ACL Injuries In Football: Evidence Based Strategies To Reduce Its Incidence

ACL injuries in football: evidence based strategies to reduce its incidence

The anterior cruciate ligament (ACL) is one of the most devastating injuries in football, given its consequences both at the team and athlete levels. Its average incidence, 0.06 per 1000 hours of athlete exposure (AE) (with the rate being around 20 times higher in competition than in training), is not high compared to other injuries such as hamstring strains (1). At the team level, this means that a squad of 25 players may suffer one ACL injury every two seasons (2). At the league level, where 10 games are played in each round, an ACL injury occurs approximately every 7 rounds of competition. This incidence appears to be higher in young female players (3), and amateur football (4), and can also increase due to a coaching change or promotion to a higher division (5).

Despite its low incidence, the ACL injury has been shown to cause the highest injury burden per 1000h AE (14.4) (1), only surpassed by hamstring strain injuries. The reason for such prolonged absences, and increased burden) is the long recovery and rehabilitation times required. Research reports an average return to competition (RTC) time of between 210 and 220 days, equivalent to at least 7 months (6,7). However, some studies suggest these periods should be extended, as delaying the RTC up to 9 months could reduce the risk of recurrence by around 51% (8), given that functional limitations (i.e., neuromuscular function and control deficits) may persist up to 2 years post-surgery.

At the club level, an ACL injury means the injured player will miss most likely miss the entire season, directly impacting performance, especially in scenarios where a key player is lost. The problem extends beyond performance, causing significant financial losses in two ways: paying salaries to a player unavailable for the rest of the season, and the devaluation of the player in a potential future transfer (9). In line with this, it has been shown that an ACL tear results in losses of around €84,500 per 1000h AE for teams in the top five European leagues, with this figure potentially skyrocketing for teams with larger budgets, and even more so for players with the highest salaries (1).

At the individual level, the ACL injury is particularly devastating. First, the increased risk of not returning to the previous level of competition is likely influenced by the lengthy recovery process, which is often complicated by persistent neuromuscular deficits that significantly impact a player’s ability to return to their physical peak. Studies show that although most players (98.4%) manage to RTC, the majority do not reach their prior performance level, with a considerable percentage either retiring, remaining without a contract, or not returning to play (10). The situation worsens for players aged over 30, with over 76% playing in lower divisions or retiring. Secondly, there is a high risk of re-injury, particularly in non-contact situations, where the recurrence rate can reach 26.9% (11). Isolated ACL injuries, without other associated injuries, further increase the risk of a second tear to 42%. Additionally, long-term complications, such as chronic knee laxity and a heightened risk of early-onset osteoarthritis, are not uncommon (12). Up to 80% of athletes may develop knee osteoarthritis, with nearly half experiencing symptomatic cases (13). Therefore, ACL injuries are among the most concerning at all levels within a football team’s structure, and efforts must be directed towards reducing their incidence.

How to identify those players at higher risk of ACL injury, and in a practical way, accessible for practitioners working in football.

To accurately understand the aetiology of injuries, it is essential to recognise that they result from a complex interaction of multiple risk factors (RF) that can be classified as modifiable and non-modifiable (14). Modifiable RF are of greater interest for injury prevention as they can be modified with target screening and training. Common study designs used for identifying RF include case-control studies, intervention studies, and prospective cohort studies, with the latter being the gold standard (14). Despite that, significant limitations still persist in such designs, such as the fact that many of them do not account for the multifactorial nature of injuries and the assumption that RF are static over time (15). Hence, effective injury prevention requires a shift in the research paradigm, integrating comprehensive and dynamic screening methods to establish first, to identify and address RF early (15,16).

In the absence of strong research evidence establishing RFs, an alternative approach has focused on identifying risky patterns (i.e., characteristics) during well-established ACL injury mechanisms, as several high-quality studies have described them in football recently (17–21). Specifically, changing of direction (COD) in defensive situations, often in response to deceptive actions from opponents (Video 1) (22), and landing from jumps have been identified as the two primary mechanisms (17,18). Many studies have aimed to identify the biomechanical determinants of knee abduction moments, tibial shear, and knee internal rotation moments, which are known to increase ACL tensile loads in such mechanisms (23,24). While this research, using gold standard technologies (i.e. 3D motion capture system, triaxial force platforms) have provided important theoretical insights into high-risk moments, their real-world application is limited due to the prohibitive cost of required equipment for football teams and the time-consuming nature of the analyses. To bridge this gap, the Landing Error Scoring System (LESS) (25) and Cutting Movement Assessment Score (CMAS) (26,27) were developed, demonstrating the highest levels of validity, reliability, and usefulness to date. Therefore, in my PhD, we proposed a framework that allows practitioners to perform ACL injury screening using only smartphones and equipment readily available in football team settings.

