Written by Darren J. Paul, Qatar
Category: Sports Science

Volume 9 | Targeted Topic - Return to Performance After ACL Reconstruction | 2020
Volume 9 - Targeted Topic - Return to Performance After ACL Reconstruction

– Written by Darren J. Paul, Qatar



An Anterior Cruciate Ligament (ACL) injury is considered a traumatic event in an athlete’s life. The expected duration of absence, high possibility of reinjury and in some cases, premature ending to an athlete’s career signifies that strategies to mitigate the risk of ACL injury are warranted.

There has been much attention directed towards testing, prevention and rehabilitation methods in relation to ACL injury. Despite positive advancements, a recent study of ACL injuries in the English Premier Soccer League over the past 15 years reported that injury prevalence has remained essentially unchanged over this period1. Of equal concern is the six-fold greater injury incidence during competition compared with training2. Considering the match stimulus in soccer (based on 1-game week) accounts for approximately 20–30% of total weekly activity; this distribution is worryingly disproportionate.  

It is increasingly evident that the mechanism of ACL injury is complex and multifactorial, influenced by the sport and individual characteristics. Movement sequences in many invasive intermittent sports (e.g. soccer), are unique and cannot be predicted with 100% accuracy. Each player will approach a situation in their own individual way, relying on a combination of intuition, experience and laws which govern the game. Indeed, when a movement is performed in a dynamic environment, under pressure and in response to an unpredictable stimulus3, the risk of ACL injury is potentially increased. This complex interaction of stimuli indicates that other factors in addition to physical attributes are needed to fully prepare athletes to return to high level competition.



The actual movement itself is only part of the mechanism; the performer-environment interactions and subsequent decision to execute the movement must also be considered in sport injury etiology. While the physical demands of sports such as soccer have increased quite substantially over recent years (greater number of sprints and increased distance covered in high intensity effort), little is known regarding the increased cognitive demand that may accompany these more intense movements. Most previous work has not factored in the behaviors (i.e., decision making), which lead to the situations in which injuries occur. By failing to appreciate vital contextual information, and simply cataloging the apparent mechanism at the time of injury (e.g., knee valgus and external rotation of the tibia), we may be limiting our ability to understand sport-specific injury mechanisms.  

Executing an unanticipated movement presents considerably different challenges to those faced when the athlete is able to predict and plan their next move, whether that be a change of direction or jump-landing task. Such pressure may include a combination of spatiotemporal constraints (e.g., a small area to operate, fast moving ball, and short time between stimulus and response), differing levels of cognitive complexity (e.g., position of teammates, anticipating and reacting to the opposition, and high criticality of the situation) and fatigue (e.g., acute fatigue from preceding play, injury to another limb). These scenarios will impact upon the athlete’s ability to execute a movement task effectively and may also predispose them to positions associated with heightened injury risk. During the most intense and demanding moments on the field, athletes may only have milliseconds to scan the surrounding environment and decide upon and execute, an appropriate movement. Indeed, slower baseline cognitive processing speeds (e.g., longer reaction times) are associated with mechanics that may result in greater ACL loading during the execution of unplanned landing and cutting maneuvers4, highlighting that such time constraints can impact perception, decision and action; and consequently, performance and injury risk. 



An ACL injury is no longer considered a ‘simple’ musculoskeletal pathology with only local mechanical or motor dysfunctions. Together with the psychological trauma and reductions in physical capacity, there is a cascade of likely events across the whole spectrum, including neurological insult to the central nervous system and reductions in the sensorimotor system that makes for a challenging return5. Therefore, only reconstructing the mechanical structures of the knee and then sending the athlete back to sport ‘when it’s time’ is likely to produce unsuccessful outcomes6. Indeed, signing off an athlete for return to play without having considered their ability to integrate perception, decision making, and action effectively within sport relevant scenarios perhaps leaves us open to the athlete being underprepared and at risk of reinjury.



