Doctor of Philosophy in Kinesiology (PhD)
Neck muscle activation in automotive collisions
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The vestibular system provides sensory information regarding linear and angular motion of the head for tasks such as spatial navigation and postural stabilization. In these dynamic environments examination of vestibular signals is experimentally difficult given current techniques. Recently, continuous stochastic stimuli have shown promise in addressing some limitations in current vestibular probes and might provide a useful tool for investigating the dynamic behaviour of the vestibular system. The purpose of this thesis is a) to develop further the stochastic stimulus format by examining the customizability of the stimulus bandwidth and the stimulus’ effectiveness in extracting dynamic responses, and b) to use these advancements to explore dynamic vestibular function during locomotion and head rotation. Exploration of the customizability of stimulus bandwidth revealed that a single broad bandwidth stimulus provides similar information to the sum of a series of sinusoidal stimuli or narrow bandwidth stimuli, but in much less time, and that stimulus bandwidth can be modified, by removing frequencies below 2 Hz, to attenuate the postural perturbation created by the stimulus. In a dynamic context the stochastic stimulus was also shown to be very effective in extracting the time varying modulation of vestibular-evoked responses during motion by identifying phase-dependent vestibular responses in the gastrocnemius during locomotion. The stochastic stimulus was then used to examine vestibular modulation and suppression during locomotion and vestibular spatial transformation during head turn. During locomotion, phase-dependent modulation of vestibular responses was observed in muscles of the leg and hip. In some muscles around the ankles these responses are attenuated with increasing cadence and walking speed. Lastly the transmission and spatial transformation of these vestibular-evoked responses are not hindered by motion and the spatial transformation occurs in nearly real time during head rotation. In general, the stochastic stimulus can be customized to reduce postural sway and is effective in extracting the dynamic modulation of vestibular influence on muscle activation. The identification of widespread phase-dependent vestibular coupling in the lower limbs and continuous spatial transformation of vestibular signals demonstrates that the stochastic waveform is an effective tool for the investigation of human vestibular physiology in dynamic contexts.
Maintaining upright stance involves a time-critical process in which the central nervous system monitors postural orientation and modulates muscle activity accordingly. Visual, vestibular and somatosensory systems detect body motion that the balance controller utilizes to update standing control. The time delays between motor output and the resulting sensory feedback are expected and likely accommodated for. Consequently, we perceive whole-body movement as being a consequence of our own actions. Balance control, however, also involves processes that do not rely on conscious perception, allowing us to maintain standing balance almost effortlessly. Recent studies have demonstrated that both the perception and vestibular control of balance are modulated when sensory signals of whole-body movement do not match self-generated ankle torques. The aim of this thesis was to explore the temporal properties of the sensorimotor loops driving the perception and vestibular control of standing balance. Using a robotic balance simulator, experimentally-induced time delays were introduced between human participant’s ankle-produced torques and body movement. The first experiment used a psychophysical design to determine what delay is needed for humans to perceive a change in balance control. All participants were able to perceive a 300 ms delay with 100% success, with an average 69% correct threshold of 155 ms. In the second experiment, participants were exposed to a virtual vestibular perturbation while they balanced their body at different induced delays. Vestibular-evoked muscle responses attenuated with increasing loop delays, falling to amplitudes 84% smaller than baseline when a 500 ms delay was introduced between the produced torques and body movement. This is the first study to explore the time domain relationship between sensory and motor signals in standing, and the results reveal and describe temporal constraints of the sensorimotor control of balance. The present findings will act as springboard for studying postural control mechanisms in the future, encouraging the use of this robotic simulator to alter sensorimotor relationships during ongoing balance control. Using interventions like induced delays, we can decipher the natural processes that govern posture, and explore the adaptability and plasticity of these systems.
Re-learning to maintain standing balance in the presence of a paretic lower limb is important for many stroke survivors. Models of inter-limb adaptations of the central nervous system performing its role as the balance controller can aid the development of post-stroke balance therapies. This thesis quantifies such inter-limb adaptations in healthy participants. Two studies examine whether asymmetrically manipulating the limbs’ contributions to simulated standing balance (i.e., ankle torque gains) using a robotic balance platform can shift balance control toward a targeted limb.In the first study, virtually weakening (decreasing the contribution, or input gain, to the simulation from) a limb in the medial-lateral direction significantly shifted weight distribution, but not anterior-posterior torque variance, towards the virtually weakened limb. Asymmetrically manipulated anterior-posterior limb contributions also did not produce observable changes in torque, despite expectations for the balance controller to adapt and prefer using the virtually strengthened (gain-increased) limb.The second study further investigates manipulating anterior-posterior limb contributions and whether the balance controller is optimally adaptive. The protocol’s torque gain values, unlike those of the previous study, required the balance controller to adopt a new strategy to remain upright. The targeted limb was virtually strengthened by a factor of two (gain of two) while the other limb was virtually reversed (gain of negative one). Two measures of balance contribution were calculated using (1) root mean square torque during quiet stance and (2) the balance controller’s frequency response functions identified during perturbed stance. Over a two-day protocol with gains alternating between normal and manipulated values in each day, significant shifts of balance contributions were observed within and between days. The results demonstrate that the central nervous system can adapt inter-limb balance coordination in the absence of sensory feedback that explicitly communicates the asymmetrical manipulation of the balance dynamics. Anterior-posterior torque gain manipulations show promise as therapy for reducing balance asymmetries, which is crucial for restoring the mobility and independence of stroke survivors. As an additional mode of balance therapy, this novel method may enhance the effectiveness of existing stroke rehabilitation programs. Future work will address the applicability of this protocol to patient populations.
