A traumatic brain injury (TBI) refers to an external force to the head resulting in structural and/or functional changes to the brain; these are typically associated with acute alterations to consciousness and then transient or persisting cognitive, motor, mood and/or somatic symptoms thereafter (1, 2). A TBI can vary in presentation, depending on severity and region(s) affected (2). TBI is currently the leading cause of death in individuals under the age of 50 in Canada and the United States (2). The prevalence of TBI is increasing, largely due to the increased survival rate of motor vehicle accidents, which is in turn coupled with higher persisting physical and cognitive dysfunction even post-therapy (3).

It is estimated that over three million people with TBI in the United States are living with a significant long-term disability (2, 4, 5). Persisting impairments associated with disability include cognitive deficits (1) and neuromotor impairments (6), such as balance (7). These disabilities adversely affect the livelihood of people with TBI, resulting in loss of productivity. Finkelstein et al. (8) found that after being hospitalized, 1 out of 5 Americans with TBI did not return to work 1 year-post-injury, due to a disability, amounting to an estimated $51.2 billion in total lifetime productivity loss. TBI has become a global public health concern due to the high prevalence of lasting health challenges and is predicted to remain the leading cause of disability stemming from a neurologic disease, until at least 2031 (1, 9).

A common and debilitating challenge experienced by TBI survivors is gait-associated mobility impairment (10, 11). For instance, gait analysis has shown that compared to healthy controls, people with TBI walk slower due to reduced step length and stride length (10, 11). In addition, individuals often report instability during ambulation, revealing deficits in balance (12). As a direct result, people with TBI exhibit increased step-to-step variability (13), may walk slower (11, 12), and report difficulty navigating their environment (10, 12). Yet, despite the existence of strong evidence for persistent motor deficits after TBI when compared to other neurological populations such as Parkinson's disease and stroke, motor impairments have received less attention within the TBI population (14). Mobility challenges, most notably abnormal gait function, have been linked to decreased community engagement (7, 13) and self-rated quality of life, post-TBI (15).

In addition to gait dysfunction, people with TBI exhibit persisting cognitive impairments such as attention, concentration, learning, memory, and executive functioning deficits (16). Executive function can be defined as a group of related, yet distinct, cognitive processes that modulate information from the prefrontal cortex to produce intentional, goal-directed behavior. Models of executive function [e.g., Miyake and Friedman's model (17)] generally include three foundational components: inhibition of pre-potent responses, monitoring and updating, and mental shifting. From these components, basic cognitive processes such as attention control, cognitive inhibition, cognitive flexibility, and working memory, are derived. As these integrative functions are imperative for effective goal directed actions, they play a vital role in independently managing work, social relationships, activities of daily living, as well as re-integrating into the community, post-TBI.

Functionally, cognitive and physical impairments are often addressed independently during rehabilitation, for example by therapies that discretely focus on one (e.g., physiotherapy) or the other (e.g., speech language therapy; neuropsychology) (11). However, increasing evidence has shown that cognitive and motor processes are closely linked (11). Motor function is mediated by complex cortical, subcortical, and brainstem networks that communicate with each other and project signals via the reticulospinal pathway to the spinal cord, resulting in coordinated movement (18, 19). TBI can disrupt one or several pathways implicated in cognitive functioning, which may impede motor functioning, such as gait (20–22). For example, impaired executive function is associated with slower walking speed (20), increased incidence of falls (23, 24), and increased stride time variability (25). Furthermore, dual-tasking has demonstrated the interplay between executive function and gait, both of which are commonly affected after TBI (20, 26).

Although functional outcomes following TBI have previously been predicted by demographic variables and severity of brain injury, they have produced mixed results (27). Alternatively, there has been a new effort to focus research on cognitive sequela due to TBI. Based on the evidence that cognitive function is associated with motor impairments, especially in locomotion, the question arises if cognitive outcomes can also become predictors of motor outcomes. For example, emerging evidence shows associations between cognition and motor recovery in stroke, most pronounced for executive function (28). However, such potential connections have not yet been investigated for TBI. Therefore, this study is a novel investigation that seeks to close the existing knowledge gap to improve outcomes for successful TBI rehabilitation. If associations or predictions are found, this may present a platform for future research to integrate both cognitive and motor domains in rehabilitation exercises. Previous studies involving persons with TBI have examined cognitive and motor recovery separately. Therefore, this investigation presents a novel approach in analyzing the long-term relationship between cognitive domains, particularly executive function, and gait as one of the most critical motor outcomes during post-trauma recovery.

We sought to investigate if correlations may exist between measures of executive function and gait recovery, post-TBI. As our first hypothesis, we predicted that significant correlations would emerge in early recovery and would be maintained longer term between cognition and gait performance. Additionally, we sought to investigate the predictive power of executive functioning at 2-months post-TBI on gait outcomes at 12-months post-TBI. As our second hypothesis, we predicted that executive function in early recovery stages is a significant predictor for long-term gait outcomes.

