Because movement is controlled by the central nervous system, any type of cognitive disturbance can increase an athlete’s risk of injury. An increasing research focus on these neuropsychological variables could determine the future of rehabilitation and injury prevention.
By Charles Buz Swanik, PhD, ATC
Unintentional injuries related to sport and physical activity are very common in the lower extremity and include more than one million severe sprains annually at the knee and ankle.1-3 Many of these result from what are considered noncontact mechanisms, whereby individuals are not struck by another player, but simply move in a manner that directs excessive loads into the ligamentous structure rather than absorbing them within musculotendinous tissue.
During physical activity, the lower extremity muscles must absorb very high ground-reaction forces that are five to eight times greater than one’s body weight.1,4 Neuromuscular control determines the timing and level of muscle activation needed to stabilize joints in response to these external forces. However, a brief disruption in the cognitive processes can cause a loss of coordination or error in judgment severe enough that the altered muscle recruitment strategy places the individual at an increased risk of injury.5 Reaction time, processing speed, and conscious attention to different cognitive loads (such as visual spatial awareness or verbal communication) are often required during physical activity and may interact with personality characteristics like anxiety and fear to predispose certain individuals to these coordination errors that can contribute to unintentional injury.6-8
Moreover, sex differences have been found with regard to these neuropsychological characteristics and joint stabilization strategies. Female athletes trend lower in visual spatial task performance, whereas male athletes show deficits in verbal tasks.9,10 The risk of noncontact knee injuries is significantly higher in female athletes than in male athletes;2 although it remains conjecture, it is easy to speculate that these neuropsychological differences may influence female athletes’ predisposition to noncontact knee injuries. However, these gender differences also do not fully explain the underlying mechanism because many male athletes suffer similar noncontact injury events.1,11 It is also possible that male and female athletes are predisposed to injury, as a function of these neuropsychological characteristics, in different ways. More research on these neurocognitive variables may identify additional gender-related risk factors for noncontact lower extremity injuries, allowing for the development of appropriate strategies for screening exams, prevention and individualized rehabilitation.
The capacity of muscle tension to assist with joint stabilization is dictated by the muscle’s stiffness properties.12 Stiffness refers to the degree to which a muscle resists changes in length (change in force/ change in length). For instance, increasing muscle activation causes an increase in stiffness, which has been shown to contribute to joint stability.12,13 Neuromuscular regulation of stiffness in the lower extremity during sports and physical activity depends heavily on feed forward motor control processes. Feed-forward neuromuscular control involves anticipating joint loads and position in advance, then planning and executing preprogramed muscle activation strategies to optimally accommodate those loads and positions based on past experiences.14-16 Feed-forward control pre-activates muscles around a joint before excessive loading occurs, which will aid in absorbing large forces at initial ground contact, and in turn decrease stress on the ligaments. Several neurocognitive phenomena have been identified that have the potential to disrupt the formulation and execution of these critical muscle activation strategies.
A change in attentional demands, or loss of concentration, within the central nervous system (CNS) is a major cause of stress-related injuries.17 As the sensory complexity of one’s immediate environment increases, or a situation becomes more stressful, attention will narrow and the person’s ability to process information will decrease.18 An individual can excessively filter sensory information, but then control of the neuromuscular system will become more difficult.8,18,19 This loss of sensory information may cause an increase in the number of errors during a movement execution because attention is diverted from the joint/muscle receptor feedback to the stressful situation, making the individual overly analytical.18 During these situations, the visual field can also narrow, diminishing the ability to properly pick up important visual cues in the periphery, which can further increase the risk of injury.17
Swanik et al5 recently examined the relationship between decreased neurocognitive performance and noncontact anterior cruciate ligament (NCACL) injuries in athletes using the Immediate Post-Concussion Assessment and Cognitive Testing (ImPACT) software. Athletes completed ImPACT testing during the preseason, while they were still healthy. The results showed that those athletes who went on to suffer NCACL injuries had preseason deficits in reaction time, and lower scores for processing speed, working memory, attention, and concentration, when compared to national norms and to matched study controls who did not have NCACL injuries.5 Swanik et al concluded that situational awareness is a prerequisite to executing complex motor programs during physical activity. Increases in a person’s cognitive load (amount and speed of neural processing) may slow reaction time and alter the timely formation or execution of muscle activity, which is associated with poor coordination.5,20,21 Therefore the deficits in reaction time, processing speed, and visual and verbal memory could cause errors in judgment or a loss of coordination by disrupting neuromuscular control, which would compromise dynamic restraint and increase the risk of injury.5
Female athletes are six times more likely to suffer a noncontact knee injury than male athletes, and it is suggested that neuromuscular factors are the primary contributor; however, the involvement of neuropsychological variables has been largely overlooked.1 Neurocognitive abilities appear to be sexually dimorphic, but no research has studied their interaction with regard to joint stability, so it remains unknown whether or how the gender differences in cognitive performance may affect dynamic stabilization and injury proneness. Male subjects do tend to excel on visual spatial tasks, while female subjects demonstrate advantages on verbal and communication tasks.9,10,22,23 Both of these attributes could arguably be important to the performance of vigorous physical activity or sport competition. However, a superficial examination of these advantages/disadvantages could be deceiving. Although male subjects appear to be slightly faster than females during visual spatial tasks, female subjects are actually more accurate than male subjects, and use a more cautious approach.10 In other words, male subjects rely on a more flexible strategy, whereas female subjects use a heuristic system that stays constant.24 So, one might surmise that these cognitive abilities may interact with characteristics such as risk taking, or fear of failure, to alter an individual’s decision making processes regarding feed-forward motor control strategies. However, this theory remains largely speculative and must be explored through a series of well-controlled prospective studies, because, as it will become apparent, retrospective injury data are invalidated by recently discovered neuroplastic changes in the brain.
