During nervous system development, growing axons navigate towards their targets using signals from their environment. These signals may be biochemical or mechanical in nature; however, the role of mechanical cues in axon pathfinding in vivo, and the spatiotemporal dynamics of embryonic brain mechanics, are still largely uncharacterised. Here, I have identified a role for tissue mechanics in embryonic axon guidance in vivo, using retinal ganglion cell (RGC) axon outgrowth in the developing Xenopus laevis optic tract (OT) as a model system. Using atomic force microscopy (AFM) to map brain stiffness in vivo, I found that embryonic Xenopus brain tissue was mechanically heterogeneous at both early and later stages of OT outgrowth, i.e. just before RGC axons make a stereotypical turn in the mid-diencephalon, and when they reach their target, respectively. The final path of RGC axon turning followed a clear mechanical gradient: by the later stage, tissue rostral to the OT had become stiffer than tissue caudal to it. This mid-diencephalic stiffness gradient was an intrinsic property of the underlying brain tissue, and correlated with local cell body densities (with higher density rostral to the OT and lower density caudal to it). Crucially, inhibiting cell proliferation in vivo during OT outgrowth abolished the stiffness gradient and reduced OT turning at the later stage. Next, I developed a time-lapse AFM technique to track tissue stiffness and RGC axon behaviour simultaneously in vivo. Using this approach, I followed the evolution of the mid-diencephalic stiffness gradient during intermediate developmental stages, around the time when the OT’s caudal turn is initiated. The stiffness gradient was shallow pre-turn, but increased in magnitude during axon turning (mostly due to an increase in tissue stiffness rostral to the OT). This increase in stiffness gradient preceded the rise in OT turning angle, suggesting that the stiffness gradient is not caused by the invading axons. The observed rise in stiffness gradient correlated with stage-specific increases in local cell density, and was attenuated by blocking mitosis in vivo during time-lapse AFM measurements (which also reduced OT turning). As final confirmation that brain stiffness contributes to RGC axon pathfinding, I disrupted mechanical gradients by artificially stiffening brain tissue in vivo. Importantly, global stiffening via application of transglutaminase eliminated the mid-diencephalic stiffness gradient by increasing tissue stiffness caudal of the OT, and reduced the OT turning angle. Sustained mechanical compression of small areas using an AFM probe stiffened brain locally and repelled RGC axons, consistent with the way they turned away from rapidly stiffening tissue regions during time-lapse AFM experiments. Taken together, these results are consistent with a function for tissue mechanics in axon pathfinding in vivo.

In the last decade, traumatic brain injuries (TBIs) and spinal cord injuries (SCIs) has become one of the most scrutinised medical challenges of our time. However, the lower quality of life experienced by the sufferer and the associated socio-economic cost of both TBI and SCI remain a huge burden for society. There is currently no reliable way to evaluate the level of functional damage caused by TBI and SCI related mechanical forces without invasive examination. The types of axonopathy involved in such injuries are the combinations of coupled mechanical-electrophysiological phenomena at multiple length and time scales, extremely challenging to approach by experimental means alone. It is therefore highly desirable to complement experimental studies with computational work to further the understanding of such multiscale problems. This thesis firstly proposes a novel 3D finite element framework coupling mechanics and electrophysiology to model cellular and subcellular phenomena, such as nerve dislocation and membrane damage by micropipette. The former study shows that 1D simulations focussing solely on the stretch component of the axonal damage are unable to capture the same electrophysiological damage that a 3D framework predicts. The latter study shows that local membrane deformation can lead to electrophysiological alterations at the axonal level solely through geometrical effects and without the need to account for ion channel activity alterations. This was demonstrated for micropipette suction in a patch clamp where the consideration of the 3D flow of current was sufficient to alter its electrophysiology, offering an alternative explanation to the damage mechanism hypothesised by published experimental work. At the axonal and tissue scale, previous models have often simplified the modelling of damage by using a single axon model. It is however unclear whether an altered axonal electrophysiology can truly be representative of the compound electrophysiology of multiple axons or fibre. Three different models: axonal, fibre and tissue level models, were evaluated and compared for their ability to model macroscale electrophysiology deficits. The results of the three models suggest that the recovery of compound action potential amplitude post-mechanical stretch can not be straightforwardly scaled from axonal level to fibre level. Furthermore, the electrophysiological recovery may not be solely dependent on mechanical recovery of the tissue. This thesis identified the need for scale specific models in the context of TBI and SCI. In particular, lipid bilayer membrane geometrical distortion following mechanical insult at the subcellular scale and functional tissue alteration at the tissue scale both require a different approach. The models proposed herein successfully identify mechanisms overlooked in previous experimental literature. In order to fully capture experimental behaviour, future models will need to account for other mechanisms such as mechanoporation, reorganisation of paranodal junctions and injury related Calcium ion imbalance.

