Maintaining balance is controlled by two different processes: feedforward and feedback control. Feed-forward control is used prior to performing voluntary movements whereas feedback control is used to correct for unexpected perturbations. Studies suggested that age-related changes in postural responses may contribute to increased risk of falls in older adults. To address whether Tai Chi training can induce improved patterns of feed-forward control, voluntary arm elevations during standing were performed. Compared to age-matched controls, smaller displacements of the center of pressure were found among older adults who practice Tai Chi. This may suggest adapted feed-forward control induced by training. To investigate feedback control, perturbations were applied while walking with various arm constraints. Context-dependent modulation in response amplitude was found with changing levels of postural threat in older adults, comparable to young adults. Delayed onset latencies and frequent inhibition of Soleus may suggest less effective balance strategies employed in older adults, and an increased risk of falling. / Rehabilitation Science

This thesis examined Autonomic Nervous System activity evoked by postural instability. Autonomic reactions were assessed using measures of electrodermal activity at the surface of the skin. Perturbation-evoked electrodermal responses (EDRs) were consistently observed in conjunction with both upper and lower limb postural reactions. These autonomic responses were sensitive to perturbation amplitude, as well as experimental manipulations which did not influence the perturbation itself but which affected response execution. In particular, stimulus predictability and movement challenge modulated EDRs, although purely cognitive manipulations did not significantly affect autonomic responses. Probes examining the potential role of such autonomic potentials in compensatory balance control revealed that EDRs evoked during compensatory postural reactions were larger and more consistent than potentials evoked by purely motor or sensory stimuli, suggesting that evoked autonomic activity plays a role in compensatory behavior. While the specific role of autonomic contributions in compensatory balance control require further study, speculative models for autonomic contributions propose either feedback-based pathways for detection of instability to initiate the postural reaction, and/ or an adaptive role to higher centers important for establishing sensorimotor gain in future conditions. This thesis presents new evidence regarding basic neural mechanisms engaged in the recognition and response to postural instability, and future work may extend these findings in clinical populations with high fall incidence and offer clues as to alternative causes for falls and fall prevention.

This study used an energy budget approach to record changes in the biology of the Antarctic sea urchin Sterechinus neumayeri in relation to environmental seasonality (i. e changes in chlorophyll standing stock and seawater temperature) over an unbroken two year period. Chlorophyll standing stock showed a brief but intense bloom each austral summer which contrasted with prolonged winter minima. Benthic chlorophyll standing stock, as recorded from sediment cores showed a similar cycle. Seawater temperature varied between -1.8°C and +1.2°C. Feeding activity was highly seasonal and closely correlated to chlorophyll standing stock. Feeding ceased during the austral winter of 1997 and 1998 for 6 and 4 months respectively. Metabolism, as measured by oxygen consumption and also ammonia excretion showed strong seasonality, with relatively brief 3 to 4 month periods of elevated activity in the austral summer contrasting with prolonged winter dormancy. Laboratory studies indicated that only 10-15% of the 3 fold seasonal rise in metabolism was caused directly by temperature (Q10=2.5) and that 80- 85% was related to increased physiological activity associated with feeding. Growth rate was measured over one year and was very slow. Comparison with other studies indicated that echinoid growth rate is strongly dependent on food availability, but that maximal growth rate is limited by seawater temperature, or by a co-varying factor. S. neumayeri is an annual spawner and histology was used to describe both the vitellogenic cycle and also to calculate reproductive output. Comparison with other published studies worldwide indicated that reproductive output is highly dependent on food availability, and that maximal reproductive output is not limited by temperature. Although the overall P: B ratio was low, the ratio of reproductive production to total production was higher than expected. These results indicated that due to the low metabolic rate only 12-16% of total body energy levels were used to endure the prolonged non-feeding polar winter. The overall annual growth efficiency was greater than for warmer water species, due to the larger relative contribution to reproductive output.

