We might be falling short in our efforts to prevent ACL injury by allowing athletes to focus on how they are moving during screening.
By Sarah Marie Tighe, SPT, and Thomas Gus Almonroeder, PT, DPT, PhD
Almost 8 million athletes participate in high school sports each year in the United States,1 and an additional 490,000 compete at the intercollegiate level.2 Regrettably, acute lower-extremity musculoskeletal injuries are relatively common in sports that involve frequent high-impact maneuvers, such as landing and cutting (e.g., basketball and soccer).3 Anterior cruciate ligament (ACL) injuries are of particular concern, because they can be season- or career-ending,4 and can contribute to premature osteoarthritis of the knee.5
Prevention of ACL injury is a priority in sports medicine; evidence suggests that targeted training might reduce the risk of ACL injury in some cases.6 However, epidemiological studies of injury trends indicate that prevention efforts might have had minimal influence on the rate of ACL injury in athletes.7,8
As a result, improving our ability to identify athletes who are at risk of injury and then intervene effectively continue to be key objectives in sports medicine.9 An important step in this process might be to develop better understanding of variables that contribute to ACL injury.
What movement pattern increases ACL injury risk?
Most ACL injuries in athletes are noncontact in nature (i.e., they do not involve collision with a teammate or opponent),10-13 and typically occur during what appears to be a fairly routine maneuver that the athlete has undoubtedly performed countless times during a career. Observations from video analysis indicate that landing with limited knee flexion and marked knee abduction motion might place athletes at risk of ACL injury.14-16 A simulation study also showed that landing in this manner can place high load on the ACL.17 As a result, landing and cutting with limited knee flexion and high knee abduction motion or loading is discouraged.9
Although it appears that we have a fairly well-developed understanding of the movement pattern that places an athlete at risk of ACL injury, our understanding of the cognitive and neuromuscular factors that contribute to this high-risk movement pattern is underdeveloped.
Influence of divided attention
From a motor-control perspective, a potential challenge associated with sports such as basketball and soccer is that athletes must maneuver while simultaneously attending to their opponents, teammates, the goal, etc.18 In addition, they might also need to allocate attentional resources to manipulating the ball if they are in possession (e.g., dribbling). As a result, they are unable to fully focus on their movement (i.e., they must divide their attention).
The limited nature of attentional capacity has been characterized theoretically19 and tested experimentally.20 However, studies that have explored biomechanical variables related to the risk of ACL injury by means of laboratory-based motion analysis typically do not reflect the attentional demands of sports, because athletes are often able to fully attend to their movement during testing. Interestingly, it appears that attempts to reflect the attentional demands of the sports environment by requiring athletes to attend to a teammate, opponent, or ball might result in mechanics that are associated with greater risk of ACL injury.21-27
Three studies reported useful, enlightening findings:
- McLean and colleagues26 found that peak knee abduction angles were more than 30% higher (15.0°, compared to 11.3°) when athletes performed a lateral cutting maneuver with a simulated defensive opponent present, compared with baseline trials performed without a defensive opponent.
- Fedie and collaborators23 reported greater loading of the knee in the frontal plane when intercollegiate basketball players were required to intercept a pass shortly after cutting, compared to baseline trials during which they did not need to attend to a ball.
- Almonroeder and colleagues21 recently found that female athletes demonstrated less knee flexion and greater knee abduction when they attended to executing a basketball chest pass shortly after landing from a cut—in contrast to what was observed in trials performed without the chest pass.
These findings appear to show that the attentional demands of sports might contribute to the risk of ACL injury by limiting an athlete’s ability to fully attend to their movement.
There is also evidence that the addition of a relatively simple secondary cognitive task, unrelated to sports, might be sufficient to influence performance of common athletic maneuvers:18,28,29
- Seymore and collaborators29 recently found that soldiers completed a run-and-cut with less knee flexion when they were required to simultaneously count backwards, compared with baseline trials performed without an additional cognitive task.
- Dai et al28 also reported that athletes landed with less knee flexion and higher ground reaction forces during a land-and-jump task while counting backwards, compared with baseline trials.
These findings indicate that even a subtle change to a testing protocol—to limit an athlete’s ability to fully attend to their movement—can alter cutting or landing mechanics in a manner that could increase ACL loading.
Opportunity for further research
It appears that attempts to more fully reflect the attentional demands of the sports environment could provide additional insight into mechanics that contribute to ACL injury. Conversely, failure to account for attentional demands of sports within a testing protocol could limit its ecological validity. Continuing to develop and implement testing protocols that more adequately reflect the attentional demands of sports appears warranted.
Virtual reality. Perhaps virtual reality could allow for development of a more “game-like” testing environment and, therefore, offer opportunities for advancement in this regard.30 Virtual reality would also allow researchers to adapt the environment to the actions of the athlete (e.g., virtual defenders that react during testing), which could also promote more realistic testing. In addition, development of methods that allow for analysis of movement during competition, such as model-based image-matching31 or wearable technology (e.g., inertial measurement units),32 could also provide a better understanding of how attention influences movement in sports.
Study of the individual. Attempts to explore the influence of an individual athlete’s attentional capacity might also be an avenue for future research. It is possible that athletes who have more limited attentional capacity might be challenged to a greater extent to control their movement when they must divide their attentional resources; however, this premise has not been systematically tested.
