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<p> Burial is an important life history strategy employed by benthic fishes that has not been fully explored in its diversity by the biomechanical literature. This thesis explores the mechanism by which the Pacific sandfish buries as well as the physical limitations of the behavior. We first investigate the role of the ventilatory pump in the burial behavior of sandfish by using high-speed videography, dye, and digital particle image velocimetry (DPIV). We determined that sandfish employ a modification of the ventilatory pump, which is used repeatedly to fluidize the substrate ventral to the head. This modification of the ventilatory pump should reduce the energetic costs associated with burial as it decreases the cost of transport typically associated with &lsquo;shoveling&rsquo; substrate. Second, we investigate the physical limitations that are caused by the reliance on the ventilatory pump to fluidize substrate. We used sand beds of varying grain sizes, and therefore varied the minimum velocities of fluidization, to determine how sandfish respond variation in substrata. We determined that sandfish can bury in grains smaller than 1.00mm in diameter but were unable to bury in any substrate larger than 1.00mm. We also determined that there was an increase in the time it took sandfish to bury in those substrates smaller than 1.00mm as grain size increased. There was no change in the frequency of the behavior, however, suggesting that sandfish have very little ability to bury in larger substrates. We also determined that it is probably not the absolute velocity produced by the opercular jet that determines burial success, but the ability burying behavior to maintain the sand&rsquo;s momentum during the expansive phase that occurs between bouts of opercular jetting.</p>

<p> Intramuscular pressure (IMP), which is closely correlated with both active and passive muscle tension, may become a useful supplement to current clinical tools as a means to quantify individual muscle-generated force. A continuing challenge associated with this measure is its non-uniform distribution, which is not yet fully understood. Several studies have observed that pressure increases with muscle depth. Conservation of mass suggests that these regional pressure differences may result from non-uniformly distributed changes in local tissue volume. Therefore, the overarching goal of this work was to characterize volumetric strain distribution in skeletal muscle as a means to better understand the mechanism driving the non-uniform IMP distribution. </p><p> Three-dimensional volumetric strain distribution had not been previously quantified in skeletal muscle; therefore the bulk of this thesis work revolved around developing and validating a method for this purpose using cine Phase Contrast (CPC) magnetic resonance imaging (MRI). CPC MRI has been previously used to quantify 2D strain distribution in skeletal muscle. Fortunately, the method lends itself to 3D measurements using multiple slice data collection, but this requires a lengthy data acquisition time. We chose to develop the method during passive tension of the human tibialis anterior (TA), because passive tension is closely correlated with IMP and the motion repeatability is more readily controlled and maintained for an extended duration than active tension. </p><p> As hypothesized, volumetric strain was found to be non-uniformly distributed during passive tension of the human TA with a decreasing trend from the anterior (superficial) to the posterior (deep) muscle regions. These data align with previously observed trends of decreasing IMP near the muscle surface and may provide important insight into ideal sensor placement regions to maximize measurement repeatability. These results advance our understanding of the tension-IMP relationship in muscle by providing insight into the mechanism behind the non-uniform distribution of IMP. Furthermore, this work has strong potential to contribute to a computational model relating IMP to muscle tension by way of volumetric strain.</p>

<p> Although coordination has been identified as a fundamental element necessary for the successful achievement of walking, this aspect of gait has yet to be embraced into instrumented gait analysis, perhaps in part due to the lack of a normative reference and unfamiliarity of mathematical methods that are best suited to capture this essential behavior. Therefore, this work focused on expanding clinical gait analysis techniques by validating nonlinear methods that describe the influence of neurological control on the musculoskeletal system. This body of work investigated the coordination dynamics during gait in both prospective and retrospective subjects free of gait pathology, subjects with spastic cerebral palsy, and subjects with a lower limb amputation using motion capture and mathematical models to help elucidate the complexities of gait and enhance therapeutic interventions. This investigation quantified coordination strategies employed by an unimpaired subject when presented with various walking conditions and challenges mimicking various inhibitions associated with performing the task of swing limb advancement. Two novel indices of coordination dynamics were created to provide a concise metric and ease their inclusion into future research applications. The first normative reference dataset of these coordination measures was created from a large cohort of unimpaired subjects. While there is presently not a gold standard method for quantifying coordination during gait, the exciting correlations between the proposed measures and select clinical performance tasks indicate the coordination measures quantify essential inter-segmental coordination dynamics of walking. The theoretical pendular software model created shows swing limb advancement is not a purely passive motion, but instead an actively controlled motion. Comparisons between the various cohorts revealed the proposed measures of coordination are more suitable for characterizing motor control strategies contributing to a gait pattern, quantify organization of individual segments, identify mechanisms of change, and reveal the loci of impairment(s). The proposed measures of coordination dynamics are capable of distinguishing between different gait pathologies and patterns associated with altered limb advancement during the swing period of gait. Results from this multidisciplinary work have the strong potential to directly impact the clinical treatment of persons with aberrant coordination dynamics during gait.</p>

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