A diversity of animals move on and bury within dry and wet granular media, such as dry desert sand or rainforest soils. Little is known about the biomechanics and neural control strategies used to move within these complex terrains. Burial and subsurface locomotion provides a particularly interesting behavior in which to study principles of interaction because the entire body becomes surrounded by the granular environment. In this dissertation, we used three model organisms to elucidate control principles of movement within granular substrates: the sand-specialist sandfish lizard which dives into dry sand using limb-ground interactions, and swims subsurface using body undulations; the long-slender shovel-nosed snake which undulates subsurface in dry sand with low slip; and the ocellated skink, a desert generalist which buries into both wet and dry substrates. Using muscle activation measurements we discovered that the sandfish targeted optimal kinematics which maximized forward speed and minimized the mechanical cost of transport. The simplicity of the sandfish body and kinematics coupled with a fluid-like model of the granular media revealed the fundamental mechanism responsible for neuromechanical phase lags, a general timing phenomenon between muscle activation and curvature along the body that has been observed in all undulatory animals that move in a variety of environments. Kinematic experiments revealed that the snake moved subsurface using a similar locomotion strategy as the sandfish, but its long body and low skin friction enabled higher performance (lower slip). The ocellated skink used a different locomotor pattern than observed in the sandfish and snake but that was sufficient for burial into both wet and dry media. Furthermore, the ocellated skink could only reach shallow burial depths in wet compared to dry granular media. We attribute this difference to the higher resistance forces in wet media and hypothesize that the burial efficacy is force-limited. These studies reveal basic locomotor principles of burial and subsurface movement in granular media and demonstrate the impact of environmental interaction in locomotor behavior.

In addition to its intrinsic importance during quiet standing, posture also serves as the background for a wide variety of other critical motor tasks. The hierarchical nature of the motor control system suggests that the different layers may be responsible for different aspects of posture. I tested the hypothesis that spinal reflexes are organized according to optimal principles of stability, control accuracy, and energy. I found that there were no globally stable muscle activation patterns for muscles operating near optimal fiber length, suggesting that the intrinsic viscoelastic properties of muscle are insufficient to provide limb stability. However, for stiffer muscles a stable limb could be created by selectively activating muscles based on their moment-arm joint angle relationships. The optimal organization of length and velocity feedback to control and stabilize the endpoint position of a limb could not be produced from a purely muscle controller, but required neural feedback to improve endpoint performance, reduce energetic cost, and produce greater coordination among joints. I found that while muscles at near optimal fiber length were insufficient to provide limb stability, the length feedback provided by the autogenic stretch reflex was sufficient to stabilize. Length feedback was also sufficient to produce the directional tuning of muscle activity and constrained ground reaction forces as is observed in experiments. These results have implications for controlling powered prosthetic devices, suggesting that subdividing the responsibility for stability among hierarchical control structures will simultaneous improve stability and maneuverability of the devices.

The overarching goal of this dissertation is to refine methods employed for assessing neuromuscular changes and associated power/force outputs during various perturbations of fatigue, direct or perceived, induced by either exercise or nutritional interventions, with associated performance outcomes. To address this goal, we collected physiological and biomechanical data from subjects across a set of experiments designed to induce different levels of fatigue by the implementation of various exercise and nutritional interventions to cause various levels of fatigue in an ecologically valid manner. The data sets were collected during a single joint task and during cycling trials. During these experimental trials, we collected measures of kinetics (force and cycling power) as well as muscle activation (EMG) and physiological measures (heart rate, rating of perceived exertion, blood lactate, blood glucose, ventilation, oxygen uptake and carbon dioxide production) to investigate the overall performance, as well as potential mechanisms for improved performance related to the exercise and nutritional interventions. In order to substantially enhance the collection of cycling kinetics and kinematics, we have developed an innovative sensor that improved the measurement resolution (temporal and spatial) of a commercial research grade power meter. Using these improved measures alongside advanced muscle activity analysis, we could ameliorate an experimental framework that could be used to investigate changes in fatigue and coordination pattern associated with exercise and nutritional interventions. Investigation of the effects of a CHO mouth rinse vs. placebo on force and muscle activity during a very short (<3 min) neuromuscular demanding fatiguing trial demonstrated a consistent change in EMG median frequency related to increased fatigue in both experimental conditions, providing little evidence of change in neuromuscular strategy associated with CHO mouth rinse. Further investigation explored the effects of a CHO mouth rinse vs. placebo using fundamental physiological measures of neuromuscular activation and overall performance measures during an ecologically valid late endurance cycling time trial. Our results demonstrated that while there was no overall effect noticed for time to completion, there was a significant decrease in performance in the time to complete various components of the time trial during the placebo trial only. Muscle activity of the lower leg (MG and SOL) demonstrated a modification in frequency only evident during the placebo condition. Application of principal component analysis to power output and the EMG intensity profiles of the muscles of the lower leg during the pedal cycle revealed a more detailed understanding of the effect of CHO mouth rinse on performance during cycling. The average power output profile in WASH showed an earlier onset in the pedal cycle, greater duration and higher amplitude versus PLA during the TT. Additionally, only the PLA condition showed a significant increase in muscle activation throughout the time trial, which could be evidence of fatigue. This dissertation shows for the first time that CHO mouth rinse may have a substantial effect on the maintenance of power while mitigating the impact of neuromuscular fatigue, in late endurance performance, further strengthen our assertion that CHO may, in fact, minimize the changes in performance that are associated with fatigue during late endurance fatiguing events. / Graduate

No description available.

