School of Public Health
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The collections in this community comprise faculty research works, as well as graduate theses and dissertations.
Note: Prior to July 1, 2007, the School of Public Health was named the College of Health & Human Performance.
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Item A MECHANISTIC APPROACH TO POSTURAL DEVELOPMENT IN CHILDREN(2011) Bair, Woei-Nan; Clark, Jane E; Kinesiology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Upright standing is intrinsically unstable and requires active control. The central nervous system's feedback process is the active control that integrates multi-sensory information to generate appropriate motor commands to control the plant (the body with its musculotendon actuators). Maintaining standing balance is not trivial for a developing child because the feedback and the plant are both developing and the sensory inputs used for feedback are continually changing. Knowledge gaps exist in characterizing the critical ability of adaptive multi-sensory reweighting for standing balance control in children. Furthermore, the separate contributions of the plant and feedback and their relationship are poorly understood in children, especially when considering that the body is multi-jointed and feedback is multi-sensory. The purposes of this dissertation are to use a mechanistic approach to study multi-sensory abilities of typically developing (TD) children and children with Developmental Coordination Disorder (DCD). The specific aims are: 1) to characterize postural control under different multi-sensory conditions in TD children and children with DCD; 2) to characterize the development of adaptive multi-sensory reweighting in TD children and children with DCD; and, 3) to identify the plant and feedback for postural control in TD children and how they change in response to visual reweighting. In the first experiment (Aim 1), TD children, adults, and 7-year-old children with DCD are tested under four sensory conditions (no touch/no vision, with touch/no vision, no touch/with vision, and with touch/with vision). We found that touch robustly attenuated standing sway in all age groups. Children with DCD used touch less effectively than their TD peers and they also benefited from using vision to reduce sway. In the second experiment (Aim 2), TD children (4- to 10-year-old) and children with DCD (6- to 11-year-old) were presented with simultaneous small-amplitude touch bar and visual scene movement at 0.28 and 0.2 Hz, respectively, within five conditions that independently varied the amplitude of the stimuli. We found that TD children can reweight to both touch and vision from 4 years on and the amount of reweighting increased with age. However, multi-sensory fusion (i.e., inter-modal reweighting) was only observed in the older children. Children with DCD reweight to both touch and vision at a later age (10.8 years) than their TD peers. Even older children with DCD do not show advanced multisensory fusion. Two signature deficits of multisensory reweighting are a weak vision reweighting and a general phase lag to both sensory modalities. The final aim involves closed-loop system identification of the plant and feedback using electromyography (EMG) and kinematic responses to a high- or low-amplitude visual perturbation and two mechanical perturbations in children ages six and ten years and adults. We found that the plant is different between children and adults. Children demonstrate a smaller phase difference between trunk and leg than adults at higher frequencies. Feedback in children is qualitatively similar to adults. Quantitatively, children show less phase advance at the peak of the feedback curve which may be due to a longer time delay. Under the high and low visual amplitude conditions, children show less gain change (interpreted as reweighting) than adults in the kinematic and EMG responses. The observed kinematic and EMG reweighting are mainly due to the different use of visual information by the central nervous system as measured by the open-loop mapping from visual scene angle to EMG activity. The plant and the feedback do not contribute to reweighting.Item On the Development of Postural Stability During Infancy as a Process of Growth and Active, Exploratory Sensorimotor Tuning(2007-12-04) Metcalfe, Jason Scott; Clark, Jane E; Kinesiology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The process by which humans stabilize bipedal stance represents a confluence of changes associated with musculoskeletal maturation and experience-based sensorimotor learning. While investigations have documented a variety of changes with increased bipedal experience, such as reduced velocity and frequency of postural sway and concomitant refinements in muscle activation sequences, the extent to which these changes may be ascribed to growth versus learning processes has not been well characterized. For example, reduced sway frequency is a natural consequence of increasing body height but alternatively, may be explained by active modulations in motor commands specifying the timing and magnitude of muscular activation sequences. It is clear that both types of influences are needed to explain postural development. However, a parsimonious framework for understanding and explaining postural development has yet to be clearly articulated and validated against empirical observations. As such, the purpose of this dissertation was to initiate the development of such an account through a combination of empirical and computational studies. In this dissertation, data are presented from a longitudinal study of upright posture in infants ranging from the onset of independent sitting until 9 months of walking experience; this dissertation focused on the particular period spanning from walk onset onward. Infants participated in a quiet stance task involving hand contact with a surface that was either static or dynamic as well as an independent stance condition. Empirical analyses were performed to estimate the statistical properties of sway and characterize adaptations to static and dynamic manipulations utilizing the touch surface. An unexpected lack of significance for sway magnitude was observed in all conditions. Robust effects, however, were found across measures of rate properties of sway. Taken in the context of previous literature, the empirical observations were used to guide a final study utilizing computational techniques to test hypotheses regarding potential sources of change in postural development. First, the mechanical and computational requirements for postural stabilization were systematically assessed through a review of extant models of both stance and motor learning. Armed with insights from this review, the final study examined an autonomous reinforcement learning algorithm, that was designed to capture the essence of how a human may stabilize his or her posture under the tutelage of exploratory action. Simulation results provided evidence in support of conclusions regarding changes in rate-properties of postural sway and underlying associations with physical growth as well as calibration of both sensory and motor system parameters. Further, simulations emphasized the importance of inclusion of noise in biologically-relevant aspects of the model, such as in sensory and motor processes, as well as the need to consider physical morphology as a primary constraint on sensorimotor learning in the context of upright postural development.