Recurrent joint instability
Contemporary theories suggest that repeated sprains permanently stretch ligaments and denervate mechanoreceptors, which interferes with the optimal muscle activation strategies necessary to restore joint stability and avoiding disability.20,25 However, 10% to 50% of patients with major ligament injuries present with laxity and a loss of joint sensation but are somehow able to cope, preserving a physical activity lifestyle by apparently compensating with newly learned muscle activation strategies (these patients have been described as “copers”).26 Other injured patients complain of continued joint instability, characterized by a sudden, uncontrolled “giving way”, yet may not exhibit the laxity indicative of “over-stretched” ligaments or the loss of joint awareness that is assumed to occur. This evidence implies that essential neurophysiological attributes influencing the maintenance of joint stability have been overlooked and may explain the divergent functional outcomes of coper and noncoper patient populations.
Recently, several studies have provided strong evidence that the brain undergoes permanent changes to its anatomy as a result of ligament injury, which may explain the differential outcomes of patients. Ligament sensory signals that reach the cerebral cortex give an individual a perception of motion or load, and are used through executive functions to facilitate learning of new, compensatory feed-forward movement strategies. When a ligament injury occurs, it appears the brain may experience a form of sensory deprivation where this information (stimulus) is lacking. Central nervous system reorganization then occurs in response to these deficits, and local neurons begin to encroach upon this region to maximize function.27-29 This phenomenon has been confirmed using several techniques including electroencephalography (EEG), functional magnetic resonance imaging (fMRI), and somatosensory evoked potentials (SEPs); and may explain the success or failure of patients during recovery.
Interestingly, Courtney et al27 found that the individuals with severe knee ligament injuries who could return to sports without reconstructive surgery (high-functioning copers) had abnormal SEPs, suggesting that they were the patients who may have experienced significantly more reorganization than patients who could not “cope” with the injury. However, Kapreli et al30 found increased activity in the contralateral pre-motor areas of non-coper patients using functional MRI. Moreover, patients with knee ligament sprains have increased activity in the part of the brain where visual information is processed, and task complexity or focused attention is involved.29,30
All combined, these data suggest that patients with severe ligament injuries may compensate for the sensory deficits by reorganizing their central nervous systems to rely more on visual information and heightened attention towards task performance. These neuroplastic changes also progress differently in certain patient groups, which may ultimately determine success or failure in their ability to stabilize the joint through altered neuromuscular control strategies.
The extent to which these changes in brain neurophysiology also affect behavior must also be considered. Levy et al31 investigated the relationship between mental toughness, sport injury beliefs, pain, and adherence to rehabilitation. Mental toughness was not found to be associated with injury coping appraisals, but more mentally tough participants perceived their injury to be less severe and felt less susceptible to further injury. This suggests that patients with severe joint sprains may also demonstrate a fear of reinjury when attempting vigorous physical tasks, and supports the possibility that psychological factors may account for the differences in ability among potential coper and noncoper groups. Fear of movement and injury has been investigated in subjects after ACL reconstruction.32 In a retrospective survey three to four years after injury, subjects who were not able to return to their preinjury activity levels reported a greater fear of reinjury, and this fear of was significantly related to return to sport and decreased athletic intensity. Chmielewski et al29 found that fear of reinjury was associated with function after ACL reconstruction only during the timeframe (six to 12 months post-surgery) when patients would be returning to sports. Not surprisingly, bodily pain was also a significant predictor of function, which would have a profound effect on the individuals choice of movement strategies Chmielewski et al.7
The last decade of lower extremity research has produced an abundance of biomechanical data regarding injury mechanisms. However, the movement anomalies related to unintentional, noncontact injuries are still considered motor behaviors, under control of the central nervous system. Several characteristics of CNS performance such as reaction time, processing speed, memory and visual spatial awareness have been linked to injury, and some display a gender bias. These facts warrant support for future research to determine their usefulness in prevention and rehabilitation programs. Furthermore, CNS reorganization appears to occur after lower extremity injury, but proceeds differently in coper and non-coper populations, which suggests that much greater attention to neuropsychological factors may complement traditional rehabilitation programs and maximize patients’ return to sport and physical activity.
Charles Buz Swanik, PhD, ATC, is an associate professor in the department of kinesiology and applied physiology at the University of Delaware.
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