The Melnikov method is applied to a model of parametrically generated cross-waves in a long rectangular channel in order to determine if these cross-waves are chaotic. A great deal of preparation is involved in order to obtain a suitable form for the application of the Melnikov method. The Lagrangian for water waves, which consists of the volume integrals of the kinetic energy density, potential energy density, and a dynamic pressure component, is transformed to surface integrals in order to avoid constant conjugate momenta. The Lagrangian is simplified by subtracting the zero variation integrals and written in terms of generalized coordinates, the time dependent components of the crosswave and progressive wave velocity potentials. The conjugate momenta are calculated after expanding the Lagrangian in a Taylor series. The Hamiltonian is then determined by a Legendre transformation of the Lagrangian. Ordinarily, the first order evolution equations obtained from derivatives of the Hamiltonian are suitable for applications of the Melnikov method. However, the crosswave model results in extremely complicated evolution equations which must be simplified before a Melnikov analysis is possible. A sequence of seven canonical transformations are applied and yield a final set of evolution equations in fairly simple form. The unperturbed system is analyzed to determine hyperbolic fixed points and the equations describing the heteroclinic orbits for near resonance cases. The Melnikov function is calculated for the perturbed system which must also satisfy KAM conditions. The Melnikov results indicate the system is chaotic near resonance. Furthermore, the heteroclinic orbits, about which chaotic motions occur, are transformed back to the original set of variables and found to be extremely complicated; this orbit would be impossible to determine analytically without the canonical transformations. The theoretical results were verified by experiments. Poincare maps obtained from measurements of the free surface displacement indicate both quasi-periodic and chaotic motions of the water surface. Power spectra and time series of the water surface displacement are also analyzed for chaotic behavior, with less conclusive results. Stability diagrams of cross-wave generation confirm behavior consistent with parametric excitation. / Graduation date: 1994

Purpose: Biomechanical differences between people with low back pain (LBP) and healthy controls have been shown previously. LBP has been associated with standing postures in occupational settings. A transient pain-generating model allows for comparisons between pain developers (PD) and non-pain developers (NPD). The first objective was to utilize a multifactorial approach to characterize differences between PD and NPD individuals. The second objective was to investigate the impact of exercise on LBP during standing. Methods: Forty-three participants without any history of LBP volunteered for this study. Participants performed pre- and post-standing functional movements and 2-hours of standing. Continuous electromyography (EMG) data were collected from 16 trunk and hip muscles, kinematic and kinetic data were used to construct an 8-segment rigid link model. Vertebral joint rotation stiffness (VJRS) measures were calculated. Participants completed visual analog scales (VAS) rating LBP every 15 minutes during the 2-hr standing. Participants were classified as PD or NPD based on greater than 10 mm increase in VAS. Participants were assigned to exercise (EX) or control (CON) groups. All participants returned for a second data collection following 4-weeks. Results: Forty percent of participants developed LBP during standing. The PD group had elevated muscle co-activation prior to reports of pain (p < 0.05). Following standing, there was a decrease in VJRS about the lateral bend axis during unilateral stance. PDEX had decreased VAS scores during the second data collection (p = 0.007) compared with PDCON. Male PDEX had decreased gluteus medius co-activation during standing (p < 0.05). Between-day repeatability for the CON groups was excellent with intraclass correlation coefficients > 0.80 for the majority of the outcome measures. Conclusions: There were clear differences between PD/NPD groups in muscle activation patterns, prior to subjective reports of LBP, supporting the hypothesis that some of the differences observed between these groups may be predisposing rather than adaptive. An exercise intervention resulted in positive changes in the PD group, both in subjective pain scores as well as muscle activation profiles. Elevated muscle co-activation in the first 15-30 minutes of standing may indicate that an individual is at increased risk for LBP during standing.

Purpose: Biomechanical differences between people with low back pain (LBP) and healthy controls have been shown previously. LBP has been associated with standing postures in occupational settings. A transient pain-generating model allows for comparisons between pain developers (PD) and non-pain developers (NPD). The first objective was to utilize a multifactorial approach to characterize differences between PD and NPD individuals. The second objective was to investigate the impact of exercise on LBP during standing. Methods: Forty-three participants without any history of LBP volunteered for this study. Participants performed pre- and post-standing functional movements and 2-hours of standing. Continuous electromyography (EMG) data were collected from 16 trunk and hip muscles, kinematic and kinetic data were used to construct an 8-segment rigid link model. Vertebral joint rotation stiffness (VJRS) measures were calculated. Participants completed visual analog scales (VAS) rating LBP every 15 minutes during the 2-hr standing. Participants were classified as PD or NPD based on greater than 10 mm increase in VAS. Participants were assigned to exercise (EX) or control (CON) groups. All participants returned for a second data collection following 4-weeks. Results: Forty percent of participants developed LBP during standing. The PD group had elevated muscle co-activation prior to reports of pain (p < 0.05). Following standing, there was a decrease in VJRS about the lateral bend axis during unilateral stance. PDEX had decreased VAS scores during the second data collection (p = 0.007) compared with PDCON. Male PDEX had decreased gluteus medius co-activation during standing (p < 0.05). Between-day repeatability for the CON groups was excellent with intraclass correlation coefficients > 0.80 for the majority of the outcome measures. Conclusions: There were clear differences between PD/NPD groups in muscle activation patterns, prior to subjective reports of LBP, supporting the hypothesis that some of the differences observed between these groups may be predisposing rather than adaptive. An exercise intervention resulted in positive changes in the PD group, both in subjective pain scores as well as muscle activation profiles. Elevated muscle co-activation in the first 15-30 minutes of standing may indicate that an individual is at increased risk for LBP during standing.

