Re-engaging the Brain in ACL Rehabilitation: Restoration or Compensation of Motor Control

Recognizing the role of cognition in sport

Imagine yourself as a point guard in a chaotic basketball game dribbling the ball up the court. Your team is down by 2-points with 15 seconds left in the game and no timeouts. The crowd is chanting, teammates on the bench are cheering, and the coach is calling a play from the sideline. As you cross the half-court line you fixate your eyes just beyond your defender’s shoulder keeping them within your central line of sight while simultaneously your peripheral vision analyzes the position of both your teammates and their defenders, and you hear your coach call the play. You gaze up toward the clock, 8 seconds remaining in the game. Your teammate approaches and sets a screen on your defender. You cross over, pull-up for a 3-pointer… “swish”. (Note: not once have you cognitively attended to the complex motor coordination of dribbling the basketball while jogging, or where your knee was positioned when you crossed over dribbling off the on-ball screen.) In this brief example, you (the basketball player) would have engaged in over 15 cognitive-motor dual-task scenarios requiring both accurate integration of relevant sensorimotor stimuli (auditory, visual, somatosensory) and inhibition of irrelevant stimuli to not only execute a complex motor skill (shooting the ball) while under immense cognitive stress, but also maintain autonomous motor control to avoid injury risk movement patterns.

The ability to maintain motor control while engaged in continuous cognitive-motor dual-tasking in sport reflects the sheer processing power of our nervous system. Now reflect on your rehabilitative exercise prescriptive patterns. Do the 3 sets of 10 repetitions leg press or predictable agility ladder drills resemble or even come close to neural processing your athlete must do upon return to sport? Furthermore, are the strongest athletes always the most coordinated? Or injury resistant? The restoration of muscle function (strength, power, hypertrophy, etc.), although imperative to successful rehabilitation, is only one aspect of bridging the gap between rehabilitation and sport performance.

ACL injury is a sensorimotor coordination error under cognitive stress

Sports rehabilitation specialists have the daunting task to not only return an athlete to functions of daily living but attempt to bridge the wide gap between recovery from an injured state to maximizing performance. The inability to successfully restore sports performance after ACLR is apparent in rehabilitation literature with reinjury (graft rupture or contralateral tear) as high as 33% in youth athletes. 1 While a common assertion is that this failure is due to inadequate restoration of physical abilities when returning to sport. A recent series of publications demonstrate that even recovery of muscle strength, symmetric functional performance and patient-reported function is not sufficiently protective on their own to remove non-contact reinjury risk. 2–5

To a clinician this may paint a disappointing picture indicating that what we may consider the fundamentals of injury recovery, the restoration of joint function and strength of the surrounding joint musculature, is not the end goal but just the first step in a long and uncertain journey back to sport performance. However, recent evidence merging motor learning, cognitive neuroscience, psychology, and motor skill acquisition is providing clinicians with new tools to repaint this picture and propel patients beyond recovery of health toward optimized performance. 6–8

To utilize these new tools in rehabilitation and improve sensorimotor control when returning to sport after ACL injury requires resolving a fundamental gap for many practitioners, that is recognizing the role of cognitive processing to maintaining injury-resistant movement coordination. 9 The prevalence of non-contact ACL injuries should make the role of the nervous system and maintaining movement coordination in the face of cognitive challenges readily apparent. (10–12)

This sensorimotor error nature of the ACL injury event is exemplified by most ACL injuries occurring during games, when other players or the ball is in close proximity and when the athlete’s attention is very clearly not on their movement performance.11,13,14 Thus, as cognition and attention relative to sensorimotor control contribute to ACL injury etiology, the typical practice of focusing primarily on joint or muscular factors in isolation leaves key components for injury prevention and recovery out of the typical rehabilitative framework.

Overlooking the role of cognition in ACL injury and recovery

ACL injury risk is certainly mediated by biomechanical variables however, movement does not occur in isolation and what we consider injury-risk movements (knee valgus, stiff landing) are secondary to athlete attentional and cognitive resources as much as muscular forces or joint alignment. Mounting evidence indicates baseline neurocognitive ability contributes to injury-risk biomechanics when moving under reactive stimuli15,16 and deficits in specifically visuospatial cognition increase non-contact ACL injury-risk.17,18 Prospective neuroimaging data also indicate reduced brain connectivity may contribute to the motor coordination errors that result in injury.19,20 Further implicating central nervous system processing mechanisms which play a role in ACL injury is data demonstrating isolated concussion impairs motor coordination and contributes to ACL injury risk.21 Despite ACL reconstruction and typical rehabilitation deficits in motor coordination persist and are exacerbated under cognitive challenges that demand visual attention.22 The decreased ability to dual-task after injury is likely secondary to athletes redistributing reliance for motor control from original implicit (automaticity of movement) and sensory processes to vision and explicit (cognitive planning of movement) processes.23,24 In other words, athletes at risk for and after ACL injury are engaging neural resources typically utilized to visually process their environment, plan action and engage in executive functioning as compensation to maintain motor coordination (i.e. thinking about their body movements instead of thinking about their intended environmental interaction or movement objective). This allowed compensation strategy in rehabilitation can easily breakdown when suddenly challenged in sport and those compensations are no longer available as the sport activity now demands their focused attention and not the knee joint exercise.25

