV/EHU (Euskararen arloko Errektoreordetza). REFERENCES 1. Drawer S, Fuller CW. 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