Traditionally, physical and cognitive impairments post-TBI have been addressed separately, and therefore, their rehabilitation processes have occurred independently (11). There is increasing evidence suggesting that physical processes such as ambulation require higher order cognitive inputs, including executive function and attention (11). Therefore, it is imperative to understand the relationship between cognitive function and gait post-TBI in order to create effective rehabilitation protocols for both cognitive and physical function. This study presents new data to elucidate the relationship between cognitive functioning and gait post-TBI with the aim to provide preliminary evidence for the creation of effective rehabilitation protocols that addresses cognitive and physical function in an integrated fashion. The current investigation shows evidence for pre-rehabilitation and post-rehabilitation correlations and predictions that exist between measures of gait variability and gait velocity and executive function and attention. The results offer several key findings.

First, in the early stages of recovery, levels of cognitive function and gait function seem closely correlated. TMT-B and DS-F scores significantly correlated with both SP and MP gait speed measures; and reductions in temporal stride variability, the latter being a key indicator of gait control.

Second, long-range correlations persist between cognition and gait at 2- and 12-months, respectively. Higher executive function status in early recovery is associated with long-term improvements in gait variability and higher attention function in early recovery and is associated with long-term improvements in gait velocity.

Finally, our regression analysis shows scores on the TMT-B as a significant predictor of spatial and temporal gait control variability as well as gait speed, even when controlled for age, gender, and influence of outlier observations. Although a predictive relationship was found between the independent variables, the low adjusted R² values require a cautionary interpretation of the limited precision of the predictions.

Previous studies have shown connections between executive and attention function and motor control, including gait functions (3) [e.g., (20, 22, 44–46)]. While the connections between cognition and motor function have been mostly investigated in cohorts of healthy elderly persons and persons with stroke and Parkinson's disease, our study for the first time shows similar connections in TBI recovery. Furthermore, we were able to elucidate a predictive role for early positive cognitive recovery, especially for executive function, for improvement of critical kinematic gait variables in long-range gait control after 1 year.

There is a consistent body of research showing the importance of physical ability for maintaining cognitive function. However, our data show that there is also a reversed role for cognitive function predicting long-term motor outcomes. One challenge associated with TBI treatment and rehabilitation is that unlike other neurological conditions, there is not one common mechanism of injury (3). The many ways one can obtain a TBI and present clinically lead to a plethora of diverse symptoms. This study has shown that cognition is predictive of gait, across a spectrum of TBI injuries. This evidence may lead to a more holistic strategy in early TBI rehabilitation to design rehabilitation exercises that integrate appropriate cognitive and motor challenges in re-learning protocols. Early studies examining dual task intervention programs for individuals with acquired brain injury have demonstrated a lack of generalizability (47). Further studies are required to assess the efficacy of concurrent cognitive-motor training regimes.

Our data confirm that at least at a functional and goal-directed level, walking involves executive and attention capabilities, and if those capabilities are better preserved post-injury, they can be important predictors for long-term improvements of walking ability. What mechanisms may account for the associative and predictive relationship? Poor cognition is typically associated with poor rehabilitation outcomes. One of the suggested mechanisms borrows from stroke research as a model for acquired brain injury and postulates that poor cognition alters implicit and explicit learning abilities which are important for recovery in stages of adaptation as well as remediation on an impairment level (19, 48). This mechanism may also underlie the outcomes in our present TBI study. On the neural level, studies have pointed toward possible pathways that link cognition and motor outcomes at alterations in the neurochemical milieu (49). Thus, within the context of shared pathways in neurotransmitter signaling, recovery of cognitive impairment may also provide a more favorable neurochemical milieu for functional recovery of motor control, which in our case would specifically pertain to gait mobility (50). Finally, our model data may suggest that they share representations in the brain, e.g., in tertiary association areas, or share a common network characterized by long-range internetwork connectivity (51).

Our study presents novel data in regard to significant associations and predictions, however, with limited model fit, between cognitive and motor recovery in TBI. These associations and predictions have not been previously researched. Therefore, as opposed to traditional approaches, we suggest that further investigations address cognition and motor control in an integrated fashion based on a “neural internetwork” model of rehabilitating impaired brain function.

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author/s.

The studies involving human participants were reviewed and approved by Toronto Rehabilitation Institute. The patients/participants provided their written informed consent to participate in this study.

VV: conceptualization, design/methodology, writing original draft, data collection, and data analysis. KP: conceptualization, design/methodology, and review and editing. LC and TH: data analysis. CS: design/methodology and data collection. RG: review and editing. MT: conceptualization, design/methodology, writing original draft, data analysis, and supervision. All authors contributed to the article and approved the submitted version.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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