Motor control of any movement task involves the integration of neural, muscular and skeletal systems. This integration must occur throughout the sensorimotor system and focus its efforts on controlling the system endpoint, e.g. the foot during locomotion. A person with a uni-lateral trans-tibial amputation has lost the foot, ankle joint, and muscles crossing those joints, hence the residuum becomes the new terminus of the motor system. The amputee must now adjust to the additional challenges of utilizing a compromised motor system as well as the challenges of controlling an external device, i.e. prosthesis, through the mechanical interface between the residuum and prosthetic socket. The obvious physical and physiologic asymmetries between the sound and amputated limbs are also involved in strategies for locomotion involving kinematic and kinetic asymmetries (Winter&Sienko, 1988). There are many questions as to why these asymmetric locomotor strategies are selected and what factors may be influencing that strategy. Factors influencing a change in locomotor strategy could be related to 1) the central nervous system accounting for the loss of sensorimotor feedback, 2) the altered mechanics of this new human/prosthetic system, or some combination of these factors. Understanding how the human motor system adjusts to the amputation and to the addition of an external mechanical device can provide useful insight into how robust the human control system may be and to adaptations in human motor control. This research uses a group of individuals with a uni-lateral trans-tibial amputation and a group of intact individuals using an Ankle Foot Orthosis (AFO) performing a cycling task to understand the "motor adjustments" necessary to utilize an external device for locomotion. Results of these experiments suggest 1) the motor system does account for the activation-contraction dynamics when coordinating muscle activity post amputation, 2) the motor system also changes joint kinetics and muscle activity, 3) these changes are related to control of the interface between the limb and the external device, and 4) the motor system does not alter kinetic asymmetries when kinematic asymmetries are minimized, contrary to a common practice in rehabilitation (Kapp, 2004). Results suggest that control of the external device, i.e. prosthesis or AFO, via the interface between the limb and the device reflect "motor adjustments" made by the nervous system and may be viewed in the context of tool use. Clinical goals in rehabilitation currently focus on minimizing gait deviations whereas the clinical application of these results suggest these deviations from normal locomotion are motor adjustments necessary to control a tool, i.e. prosthesis, by the motor system. Examining amputee locomotion in the context of tool use changes the clinical paradigm from one designed to minimize deviations to one intended to understand this behavior as related to interface control of the device thereby shifting the focus to improving function of the limb/prosthesis system. Kapp SL. (2004) Atlas amp limb def: surg pros rehab princ. 3rd ed: 385 - 394. Winter&Sienko. (1988) J Biomech, 21: 361 - 367.

No description available.

Necessary for effective ambulation, head stability affords optimal conditions for the perception of visual information during dynamic tasks. This maintenance of head-in-space equilibrium is achieved, in part, by the attenuation of the high frequency impact shock resulting from ground contact. While a great deal of experimentation has been done on the matter during steady state locomotion, little is known about how head stability or dynamic visual acuity is maintained during asymmetric walking. In this study, fifteen participants were instructed to walk on a split-belt treadmill for ten minutes while verbally reporting the orientation of a randomized Landolt-C optotype that was projected at heel strike. Participants were exposed to the baseline, adaptation, and washout conditions, as characterized by belt speed ratios of 1:1, 1:3, and 1:1, respectively. Step length asymmetry, shock attenuation, high (impact) and low (active) frequency head signal power, and dynamic visual acuity scores were averaged across the first and last fifty strides of each condition. Over the course of the first fifty strides, step length asymmetry was significantly greater during adaptation than during baseline (p d =2.442). Additionally, high frequency head signal power was significantly greater during adaptation than during baseline (p d =1.227), indicating a reduction in head stability. Shock attenuation was significantly lower during adaptation than during baseline (p d =-0.679), and a medium effect size suggests that dynamic visual acuity was lower during adaptation than during baseline as well (p =0.052; d =0.653). When comparing the baseline and adaptation conditions across the last fifty strides, however, many of these decrements were greatly reduced. The results of this study indicate that the locomotor asymmetry imposed by the split-belt treadmill during the early adaptation condition is responsible for moderate decrements to shock attenuation, head stability, and dynamic visual acuity. Moreover, the relative reduction in magnitude of these decrements across the last fifty strides underscores the adaptive nature of the locomotor and visuomotor systems.