Many ACL injury risk reduction programs have been developed for soccer players and athletes in other sports7. These training programs can be broadly categorized as balance, plyometric, strength, and change of direction training; notably all isolated physical components. Despite the efforts of practitioners and scholars it is reasonable to suggest that some of these programs may not be effective in addressing the necessary realms of sporting performance. Programs typically take the form of either long-duration neuromuscular training or short-duration warm-up programs, and a multi-faceted approach that includes the majority, if not all of these components is the most commonly used. A deficiency in one or more of these physical capacities is often blamed as a contributing factor for ACL injury, despite the difficulties in isolating the individual components. However, the disproportionate focus on physical and technical capacities in isolation might be due to the relative ease of controlling and measuring this type of training, rather than their superior deterministic abilities. There currently seems a bias toward assessment and monitoring of variables which are easy to measure, rather than what is important. For example, screening methods that include visual assessment of control of the knee during a slow, single leg squat or hop are likely very different to movement variability during an unplanned direction change task while fatigued and under high cognitive loads. While the former provides useful information regarding generalized physical capabilities and should not be discounted, the latter is also difficult to control and obtain sufficient data reproducibility. Nonetheless, the apparent mismatch between current assessment protocols and the chaotic scenarios encountered in match play with emerging and constantly changing environmental constraints may in part be a contributing factor to our limited understanding of risk factors for injury.



Despite the best intentions of training, the overall physical, psychological and emotional demands of a competitive match are a unique event that are unlikely to be fully replicated (Figure 1). At best, it is arguable that most of the specific drills and exercises performed are characterized by simplistic reactive responses as opposed to complex decision making which more closely represents what an athlete may face on the field8. Comparing this to unplanned tasks in an actual match, where players need to make many decisions quickly while immersed in a complex and dynamic situation highlights the disparity between preparation and realization of sport-specific training. Regarding athlete preparation, if there was a large mismatch in the amount of high-speed running exposure performed between training and a match, then it would be stressed that the athlete is underprepared, and more training is needed. In a similar regard, an important objective in training is to include situations which require perception-decision-action coupling that closely resembles those of actual match activity. Subsequently, it is unlikely that current planned physically dominant drills adequately challenge the cognitive abilities of players, particularly those competing at the highest level. Using an analogy from the sport of motor racing, the best car on the grid (physical attributes), without a highly skilled driver controlling the dashboard (perception-cognitive), will provide no chance of harnessing the car’s full potential, and there is every chance of crashing it!  

It is important to acknowledge that for a significant portion of rehabilitation following ACL injury / reconstruction, the athlete is not exposed to the sport-specific stimulus that drives skill acquisition via perception, decision action coupling. The prolonged absence of sport-specific motor skills may result in task-specific detraining. Anecdotally, on return to team training, players have referred to this as a perception of under preparedness, feeling “rusty” or “not up to match pace.” Physically, they may be stronger and fitter than preinjury, but a lack of appropriately integrated cognitive load in the rehabilitation process leaves them feeling unable to “read the game” and use their physical capacities effectively. This may result in the player feeling confident that they are ready to return to training, but not to complete the most challenging of tasks, or to return to competition. It is also possible that a returning player will push themselves beyond their usual efforts to remind fellow players and coaches of their ability to make a positive contribution to the team. This combination of blunted decision-making abilities and increased volitional exertion might increase the risk of injury.



The selection and progression of an exercise (modality, intensity, and duration) is usually determined by the conditioning staff and tends to lean toward progression of the physical aspects. However, another pertinent question should also be asked - is cognitive load progressed appropriately? To illustrate this point, consider an example of a 1 versus 1 drill, often included in the final stages of end-stage rehabilitation before the player enters full training. Although this training drill integrates relevant decision making and may exceed the physical demands encountered in team training, it neglects many other factors. Because of the challenges of an applied environment (e.g., limited human resources), the player will typically perform against a familiar and less trained opponent (rehab coach), offering few basic stimuli, with little accommodation of constraints (e.g., offside, teammates, spatial, and temporal) or context (e.g., in a state of acute fatigue). Further, within normal training, players will have played against the same small pool of opponents on so many occasions that they are likely well attuned to their opponent’s movement patterns and style of play. As a result, we should also consider the principles of training variability, specificity and overload when it comes to the cognitive demand.



