Risk Factors 

Several anatomical, biomechanical and hormonal risk factors have been proposed over the years. From an anato­mical perspective, a smaller notch width or notch-width index, a smaller ACL volume, an increased tibial slope and general increased ligament laxity compared to male athletes have been associated with increased risk of sustaining an ACL injury [4 – 8]. Endogenous hormonal fluctuations during the menstrual cycle, specifically variations in the levels of oestrogen and relaxin during the follicular and ovulatory phases may increase ACL injury risk due to overall elevated ligament laxity [8 – 10]. Although there is some research into the association between oral contraceptives and a reduced ACL injury risk, currently there is not enough high-quality research demonstrating this relationship [8, 11, 12]. 

One of the typical non-contact ACL injury mechanism in female football is a change of direction combined with deceleration prior to sidestepping and landing from a jump [13, 14]. Biomechanically, several kinematic, kinetic and muscle activation patterns have been found to be related to increased risk of sustaining an ACL injury. Increased knee valgus angles at both initial contact and peak contact in landing, increased ipsilateral trunk motion combined with higher hip adduction angles, reduced knee flexion angles and limited hip flexion angles as well as increased tibial external and internal rotation angles are kinematic patterns that have all been found to be associated with higher ACL injury risk [13, 15 – 18]. In terms of kinetic para­meters, landing with a more extended knee position and erect posture at initial contact is associated with higher vertical ground reaction forces and thus increased ACL strain [13, 19 – 21]. Anterior tibial translation due to shear forces and increased external knee abduction moments combined with elevated internal rotation moments signifi­cantly contribute to ACL peak strain [13, 22]. 

Neuromuscularly, sex-specific differen­ces in muscle activation have been shown. Excessive quadriceps activation compared with insufficient hamstring co-­contraction in females may contribute to limited knee flexion angles upon landing, which may increase ACL strain. In contrast, increased hamstring co-­contraction prevents valgus stress on the knee, while gluteus maximus and medius activity reduces peak knee abduction angles and increases knee fle­xion angles upon landing [13, 23]. In female athletes, decreased activation of the vastus medialis compared to the vastus lateralis as well as decreased ­activation of the medial hamstrings compared to the lateral hamstrings have also been demonstrated. This medial-­to-lateral imbalance may explain why many females lack the ability to resist excessive abduction loads in the knee joint [13, 24].

Is the key professionalisation?

Sex disparities in access to training facilities and optimization of a crowded match calendar and travel may also play an important role in the risk of ACL injuries between males and females. Similarly, football boots, pitches quality, and football size and weight have been based on male, but not tailored around females’ characteristics [8]. 

Perhaps the most important extrinsic risk factor that exists in women’s football is the discrepancy in resources available. For example, there is a tangible gap in the quality of pitches between the women’s and men’s games. Due to poor availability of high-quality grass pitches, women are forced to play on either poor-quality surfaces or more recently, artificial turf, as seen during the 2015 FIFA Women’s World Cup [26, 27]. This is almost unheard of in the men’s game. Whilst these surfaces help mitigate cancellation of female events due to adverse weather conditions, the risk that it may pose to the female ACL is often overlooked. Indeed, findings of a recent meta-analysis published in the Orthopaedic Journal of Sports Medicine indicate that an increased risk of ACL secondary to match participation on artificial turf is only seen in female football players and not their male counterparts [28, 29].  

Additionally, female football boot technology is someway behind that seen in men’s boots. Females exhibit several intrinsic anatomical differences in foot structure that may place them at a biomechanical risk of ACL rupture. Although there is a lack of research supporting the relationship between boot choice and ACL rupture risk, recent research by the European Clubs Association has found that 80 % of female players playing in major European leagues report discomfort with their football boots. Although major football brands are making progress with their goals of better supporting the needs of female players [31], the avai­lability of female-tailored boots is poor, or at best, poorly advertised [26]. 

The discrepancy in resource availability is further highlighted by reduced accessibility to gyms to aid injury prevention programmes. This not only due to a lack of sufficient equipment within these gyms, but also the barrier to female participation underpinned by long-stan­ding issues such as gender-based norms dictated by society. It has been suggested that females have a lower training age considering the amount of time and exposure an athlete has had to structured, coached, and progressive training [8, 30]. Another key principle could be promotion of effective communication and collaboration within the wider club-­ecosystem. As part of the UEFA Women’s Elite Club Injury Study, Ekstrand et al found that risk factors for injury with the highest average importance were “lack of communication between medical staff and coaching staff” and “load on players” [32]. Although this was in the context of hamstring injuries, the same principles could be applied to ACL injuries. The medical team could collaborate directly with the sports science department to implement effective load management strategies on the same level like it is done in men’s football, and these to be communicated with the coaching staff to facilitate forward planning and implementation of such measures, including management of player’s match minutes and training schedules [33]. 

