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Author ORCID Identifier


Open Access Dissertation

Document Type


Degree Name

Doctor of Philosophy (PhD)

Degree Program


Year Degree Awarded


Month Degree Awarded


First Advisor

Richard van Emmerik

Second Advisor

Douglas Martini

Third Advisor

Joseph Hamill

Fourth Advisor

Matt Davidson

Subject Categories

Biomechanics | Cognitive Neuroscience | Motor Control | Sports Sciences


Contact sport participation exposes athletes to repetitive sub-concussive head impacts, which have been shown to elicit cortical neurophysiologic, cognitive, and motor performance alterations that have the potential to disrupt visual perception. Despite the growing concern regarding sub-concussive impacts, our understanding of their implications on motor performance and risk for further injury is limited. A stable head provides a consistent perceptual platform for the visual and vestibular sensory systems, but the effects of contact sport participation on head stability and visual perception remain poorly understood. The goal of this dissertation was to understand whether contact sport participation modifies athletes’ ability to stabilize their head in space and the cortical mechanisms associated with these modifications. To address this goal, we asked the following questions: 1) how does contact sport exposure impact an athlete’s perception-action capabilities during visually demanding locomotor tasks; and 2) whether changes in cortical activity would relate to these motor performance changes. To address our questions, a stepwise approach was taken in three studies to understand how repetitive head impact exposure affects movement control, and how this xi is related to changes in visual perception and cortical activity. First, athletes completed a series of treadmill walking tasks with varying levels of visual task constraints; these constraints increased walking task difficulty to gain insights into how contact sport exposure disrupts the underlying dynamics associated with locomotor tasks and visual perception. By examining coordination, coordination variability, local dynamic stability, and dynamic visual acuity, a more complete understanding of the effects of cumulative sub-concussive impact exposure on locomotor dynamics, and how they relate to perceptual awareness was achieved. Next, to establish whether cortical alterations were associated with changes in motor performance, cortical neurophysiology was assessed while athletes completed a series of two postural and two locomotor tasks, some of which (one postural and one locomotor) included a visually demanding cognitive challenge. We aimed to assess cortical activity in athletes during cognitive and motor challenges to further our understanding of the cortical mechanisms associated with behavioral performance in ecologically relevant environments; this was done through lab-based protocols that attempted to mimic real world environments. Results from the first study indicated contact sport exposure modifies head control. Contact athletes reduced mediolateral head displacement while increasing vertical head and trunk displacement during locomotor tasks. This may be reflective of the reduced independent head control in the transverse plane, revealed through a coordination assessment. The findings from study two highlighted group differences during more demanding fast baseline walking, as indicated by reduced vertical head local dynamic stability in contact sport athletes compared to noncontact athletes during walking at higher speed than preferred. In addition, contact athletes significantly reduced xii both upper and lower body coordination variability during locomotor tasks. This lower variability in the contact group for trunk-head coordination was observed while performing the visual Landolt-C task was imposed, while group differences in lower body variability were present across all conditions. Also, contact athletes exhibited more frequent reductions in lower body variability during visual tasks compared to noncontact athletes. While the beneficial aspects of variability may be task dependent, in the context of sport performance and visual perception, higher variability may be indicative of exploiting abundant degrees of freedom, while reductions in variability are suggestive of sub-optimal performance and/or potential for injury. These findings highlight consistent reductions in movement flexibility and adaptability that may result from contact sport participation. In study three, no statistically significant changes in dorsolateral prefrontal cortex activity were present between groups, but moderate differences were observed during postural tasks, where on average, noncontact athletes increased cortical activity in response to a visual working memory task while contact athletes did not show a response. Similarly, while no statistically significant differences in motor performance were observed, moderate effects were observed for both postural and locomotor motor performance. Specifically, contact athletes displayed greater average mediolateral Center of Mass velocities during postural tasks, and reduced mediolateral head and trunk local dynamic stability during baseline gait compared to noncontact athletes. Collectively across all three studies, no differences in dynamic visual acuity or visual working memory performance were observed. The present studies utilized treadmill walking tasks with varying levels of visual task constraints to assess the consequences of contact sport participation on visual xiii perception, motor performance and cortical neurophysiology. Contact sport athletes exhibited distinct movement dynamics compared to noncontact athletes, including changes in whole-body coordination, variability, and local dynamic stability. Contact athletes reduced independent head control and whole-body coordination variability, which may suggest a modified ability to control joint and segmental degrees of freedom independently. Similarly, while limited within task differences were observed, how athletes responded to the changing constraints differed based on speculated repetitive sub-concussive head impact exposure. Despite prior reports, which identify cortical neurophysiologic alterations associated with increased head impact exposure, we observed no statistically significant group differences in dorsolateral prefrontal cortex oxyhemoglobin concentration changes in response to visual working memory and locomotor tasks imposed in the present study. These findings collectively underscore the intricate nature of the effects of subconcussive head impact exposure on cortical neurophysiology, motor performance, cognition, and visual perception. While visual task performance did not differ, contact athletes demonstrated reductions in independent segment control and movement adaptability during visually demanding motor tasks, which could potentially heighten the risk of injury during sport-specific tasks, though further study is needed.