Swanik and colleagues33 reported that athletes who go on to sustain a noncontact ACL injury demonstrated relatively poor baseline performance on a clinical test of cognitive functioning (using Immediate Post-Concussion Assessment and Cognitive Testing [ImPACT]), in comparison to athletes who remain uninjured. This finding appears to indicate that cognitive factors might contribute to the risk of ACL injury. However, the testing battery used by Swanik did not specifically assess attentional capacity; rather, it was designed to test cognitive processing speed and memory.
Considering the demands of the sports environment, attempts to relate the individual athlete’s attentional capacity to risk of ACL injury might also be a worthwhile avenue of research.
Cognitive study. Sports medicine professionals have highlighted the need to develop a better understanding of the role that cognitive factors (including attention) play in noncontact ACL injury.34 We agree that continuing to investigate the potential contribution of task-related cognitive demands and individual cognitive attributes is warranted. Optimally, these efforts will involve experts from a variety of disciplines.
Relevance of screening tools to assess risk of ACL injury
Development of screening tools that can identify athletes who are at risk of injury is undoubtedly a major component of prevention. Assessment of lower-extremity mechanics during performance of a drop vertical jump is a common approach to screening for risk of ACL injury.35-37 For this task, athletes drop or jump from a box, land on both feet, and immediately perform a vertical jump. Studies attempting to relate baseline landing mechanics to subsequent ACL injury have yielded inconsistent findings (i.e., some researchers have identified mechanics during the drop vertical jump that predict injury; some have not).38-42
Part of the reason for the inconsistent or limited predictive validity of the drop vertical jump might be that athletes are able to fully attend to their movement during testing—a situation that does not reflect the attentional demands of sports. Studies that have modified the standard drop vertical jump by including an overhead goal that requires attention during performance (e.g., a ball suspended overhead) have found that athletes demonstrate less knee flexion and greater knee abduction than they do in trials performed without an overhead goal.22,27 It is possible that our ability to identify athletes at risk of ACL injury would improve if we required them to divide their attention during testing. Although including sport-related “distractors” seems logical, relatively simple secondary cognitive tasks (e.g., counting backwards) may offer a sufficient challenge.
Programs for preventing ACL injury might also benefit from considering the attentional demands of sports. Providing athletes with instruction to alter their movement pattern (e.g., “Bend your knees when you land!”) is recommended for preventing ACL injury.9
Visual feedback training has also been explored, and appears to show potential for reducing mechanics associated with the risk of ACL injury.43-45
Summing up: Are benefits of training transferable to play?
Sports medicine professionals must 1) consider that athletes will be challenged to attend to movement-related cues emphasized in training when they compete and 2) not assume that improvements in performance made in a controlled training environment will carry over to movement during competition. It seems likely that training of this nature might need to be sufficient enough to achieve some level of automaticity before there will be transfer; however, the type or amount of training necessary to achieve stable changes in a movement pattern requires additional exploration.
Sarah Marie Tighe, SPT, is a student in the Doctor of Physical Therapy Program, Department of Kinesiology, University of Wisconsin-Milwaukee. Thomas “Gus” Almonroeder, PT, DPT, PhD, is Assistant Professor, Doctor of Physical Therapy Program, Rinker-Ross School of Health Sciences, Trine University, Fort Wayne, Indiana.
Disclosures: The authors have nothing to disclose.
- 2016-17 High School Athletics Participation Survey. 2017; Indianapolis, IN: The National Federation of State High School Associations. www.nfhs .org/ParticipationStatistics/PDF/2016-17_Participation_Survey_Results.pdf. Accessed March 31, 2018.
- NCAA recruiting facts. March 2018; Indianapolis, IN: National Collegiate Athletic Association. www.ncaa.org/sites/default/files/Recruiting%20Fact% 20Sheet%20WEB.pdf. Accessed March 31, 2018.
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- Tirabassi J, Brou L, Khodaee M, Lefort R, et al. Epidemiology of high school sports-related injuries resulting in medical disqualification: 2005-2006 through 2013-2014 academic years. Am J Sports Med. 2016;44(11): 2925-2932.
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- Beck NA, Lawrence JTR, Nordin JD, et al. ACL tears in school-aged children and adolescents over 20 years. Pediatrics. 2017;139(3). pii: e2016 1877.
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- Hewett TE, Torg JS, Boden BP. Video analysis of trunk and knee motion during non-contact anterior cruciate ligament injury in female athletes: lateral trunk and knee abduction motion are combined components of the injury mechanism. Br J Sports Med. 2009;43(6):417-422.
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- Almonroeder TG, Kernozek T, Cobb S, et al. Divided attention during cutting influences lower extremity mechanics in female athletes. Sports Biomech. 2017(Nov 13):1-13.
- Almonroeder TG, Kernozek T, Cobb S, et al. Cognitive demands influence lower extremity mechanics during a drop vertical jump task in female athletes. J Orthop Sports Phys Ther. 2018(Jan 10):1-22.
- Fedie R, Carlstedt K, Willson JD, Kernozek TW. Effect of attending to a ball during a side-cut maneuver on lower extremity biomechanics in male and female athletes. Sports Biomech. 2010;9(3):165-177.
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