The Role of the Brain in ACL Injury

When we think of ACL injury, phrases such as dynamic knee valgus and quadriceps-to-hamstrings strength are the first that come to mind, among other potential biomechanical and neuromuscular risk factors. As a lower extremity biomechanist, I initially believed that focusing our efforts on lower extremity function and strength would solve our big ACL injury problem. However, my dissertation research (the relationship between sports-related concussion and lower extremity injury)6–8 has led me to believe we are largely ignoring another important area when it comes to ACL injury…the brain! In this blog, I discuss how the brain plays a crucial role in ACL injury by providing both theoretical and data-driven rationales. While the research on the relationship between the brain and ACL injury is still in its relative infancy, there are a lot of exciting avenues that can be taken to help coaches and clinicians further refine risk reduction strategies for their athletes.

Before diving into some of the research, we should first take a step back and consider the complexity of a dynamic sporting environment. Take for example, an American football running back who is about to receive a handoff during a game. Some of the key neurocognitive components may include:

Remembering the blocking scheme of the linemen based on the play call – working memory
Receiving the ball from the quarterback while scanning the field – dual-tasking
Spatial recognition of the opposition and their changing position – visual attention and multiple object tracking
Avoiding oncoming tacklers – reaction time and processing speed

All of this must be completed in seconds or less! We take for granted how truly demanding a sporting environment is from the standpoint of perceiving stimuli and subsequently acting upon this information. To complete any given movement in sport, an athlete interacts with their environment based upon the nature of stimuli, the complexity of neuromuscular action, and the number of response options (Figure 1).

Figure 1. Interplay between the performer and environment.

Non-contact ACL Injury is an Error in Sensorimotor Integration

The sensory information from both internal and external sources during sport must be received, interpreted, and organized correctly and efficiently for task-specific motor output to avoid injury. This is referred to as sensorimotor integration. In any given situation on the field, an athlete possesses a certain “bandwidth” for perceiving sensory information. Visual, auditory, tactile, and proprioceptive information quickly fill this bandwidth, requiring athletes to be able to identify what stimuli are relevant and irrelevant to the task. Slower processing of this sensory information may not allow adequate time to initiate an appropriate and protective neuromuscular response within the temporal and spatial constraints of sport, leading to high impact loads on the ACL and other lower extremity structures (Figure 2). Instead of thinking of ACL injury (of those that are the result of a non-contact mechanism) as a biomechanical or strength issue, we should consider it more so to be an error in sensorimotor integration processes. The sensory information from both internal and external sources during sport must be received, interpreted, and organized both correctly and efficiently for task-specific motor output to avoid injury.

Figure 2. Conceptual framework for slow sensory processing as a risk factor for ACL injury.

Attention and Lower Extremity Biomechanics

To give mechanistic insight into how neurocognitive performance can contribute to actual ACL injury, let’s first look at some of the attention and lower extremity biomechanics research. Most previous ACL injury risk assessments consist of 1) pre-instructed tasks in which athletes know every movement component beforehand (“step off the box, land with two feet, and jump up as high as you can”), and 2) the ability to focus their attention on the movement task without regard for the surrounding environment. However, we know this is not exactly representative of actual sport, as these tasks generally do not fill up the neurocognitive “bandwidth” discussed earlier. On the field, athletes are generally performing in conditions where attention is divided among tasks that impose significant temporal and spatial constraints.

When we assess athletes during tasks that divide attention, there is a competition for resources between the actual motor task and a secondary neurocognitive stressor. Usually, athletes will sacrifice lower extremity stability to complete or maintain neurocognitive performance. During dual-task gait conditions (e.g., counting the months backwards or doing mental math problems), athletes with a concussion history walk slower and sway more in the frontal plane compared to gait conditions without a concurrent cognitive task.10,15 When completing sports-specific tasks under divided attention (e.g., passing a basketball, reacting to an unanticipated stimulus), athletes tend to demonstrate less knee flexion,4 greater knee valgus,2,4,23 and increased ground reaction forces,3 all biomechanical loading patterns that place greater stress on the ACL and heighten the risk for injury. Additionally, tasks that are unanticipated (providing information during task performance as opposed to beforehand) tend to place athletes at greater risk for ACL and lower extremity injury.9

There are a variety of ways practitioners can implement attentional stressors within a dynamic ACL injury risk screening. These may include memorizing a list of words/numbers, Stroop color-word tasks that determine movement direction, and passing/receiving a sports-specific object. Essentially, any way to fill an athlete’s “bandwidth” while performing a dynamic motor task will provide us better information as to how an athlete is processing sensory information and performing concurrent action.