Can an ACL injury be predicted?

Jauhiainen et al. used an extensive screening test battery of 880 female elite athletes to investigate the predictive potential of multiple predictive machine learning methods on the risk of sustai­ning an ACL injury. Despite analysing a large prospective data set, the predictive ability was too low for ACL injury risk assessment in clinical practice. The reasons for the low predictive model might have been due to the failure to record training and match load data, including short term changes in physical variables and training loads. Therefore, further studies are needed to investigate what exact type of data and machine learning approaches should be used for more accurate injury prediction [25]. 

Conclusion

Over 20 years of research has failed to combat the disparity in the burden of ACL injuries between male and female footballers. Future work should look to define specific ACL risk reduction mea­sures and demonstrate how these might be implemented in the elite football setting. To do this, we should not focus only upon the specific intrinsic risk fac­tors in women footballers, but also on the environmental and psychosocially driven risk factors that currently exist [8, 33].

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The recently published paper from Ekstrand et al. describes the incidence of hamstring injuries in male professional football players over consecutive 21 seasons (2001/02 to 2021/22). The authors also analyse the time trends of hamstring muscle injuries over the most recent eight seasons (2014/15 to 2021/22) and in addition, they outline hamstring injury location, mechanism and recurrence rate. 

Data was collected from the Elite Club Injury Study (ECIS). The individual player exposure and the time-loss in­juries of 3909 players were recorded by the medical staff of 54 football teams from 20 European countries, which all qualified for the UEFA Champions League (UCL) group stage. 

A hamstring injury was defined as a ’complaint sustained by a player that resulted from a football match or football training and led to the player being unable to take full part in future football training or match play‘. In the beginning injuries were solely classified by using the Orchard Sports Injury Classification System (OSICS); however, since 2011/12 the Munich Muscle Injury Classification was also used to classify hamstring injuries. Between 2001/02 and 2021/22, 2636 hamstring injuries were documented representing 19 % of all reported injuries. Compared to the first recorded season, the proportion of diagnosed hamstring injuries and the proportion of all injury absence days caused by hamstring injuries have doubled by 2021/22 (from 12 % to 24 % and from 10 % to 20 % respectively). Speci­fically during the last 8 consecutive seasons the incidence (Number of injuries per 1000 player hours, 6,7 % annually) and the burden (Number of lay-off days per 1000 player hours, 9.0 % annually) of hamstring injuries during training and match play have increased significantly. The hamstring injury incidence was 10 times higher during match play than during training and the median lay-off following a hamstring injury was 13 days. Overall it was reported that during a season 20 % of players missed training or match play due to a hamstring injury and 8 hamstring injuries can be expected in a 25-player squad per season. In addition, there were more structural than functional injuries classified, whereby these structural injuries were associated with a longer lay-off time than the functional ones (median absence 17 vs. 6 days). 

In terms of the injury mechanism and location, this study confirmed the existing knowledge that sprinting was the most common mechanism and the biceps femoris injuries were more frequent than semitendinosus and semimembranosus injuries. However, further details concerning which phase of sprinting (i.e., terminal swing, early stance) or which exact part of the biceps femoris (i.e., proximal, distal, T-junction, long/short head or myofascial, myotendinous, or tendinous) were not described. Regarding the recurrence rate, 18 % of all recorded hamstring injuries were recurrences, from which early recurrences (within 2 months) made up 69 %. Recurrences were nine times more likely to occur in matches than in training. An increased recurrence rate specifically within the first 2 months could indicate that return to full training/competition may be too early for some types of injuries. Taking into consideration that the healing time of different kind of tissues takes longer than others (i.e., connective tissue), healing may not be sufficiently completed to sustain certain thresholds of high-speed exposure. The authors hypothesize that the reason for the increasing number of hamstring injuries could be because the intensity of elite’s football matches has significantly increased, which has led to more high intensity activities in professional footballers compared to the past. Another explanation may be related to a more crowded match calendar of players associated with fewer training sessions during the (pre-) season period considering that more training sessions may lower the injury risk. In summary, this publication highlights the increa­sing burden of hamstring injuries in elite football. More effort is needed to understand better how to prevent the first injury and how to manage injured players more effectively to reduce the risk of re-injuries.