Specifically, the LESS (Table 1) (25) and the CMAS (Table 2) (27) assess the presence or absence of kinematic factors commonly associated with potential hazardous mechanical loads during drop vertical jumps (DVJ) (Figures 1 and 3A) and COD (Figures 2 and 3B) tasks, respectively. The higher the score, the poorer the movement quality in both tools. For this, the only equipment required are balls, cones, tripods, a box for the DVJ, 3 smartphones and a ball suspension stand (Figure 4). In our research, the ball was included (28) to increase the external validity of the tests (29). Video 1 and Video 2 represent examples of the assessments that can be performed using the tools and the free software, Kinovea, which is used retrospectively for grading and profiling the videos.

Several practical applications arise from such analyses in the context of a football team. For instance, these tests can be conducted at the start of the preseason to establish benchmarks for return-to-play processes. It is well known that many long-term injuries, such as ACL injuries, require the re-establishment of movement quality in dynamic activities, as neurocognitive injuries, can negatively impact co-ordination, proprioception, and motor control of the affected joints (22). Furthermore, with these tools, as historical data is accumulated, practitioners can perform prospective analyses to identify which metrics are most informative for ACL injuries and potentially for other knee and ankle ligament injuries, as movement quality may also have an influence on them. Additionally, with repeated measurements every 2-3 months, practitioners can better understand the fluctuation of risky patterns throughout the season, assessing whether their interventions are effective in reducing them and identifying specific moments when further interventions may be needed.

Which strategies are effective, efficient, and easily implementable to improve movement quality and reduce ACL injury risk?

Among the various proposed risk factors predisposing athletes to ACL injuries, those that are modifiable through individually designed interventions have garnered the most interest. For the prevention of ACL injuries in sports generally, and in football specifically, the literature suggests that several components may be effective in reducing injury incidence (30). Strength training (31), with a particular focus on musculature considered synergistic to the ACL (e.g., hamstrings, soleus, gluteus medius) (32) and lumbopelvic stabilisers (e.g., core musculature), as well as plyometric training, have shown promise in reducing risky movement patterns (33). Other comprehensive programmess, as the FIFA 11+, seem to be effective although poorly implemented in real practice due to some limitations, such us the boredom associated with the programme, the time required to be completed (20-25 min) and the soreness and fatigue caused by exercise contained in part 2 (34). As the mechanisms of injury have become well understood through recent studies, technique modification programmes aimed at improving full-body biomechanics during such movements have also emerged as interesting interventions, leading to promising results (35).

However, for any intervention to be successfully implemented, it must not only be effective but also time-efficient (36). Football performance depends on a wide range of factors, with technical and tactical skills being their main key determinants (37). While football does not demand extraordinary levels of any specific physical quality, players must maintain a well-balanced base of various attributes (e.g., strength, speed, endurance) to perform optimally over the course of 90 minutes of gameplay (38). Considering the importance of availability for performance, injury prevention is a priority, especially in high-performance environments. However, ACL injuries are not the only concern. Other injuries, such as hamstring, quadriceps, and adductor strains, as well as ankle sprains, also carry a high burden and require additional interventions (1). Yet injury prevention programmes must often be designed within time constraints in mind to ensure their effectiveness in real-world settings (39). On the other hand, equipment limitations, particularly in non-professional football leagues, further highlight the need for practical, easily implementable interventions that address the main mechanisms of ACL injury in football, such as change of direction (COD) and landing from a jump.

For that purpose, the Safe Landing programme was developed as a six-week, technique modification-based intervention that combined jump-landing, plyometrics, and COD exercises (40), with an emphasis on feedback, given its critical role in modifying risky movement patterns (33,41). The programme duration was chosen to align with the standard length of the preseason in football. Its short session time (approximately 9 minutes per session, 3 sessions per week) makes it suitable for inclusion as part of the warm-up routine. Typically, this is where practitioners will have regular access to provide a conditioning stimulus for players across the microcycle. The intervention’s simplicity—where the volume remains fairly consistent (30 jumps and 20-30 CODs per week, Table 3) and progression is based on the complexity of drills (e.g., progressing from shallow to sharp CODs and from anticipated to unanticipated movements)—coupled with minimal equipment requirements, makes it highly implementable. The quality of the feedback, based on implicit learning and an external focus (Table 4), was one of the keys to the intervention.