Preparing a team sport athlete may be as much about training the brain as it is movement technique and physiological adaptation. Perceptual and cognitive load must be viewed with the same level of importance as the physical components of performance that we devote so much of our time towards. Therefore, training should include exercises to modify possible injurious movement patterns (action) and include drills to improve aspects of perception and decision making. It is highly likely that some athletes lack the ability to identify relevant cues (perception) which may cause a cascade towards compromised decision-making, and in turn lead the athlete to perform “emergency maneuvers,” being the only solution available to carry out the action.

In a similar regard that good movement technique and appropriate training load are considered important in the gym, the focus should not solely be on reaction and response time, but rather also include accuracy and error rate. Monitoring an athletes’ agility success rate during progressively more game like training scenarios may provide practitioners with an enhanced appreciation of the player’s readiness to train; this could be a very interesting avenue for future research. Sometimes the effect of motor task difficulty on cognitive performance as an error rate (inappropriate execution of movement in response to a specific stimulus) would be masked with a delay in reaction time. Therefore, simultaneous assessment of reaction time and error rate could provide a broader understanding regarding cognitive performance during more complex undertakings such as dual tasks.  

We have recently indicated that the current assessment protocols used to measure change of direction ability for ACL reconstruction patients are likely unsuitable9. This notion is underpinned by literature showing differences in competitive vs. non competitive, planned vs. unplanned, fatigued vs. non fatigued, as well as the effects of assessment characteristics (e.g. cutting angle, approach velocity, technique, visual disturbances, dual cognitive task, double stimulation). In a similar regard, training that incorporates visual or neurocognitive processing, such as ball tracking or engaging other players, task complexity (reaction and decision making), anticipatory aspects, and cognitive load (dual task) are an important component of the program. The ability to identify when and how to adjust attention can be taught during training by increasing a player’s awareness as to what type of information he needs to direct his attention to in different situations. A proposal may be to include some youth academy players when performing selected game-based drills with the returning player(s). This will challenge the player(s) and expose them to stimuli (movements) from an opponent that is less familiar and allowing for a more competitive environment. This would be expected to mutually benefit both the returning player(s) and serve as a learning curve for the youth players’ development. Also, because of time pressures to return to competition, the window for progressing toward game-like cognitive load between return to training and full return to competition is often small.

Despite the best intentions, it is unlikely that players will ever be truly prepared to return to performance, until they are faced with that given situation. Realistically, players are likely to only fully demonstrate their physical, mental, emotional and cognitive proficiency when required, rather tending to play within the constraints of the given situation. The impact of actual competition on execution of movement has been shown in an interesting study by Spittle et al.10They found basketball players performed more pass decisions during high decision criticality situations (classified by a remaining time of 60 seconds or less and score differentials of 2 points or less) and more shoot decisions during low decision criticality situations (classified by remaining time of 5 minutes or more and score differentials of 5 points or more). Unfortunately, it is unlikely that we will be able to fully replicate these environments since competition performance will be influenced by a myriad of contextual factors such as score line, opposition and period of match. However, our role is to bridge the gap between rehabilitation and performance as best we can by exposing athletes to a wide range of relevant tasks and environments in which they are able to re-define their movement skills in tasks that at least in part, represent their sporting environment. 



