Multi-joint coordination underlies upright postural control
This dissertation sought to understand whether the potential flexibility provided by a redundant motor system is actually used to control upright posture and to understand the role of that motor redundancy in facilitating the performance of multiple tasks concurrently while standing.
The method used to address this question, the uncontrolled manifold (UCM) approach, provided a means to determine how the positional stability of the whole body, measured via variability of the COM, is stabilized by coordination of the redundant joints of the body. The first study of this dissertation tested the hypotheses that all major joints along the body's longitudinal axis are equally active during quiet standing and that their motions are coordinated to stabilize the spatial positions of the COM and head. The main results of the experiment designed to address this question showed that many joints along the body's longitudinal axis had greater moment-to-moment variability than did the ankle or hip joints (Hsu et al. Journal of Neurophysiology, 97(4):3024-35, 2007, which are presumed in inverted pendulum models to account for most of postural sway. Moreover, the variance of all examined joints was structured largely to stabilize upright posture during quiet standing, indicating that the COM was an important control variable for upright postural stability. That is, overall variance of joint motion which did not affect COM position was substantially and significantly higher than variability of joint motion leading to COM position variability. Moreover, elimination of visual information led to greater joint motion variance which had little effect on the COM position. Instead, the increased variance reflected the use of an ensemble of joint coordination patterns that stabilized the COM position.
The purpose of the second study was to investigate whether and how the available motor redundancy is utilized when additional tasks are performed concurrently. Subjects executed a targeting task alone or in combination with an additional ball-balancing task while standing. The results of UCM analysis of joint variance revealed that the joints were coordinated such that their combined variance had minimal effect on the COM position. The component of joint variance that had no effect on the COM position increased selectively when the task was made more difficult by adding an additional ball-balancing task and when performing the targeting task to a smaller sized target. Similar results were observed when examining joint variance with respect to control of the hand path.
The second study provided evidence to support the hypothesis that a major advantage of a neural control scheme which takes advantage of motor redundancy is to allow performance of multiple tasks simultaneously without any one task unduly interfering with another. The third study attempted to investigate this hypothesis further by examining the effect of artificially eliminating knee and lumbar-thoracic joint motions on postural control when the arms performed targeting tasks concurrently in standing. Subjects performed a targeting task alone or in combination with an additional ball-balancing task while standing with free joint motions (unconstrained condition) and with restricted joint motions (constrained condition). The results of UCM analysis again revealed that the joints were coordinated such that their combined variance had a minimal effect on the stability of the COM position. However, the component of joint variance reflecting the use of motor abundance decreased significantly when subjects performed the combined task with their joint motions constrained. Moreover, the component of joint variance that leads to COM variability tended to increase with a reduction in joint DOFs. Similar results were observed when examining control of the hand's path. Therefore, the results are generally consistent with those of the previous study indicating that reducing the number of DOFs available to stabilize the COM results in greater difficulty coordinating the joints to stabilize the COM when multiple tasks must be performed simultaneously.
This dissertation improves our understanding of multi-DOF coordination of postural control. This knowledge provides a basis for developing improved tools for evaluation and treatment of patients with sensorimotor deficits leading to balance disorders and many provide important insights for the development of new training procedures to help reduce the risk of falls in the elderly, suggesting that the development of training programs that help patients explore the use of motor redundancy may help improve their postural stability. (Abstract shortened by UMI.)