A programme’s high compliance rate is fundamental for modifying players technique. The 93% compliance in the middle of the competitive season likely contributed to the moderate-to-large significant improvements observed in COD (Figure 5) and jump mechanics (Figure 6), without compromising performance in these actions. Although performance considerations might seem secondary in injury prevention programmes, neglecting the performance-injury trade-off reduces the likelihood of implementation (43). Thus, the Safe Landing programme presents itself as an attractive ACL injury prevention initiative that, due to its characteristics, can be utilised by practitioners at any level of football to enhance the movement quality of players. Its six-week duration and short session times make it feasible to incorporate into preseason warm-ups, with the dual objectives of (1) enabling players to benefit from its protective effects during the most demanding part of the season (i.e. when competition begins) season, and (2) allowing players to get adapted during preseason, a time when adaptation takes priority over freshness for competition, as the intervention’s initial week may cause delayed onset muscle soreness (DOMS), which could be problematic if official games are scheduled during that period.

Future perspectives on injury prevention in football.

Football’s physical demands have increased in recent years (44), and the growing frequency of congested match schedules at the elite level has made injury prevention a critical component of training microcycles. The proposed intervention, the Safe Landing, has proven effective and efficient in improving movement quality, though it has not been enough to simultaneously enhance player’s performance. To maximise athlete buy-in and compliance, interventions should ideally be designed to improve both ACL injury risk factors and key performance determinants, such as change of direction (COD) or jumping. The challenge here lies in creating interventions that fit within the time-constrained routines of football teams—meaning they must be both time- and equipment-efficient—while simultaneously being comprehensive enough to promote the desired adaptations (36).

Moreover, interventions should target risk factors common to multiple types of injuries. If they are designed to prevent only one specific injury—such as the ACL, which is, although potentially devastating, relatively rare—we would need to develop separate interventions for every injury of concern, complicating its implementation. In the case of Safe Landing, since abnormalities in movement quality during essential drills like COD and jumps may predispose athletes to other injuries beyond ACL tears (e.g., knee or ankle sprains), it could be a promising solution for addressing joint injuries in general. However, further research is required to confirm this.

Technological innovations, particularly in Sports Science, are rapidly evolving. While the latest tools for identifying risky movement patterns represent significant advancements compared to traditional 3D motion capture systems in terms of the external validity of the evaluations, they still require considerable time to acquire the necessary expertise for conducting valid and reliable assessments. New technologies, such as the OpenPose (45) and OpenCap (46) (Figure 8) systems, which use only two smartphones and leverage artificial intelligence and machine learning, now enable integrative and complex biomechanical analyses that were previously restricted to highly specialised laboratory conditions and may offer solutions to such problems. Furthermore, some of the latest-generation smartphones are equipped with LiDAR technology, potentially enabling augmented reality analyses, which may have a positive impact in clinical and non-professional settings.

Additionally, these technologies will potentially allow practitioners not only to conduct quicker and more accurate assessments, but also to create extensive databases that were unimaginable just a few years ago. This Big Data, combined with advancing artificial intelligence, may support emerging innovative approaches and challenge current paradigms, potentially driving the development of new methodologies and, ultimately, significantly improving the current injury prevention framework within the real-world context of football.

TAKE-HOME MESSAGES

The Safe Landing is an efficient technique-modification programme consisting of plyometrics, jump-landing and COD drills that, with only three 9’ sessions per week, as a part of the warm-up, is effective in improving movement quality of football players. Its duration (6 weeks) allow it to be perfectly fit in the preparatory preseason phase of a football team. The rapid development of new technologies and the emergence of advanced AI tools in recent years could soon elevate the field of risk factor identification in sports, paving the way for innovative and groundbreaking paradigms.

Although not quite common, ACL injury is one of the most concerning injuries in football both at team and individual level, due to its devastating consequences.

Their predisposing risk factors are not properly evidenced in the literature, although its main mechanisms have been acutely described.

The LESS and CMAS are valid, reliable and feasible scales to evaluates movement quality of football players in two of the most common mechanisms of ACL injury: COD and landings from jump.

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