Research has demonstrated the effectiveness of using video footage to educate female soccer players on safe techniques for jumping, strength and agility-based activities, resulting in an 88% overall reduction in ACL injury rate in the first year11. Recent research has also shown that watching a video of an opposing player running and executing a change of direction can produce an acute improvement in sport-specific response time12. An advantage of video training during the rehabilitation phase is that it can provide complementary “off-field” training during the early stages of rehabilitation. This may be particularly useful to promote self-awareness of their movement technique, patterns in their decision-making abilities, situations in which they perform below expectations, and an increased understanding of the rehabilitation program (Table 1). An increased awareness can then be carried forward into the rehabilitation program and may allow for a better transition into the different phases of training. 



Education, communication, and coach support are pivotal but ‘buy-in’ for injury reduction programs from players and coaches remains a problem within many clubs13. Some of the main challenges include (a) the perceived benefit of playing is greater than the risk of injury, (b) the desire of players to perform as much training as possible with the team; and (c) the belief that the prescribed exercises are unlikely to be of benefit13. Successfully implementing an injury reduction program may largely be dependent on its packaging and delivery rather than its scientific merit. If the coach holds the opinion that it does not include enough “sport-specific” activities, the probability of low compliance may increase by a staggering 81%14. Therefore, practitioners needs to explore alternative methods and strategies to reduce this resistance. 

To get the buy-in from the coach and player, where possible, injury reduction exercises should be integrated with routine sports practice, rather than being a separate entity15. Involving the players and coaches in the design and/or selection of exercises may prove a fruitful endeavor by increasing the perception of ownership. For players to be fully engaged in an injury reduction program, they must feel a degree of empowerment in the exercise selection and the program is specifically tailored to their individual needs16. A standardized program also lessens the dialogue between the players and the medical staff16. The resultant effect may be a player with greater knowledge about the training process, satisfaction of an individualized approach bespoke to their needs, and overall better buy-in for the rehabilitation and/or training program. Combining this with the experience and knowledge of the fitness and/or rehabilitation specialist makes it possible to adopt a more individualized and athlete led approach.



There is a disproportionate bias toward ACL injuries in games versus training. Given the greater exposure and assuming players are physically capable of regularly training at or above match intensity, this suggests that contextual factors, which separate games from training, play a large mediating role in ACL injury risk. Therefore, we should always consider the inherent cognitive demands present in team sports and appreciate the complex interplay between physical capacities and decision making, which ultimately determines movement, performance, and injury risk.

Injury reduction and rehabilitation should include a much broader array of drills and practice scenarios with increasing levels of cognitive complexity to ensure adequate exposure and heightened readiness to re-perform. In addition, involving players and coaches in the design and/ or selection of exercises may prove a fruitful endeavor by increasing the perception of ownership and adherence. Perceptual and cognitive load must be viewed in the same light as the physical components of performance that we devote so much of our time towards. A greater number of decision making scenarios and shorter time periods to react to those decisions are some examples that might contribute to ACL injury. Future research into injury mechanism should also consider the contextual factors surrounding the injury to ensure the chaotic complexity of match play is at the forefront of discussion.



Darren Paul Ph.D. Candidate


Aspetar – Orthopaedic and Sports Medicine Hospital

Doha, Qatar





1.    Waldén M, Hägglund M, Magnusson H, Ekstrand J. ACL injuries in men's professional football: a 15-year prospective study on time trends and return-to-play rates reveals only 65% of players still play at the top level 3 years after ACL rupture. Br J Sports Med. 1st ed. 2016 Jun 1;50(12):744–50. 

2.    Stubbe JH, van Beijsterveldt A-MMC, van der Knaap S, Stege J, Verhagen EA, van Mechelen W, et al. Injuries in Professional Male Soccer Players in the Netherlands: A Prospective Cohort Study. Journal of Athletic Training. 2015 Feb;50(2):211–6. 

3.    Brophy RH, Stepan JG, Silvers HJ, Mandelbaum BR. Defending Puts the Anterior Cruciate Ligament at Risk During Soccer. Sports Health. 2015 Apr 16;7(3):244–9. 

4.    Swanik CB, Covassin T, Stearne DJ, Schatz P. The Relationship between Neurocognitive Function and Noncontact Anterior Cruciate Ligament Injuries. Am J Sports Med. 2017 Aug 30;35(6):943–8. 