Neurocognitive Performance and Injury Risk Research

Surprisingly, the first study to identify neurocognitive performance as a risk factor for ACL injury was published nearly 15 years ago!19 Baseline pre-season neurocognitive performance on the Immediate Post-Concussion Assessment and Cognitive Testing (ImPACT) battery was assessed in 80 collegiate athletes who sustained an in-season, non-contact ACL injury compared to 80 healthy controls. The ACL-injured athletes performed significantly worse on all subsets of ImPACT (verbal memory, visual memory, processing speed, and reaction time) versus controls.19 Follow-up studies have indicated that computerized neurocognitive performance can be used to identify the risk for future lower extremity strains and sprains in collegiate athletes.20,21 These studies have utilized ImPACT and the Dynavision D2 System to measure neurocognitive performance. Overall, it appears that athletes who perform worse on neurocognitive assessments of reaction time and working memory are at greater risk for ACL and lower extremity injuries.8

At present, there have been only 4 published studies on the relationship between cognitive performance and lower extremity biomechanics associated with ACL injury. Athletes who perform slower on reaction time and working memory measures demonstrate greater ground reaction forces1,14 and dynamic knee valgus14,16 during unanticipated jump-landings and cutting maneuvers. Worse short-term and working memory also appears to associate with decreased landing stability and increased landing errors.11

Can We “Train the Brain” to Reduce ACL Injury Risk?

Figure 3. Example exercise to challenge an athlete’s sensorimotor system.

If an athlete performs poorly on neurocognitive assessments, can we “train the brain” in a manner similar to improving neuromuscular strength and/or control? Although the research is still in its early stages of identifying specific and consistent neurocognitive risk factors, it does appear that slow reaction time performance is modifiable through training interventions. In a study of collegiate football athletes, Wilkerson et al.21 demonstrated a 28% improvement in visuomotor reaction time over a 6-week training period utilizing a Dynavision D2 training system. Other systems, including Fitlight and Senaptec Sensory Station may offer athletes the ability to improve cognitive attributes such as multiple object tracking, visuomotor reaction time, attention, and working memory. However, the efficacy of these tools for reducing the risk of ACL injury require more extensive research.

An additional training strategy that may enhance neurocognitive performance (and thus reduce the risk of injury) is stroboscopic visual training glasses. When wearing these glasses, athletes perform sports-specific tasks while their vision is partially obstructed. Interesting story: one of the first documented athletes to utilize stroboscopic training is none other than the great Michael Jordan.13 MJ used strobe lights and glasses to help him during various shooting drills in order to offset the distracting effects of photographers during games. Since then, athletes and military personnel train with stroboscopic devices to help them process visual information rapidly and efficiently. Recent evidence suggests that visual obstruction training improves important cognitive skills such as anticipation,17 visuomotor reaction time,22 and working memory.5 In theory, processing visual information faster would allow an athlete adequate time to initiate an appropriate and protective neuromuscular response within the constraints of sport, leading to maneuvers that do not impart high loads on tissues such as the ACL.18 While further research is needed, visual training modalities such as the stroboscopic glasses may facilitate neuroplastic alterations in the brain that lead to enhanced neuromuscular control and ultimately reduced risk for injury.12

If a coach and/or clinician does not have access to these more expensive technologies, there are alternative training strategies that require just a little more creativity. Remember, we want our tasks to fill up as much attentional bandwidth as possible to challenge the sensorimotor system. One such example is shown in Figure 3. The athlete starts 15 yards away from the coach and is instructed to sprint towards one of four color stations based upon the word presented on a Stroop color-word card. After 5 yards (the distance can be varied to offer a more difficult peripheral visuomotor challenge), the coach holds up a card (the word blue is presented in the color red) and the athlete sprints and decelerates toward the blue station. While this is just one rather simplistic example, there are many possibilities for developing exercises that offer more challenging circumstances compared to traditional pre-determined movement tasks.

Conclusion

Hopefully this blog offers you a novel perspective on the brain’s role in ACL injury. This is certainly an exciting area of research and clinical practice that we are only starting to develop and refine. Assessing and training our athletes with an emphasis on neurocognitive performance and sensorimotor integration may enhance previously developed ACL injury risk reduction strategies.

Jason Avedesian

Jason Avedesian serves as the Director of Olympic Sports Science at Clemson University. He is responsible for overseeing sports science initiatives across all Olympic sports, including research and design, athlete monitoring, data analytics, technology implementation, and oversight of the sports science internship.
Prior to joining Clemson, Jason spent time in various research and sports science settings across adolescent, collegiate, and professional settings. Most recently, Jason was a post-doctoral researcher through the Emory University School of Medicine. Jason's research focuses on ACL injury, sports-related concussion, and how sensorimotor performance contributes to lower extremity injury in athletes.
Jason holds degrees in Mechanical Engineering and Kinesiology (Michigan State University), Biomechanics (Ball State University), and Interdisciplinary Health Sciences (UNLV).

References

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