Recent advances in microfluidic technologies have enabled integration of the functional units for biological and chemical analysis onto miniaturized chips, called Labon- a-Chip (LOC). However, the effective manipulation and control of colloidal particles suspended in fluids are still challenging tasks due to the lack of clear characterization of particle control mechanisms. The aim of this dissertation is to develop microfluidic techniques and devices for manipulating colloids and biological particles with the utilization of alternating current (AC) electric fields and acoustic waves. The dissertation presents a simple theoretical tool for predicting the motion of colloidal particles in the presence of AC electric field. Dominant electrokinetic forces are explained as a function of the electric field conditions and material properties, and parametric experimental validations of the model are conducted with particles and biological species. Using the theoretical tool as an effective framework for designing electrokinetic systems, a dielectrophoresis (DEP) based microfluidic device for trapping bacterial spores from high conductivity media is developed. With a simple planar electrode having well defined electric field minima that can act as the targetattachment/ detection sites for integration of biosensors, negative DEP trapping of spores on patterned surfaces is successfully demonstrated. A further investigation of DEP colloidal manipulation under the effects of electrothermal flow induced by Joule heating of the applied electric field is conducted. A periodic structure of the electrothermal flow that enhances DEP trapping is numerically simulated and experimentally validated. An acoustic method is investigated for continuous sample concentration in a microscale device. Fast formation of particle streams focused at the pressure nodes is demonstrated by using the long-range forces of the ultrasonic standing waves (USW). High frequency actuation suitable for miniaturization of devices is successfully applied and the device performance and key parameters are explained. Further extension and integration of the technologies presented in this dissertation will enable a chip scale platform for various chemical and biological applications such as drug delivery, chemical analyses, point-of-care clinical diagnosis, biowarfare and biochemical agent detection/screening, and water quality control.

ABSTRACT Under stationary atmosphere and on uniform depth, this paper treats the standing waves in real fluids formed by two progressive waves possessing same properties but in opposite direction. Being different from the preceding scholars who usually treated the waves in real fluids with boundary layer theory, the author uses complete Navior-Stokes Equ. to analyze the entire flow field. When dealing with the free surface dynamical boundary condition, under the equilibrium of forces, the author takes account of atmosphere pressure, shear stress and surface tension. As for the bottom condition, at first consider the perfect smooth, then no-sliding and sliding condition. After constructing the boundary conditions and the governing equation, perturbation method is used to get those of second order, and the second order solution can be derived. In addition to relative depth , the bottom-adherence affects the bottom boundary effect. No matter in progressive or standing wave fields, we can see the variation of over-shot height, the asymmetric diagrams of fluid particle¡¦s horizontal velocity with phases, the phase difference between the second and first order bottom shear. Besides, in standing wave field, the existence of second-order interaction term not only affects the flow field in the boundary layer but also the field outside it.

Thesis (Ph. D.)--University of Texas at Austin, 1999. / Vita. Includes bibliographical references (leaves 185-194). Available also in a digital version from Dissertation Abstracts.

Recovery of the ability to maintain balance during standing is one of the primary and essential goals of rehabilitation programs for individuals with Spinal Cord Injury (SCI). Regaining functionality during standing by means of a neuroprosthesis would decrease secondary complications and increase independence, and would consequently improve the quality of life of these individuals. However, the development of a standing neuro- prosthesis requires techniques to generate reliable and predictable torque vectors in the lower limbs. We proposed and tested a method based on surface Functional Electrical Stimulation (FES) and the idea that three independent muscles can form a basis that would span the joint torque vector space. We tested the proposed stimulation technique on the quadriceps muscles that produce knee extension. The results of this study suggest that the quadriceps muscle basis vectors are insufficient to cover the knee joint vector space.