During standing balance, error signals delivered to the vestibular system through an electrical vestibular stimulus elicit compensatory muscle responses in the appendicular muscles involved in the control of standing. This response is only present in muscles that are active in balancing the whole-body but not present when muscle activity is unrelated to balancing the body. Previous work has shown that visual, vestibular, and proprioceptive cues that are congruent with efferent muscle signals through a robotic simulation of standing balance elicit this vestibular reflex response. The physiological connections between the visual, vestibular and neck somatosensory systems imply that congruency between any of the three sources of information and the efferent muscle signals could elicit a vestibular reflex response, but this has not been tested due to difficulties in isolating sensory feedback during standing balance. A newly constructed robotic balance simulator enabled an experimenter to control the congruency between afferent feedback and motor actions associated with standing balance. Here, we tested whether the vestibular reflexes rely on vestibular cues of self-motion being relevant to the control of standing balance. Eight healthy subjects maintained balance on the robotic balance simulator while vestibular cues of balance were minimized. To achieve this, the robotic balance simulator maintained the whole-body stable in space while providing visual and/or lower-limb somatosensory information that was congruent with the motor control of standing. It was shown that even without vestibular cues of self-motion, a vestibular reflex response can be elicited. These results will lead to a better understanding of the vestibular control of standing balance, and may be applicable to populations with balance deficits.
Standing balance is an important unbiased indicator of concussion severity. However, limitedaccessibility to high-end technology and unreliability of simple balance assessment tools make it difficult to assess standing balance accurately outside of research laboratory settings. The objective of this thesis was to develop and validate a simple objective balance assessment tool that can provide an accurate, reliable, and affordable alternative to the currently available sideline methods. In Experiment 1, thirty healthy subjects were filmed performing the Balance Error Scoring System (BESS) while wearing inertial measurement units (IMUs) that measured linear accelerations and angular velocities from seven landmarks: forehead, chest, waist, right & left wrist, right & left shin. Each video was scored by four experienced BESS raters. Mean experienced rater scores were used to develop an algorithm to compute objective BESS (oBESS) scores solely from IMU data. oBESS was able to accurately fit and predict mean experiencedrater BESS scores using acceleration data from only one IMU located at the forehead. In Experiment 2, twenty healthy subjects wore the same network of IMUs and serially performed 12 BESS tests in a hypoxic altitude chamber, aimed at increasing the number of balance errors. Each video was scored by three experienced raters and two athletic trainers. Similarly to Experiment 1, experienced rater scores were used along with IMU data to develop the oBESS algorithm. However, because experienced raters displayed low inter-rater and intra-rater reliability, algorithm training and analyses were performed only using trials where the raters had marginal scoring differences. The oBESS was able to fit mean experienced rater scores with greater accuracy than the two athletic trainers, but not at a level commonly associated with high clinical reliability. In summary, this thesis shows that the oBESS can reliably predict total BESS
The vestibular system conveys information regarding head motion to the central nervous system (CNS). Independently, this vestibular signal of head motion does not provide an absolute reference of head motion as the frequency coding of the afferent nerves is influenced by adaptation properties and nonlinearities. The optic flow signal of head rotation from the visual system however, is spatially encoded and can function as an absolute reference. The aim of this study was to determine if a visual signal of head rotation can recalibrate an altered vestibular signal of head motion during standing balance and to investigate the underlying mechanisms of this recalibration at the muscular level.Eight healthy subjects were exposed to an electrical vestibular stimulus correlated to head movement (±0.125 mA/°/s) while standing on foam with eyes closed. This velocity-coupled vestibular stimulation (VcVS) was applied in a bipolar, bilateral orientation and depending on its polarity, resulted in the vestibular nerves coding for slower or faster head movements. Initially, this alteration of natural vestibular information destabilized subjects. During the conditioning phase, subjects opened their eyes and used visual information in combination with the new vestibular information to update their representation of self-orientation. Following this, subjects showed a significant decrease (~35%) in body sway while still receiving VcVS. The mechanisms underlying vestibular recalibration were examined by observing how visuo-vestibular recalibration affected the vestibular-evoked muscular responses. Muscle activity was recorded in five subjects using surface electromyography (EMG) bilaterally on the medial gastrocnemius and tensor fascia latae muscles. Stochastic vestibular stimulation (SVS) in combination with VcVS was delivered to evoke biphasic muscular responses. Prior to the conditioning period, the peak amplitude of the response was significantly attenuated and then returned to control levels following conditioning. Overall, these observations indicate that the vestibular system can be recalibrated by a visual signal of head rotation. This process is associated with an initial decrease in vestibular-evoked muscular responses which return to control levels once recalibration occurs. These results suggest that the CNS can modulate vestibular processes by down regulation or selective gating of vestibular signals in order to achieve vestibular recalibration.