Typical sport specific functional training is simply not enough

We know that training is specific to physiology, it is readily accepted that to gain strength you lift heavy things, to gain speed you run faster, to gain endurance you train longer. Why then not consider environmental or context specificity in training? The nervous system does not engage in quadriceps activation via the same neural activity on your treatment table that it will on the court when engaged in the myriad of other cognitive processes. Therefore, to be truly sport specific is not only to engage the same energy system, similar movements, and muscle groups, but also to consider the cognitive processing for sport as a vital aspect in rehabilitative exercise prescription. Deficits in cognitive-sensorimotor control after ACL injury may contribute to an athlete’s unpreparedness to return to their prior level of sport performance and is a gap the rehabilitation clinician should aim to bridge just as we try to recover strength, resolve pain and improve movement quality.

Does rehabilitation reinforce athletes to "overthink" motor control?

Just like training neuromuscular control of “proper” knee alignment, cognitive contributions to movement can (and should be) be trained through prevention programs and all phases of rehabilitation. Take a minute to reflect on HOW you prescribe your rehabilitation training even early-stage strength training. Do you allow your athletes to stare at their knee while completing a knee extension, or do you make them visually attend to an external stimulus such as visually tracing a target or attending to visual prompts or stimuli? The latter is a simple and effective strategy for dissociating cognitive attention from body movements and is a common sport demand (i.e. to engage musculature with visual attention directed to the environment). From our current discussion it should be readily apparent why current functional assessments, which allow for single-step processing and complete focus of attention on only maintaining neuromuscular control, fail to provide sufficient data to bridge the gap between ACL rehabilitation and athletic performance. As cognitive abilities detrain just as physical ones do, without engaging in frequent cognitive-motor dual-task challenges through rehabilitation, we allow atrophy of a vital system for performance to occur.

Our patients need to be stronger, faster, and better coordinated, not only in rehabilitation clinics/facilities but on their field/court of performance. As little as altering what the patient’s attention is directed toward during exercise can drastically change how the brain will activate for movement and influence their movement strategy.26 In the clinic, training with only one activation pattern, specifically allowing attention to be directed toward the body movement (i.e. “don’t let your knees cave in”) results in motor strategy that depends on internal cognitive representation. However, upon return to sport the patient’s natural neural model for movement coordination (implicit and externally programmed) will be less efficient since it has not been engaged for 6+ months of rehabilitation.27 The neural activity underlying movement performed implicitly, how we naturally learn and perform movement, is vastly different than explicit which is how we typically train and rehabilitate movement.28,29 Therefore, movement coordination as explicitly taught in rehabilitation may result in rapid within-session performance improvements, however, they likely foster compensatory use of cognition and vision. Ultimately, during the chaotic stress of sport the rehabilitative focused explicit compensatory motor control strategy using cognition and vision must now be used to navigate the environment and is otherwise engaged with sporting cognitive stressors. This forces a reversion back to employing the implicit (natural) movement strategy, however movement with such a neural activation pattern has been untrained for weeks to months, increasing the probability of coordination errors that can lead to injury. Thus, to limit compensatory motor control and improve rehabilitation sport specificity we must train the neurocognitive and motor control systems simultaneously.

Incorporating cognitive challenge into return to play testing

If technology such as Fitlight™ are unavailable in the clinic challenging cognitive processes can still be employed. The use of computer PowerPoints, flashcards (various colors) and even clinician hand movements can be used to induce cognitive functions displayed earlier in Table 2 (essentially replace the Fitlight™ with yourself and the colors with number hand flashes). Additionally, Ness and colleagues have demonstrated good-excellent reliability (ICC 0.85-0.99) incorporating dual-task overlay (visuospatial and working memory) to traditional hop testing using inexpensive/readily available clinical materials.32 Thus, the integration of cognitive-motor dual tasks in return to play testing batteries can be implemented simultaneous with traditional testing to limit time and resource restraints and provide both the traditional physical performance and new cognitive performance metrics in one test.