5.    Needle AR, Lepley AS, Grooms DR. Central Nervous System Adaptation After Ligamentous Injury: a Summary of Theories, Evidence, and Clinical Interpretation. Sports Medicine. Springer International Publishing; 2016 Dec 22;:1–18. 

6.    Needle AR, Rosen AB. Ligament Injury Changes Brain Function: Now Let's Think About It. . . Geisler PR, editor. Athletic Training & Sports Health Care. 2017 Sep 1;9(5):198–9. 

7.    Taylor JB, Waxman JP, Richter SJ, Shultz SJ. Evaluation of the effectiveness of anterior cruciate ligament injury prevention programme training components: a systematic review and meta-analysis. Br J Sports Med. 2nd ed. BMJ Publishing Group Ltd and British Association of Sport and Exercise Medicine; 2015 Jan;49(2):79–87. 

8.    inder RA, Davids K, Renshaw I, Araújo D. Representative learning design and functionality of research and practice in sport. Journal of Sport and Exercise Psychology. 2011 Feb;33(1):146–55. 

9.    Marques JB, Paul DJ, Graham-Smith P, Read PJ. Change of Direction Assessment Following Anterior Cruciate Ligament Reconstruction: A Review of Current Practice and Considerations to Enhance Practical Application. Sports Medicine. Springer International Publishing; 2019 Sep 16;:1–18. 

10. Spittle M, Kremer P, McNeil DG. Game situation information in video based perceptual decision making: the influence of criticality of decisions. Physical Education and Sport. 2010 Jul 12;8(1):37–46. 

11. Mandelbaum BR, Silvers HJ, Watanabe DS, Knarr JF, Thomas SD, Griffin LY, et al. Effectiveness of a Neuromuscular and Proprioceptive Training Program in Preventing Anterior Cruciate Ligament Injuries in Female Athletes. Am J Sports Med. 2017 Aug 30;33(7):1003–10. 

12. Holding R, Meir R, Zhou S. Can Previewing Sport-Specific Video Influence Reactive-Agility Response Time? International Journal of Sports Physiology and Performance. 2016 May 26;12(2):224–9. 

13. McCall A, Dupont G, Ekstrand J. Injury prevention strategies, coach compliance and player adherence of 33 of the UEFA Elite Club Injury Study teams: a survey of teams’ head medical officers. Br J Sports Med. 2016 Jun 1;50(12):725–30. 

14. Soligard T, Nilstad A, Steffen K, Myklebust G, Holme I, Dvorak J, et al. Compliance with a comprehensive warm-up programme to prevent injuries in youth football. Br J Sports Med. 2010 Sep 6;44(11):787–93. 

15. Finch CF, Diamantopoulou K, Twomey DM, Doyle TLA, Lloyd DG, Young W, et al. The reach and adoption of a coach-led exercise training programme in community football. Br J Sports Med. 2014 Mar 23;48(8):718–23. 

16. Kristiansen JB, Larsson I. Elite professional soccer players’ experience of injury prevention. Cogent Medicine. Cogent; 2017 Oct 20;17:1–15.


Header image by Tim Bayman(Cropped)

Figure 1: A hypothetical model of the constraints and demands for different football related scenarios of match and training: a) Competitive 90 min 11 a side match; b) Small sided games training; c) High intensity running drill. NOTE: Cognitive and perceptual factors = e.g. number of stimuli, complexity of stimuli. Physical factors = e.g. energetic and mechanical demand. Environmental factors = e.g. match location, weather. Contextual factors = e.g. match importance, situation criticality.
Table 1: Examples of training tools and methods that may be used in accordance with the different stages of the rehabilitation phases, as per the hypothetical model.


Volume 9 | Targeted Topic - Return to Performance After ACL Reconstruction | 2020
Volume 9 - Targeted Topic - Return to Performance After ACL Reconstruction

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