This thesis investigates the processes of human postural control. How do we keep still? In any action, the force required changes with limb position. This force-position relationship is elastic stiffness. Holding a desired posture requires muscle activation that accounts for this load property. The studies here examine the physiological processes of postural control as elastic stiffness changes. Psychophysical studies show that thresholds for detecting differences in load stiffness are large relative to those normally encountered. To discriminate stiffness, subjects made consistent movements and judged the force required and detection thresholds followed rules for force perception. For the purposes of postural control, stiffness per se is not a variable of primary interest. The effects of load stiffness on postural stability were investigated using a pendulum that allowed independent control of load force and stiffness. Postural stability varied with load stiffness and this effect was independent of load force. Performance deteriorated as load stiffness became more negative. Load-dependent changes were at low frequencies only, suggesting neural processes operating at long latency, and perhaps ???volitional??? tracking, are the key to postural control. Imposed perturbations evoke patterns of muscle activation reflecting the state of the neural pathways of postural control. Stretch responses obtained while subjects held different loads show that the short-latency spinal reflex and the long-latency functional reflex in the active flexor muscle are unaffected by load stiffness. However, a stereotyped response observed after stretch-reflex latency varied systematically with load stiffness, as did reciprocal activation of the antagonist extensor muscle. The long-latency reflex appears to be a part of a coordinated reciprocal response of antagonist muscle pairs. Adapting to load properties involves modulating these later neural responses. A method was developed, based on ultrasound, to track changes in muscle and tendon length associated with small postural movements. The relationship between wrist angle and muscle and tendon length in the active muscle changed with load stiffness. Particularly with negative-stiffness loads, the wrist moves on the end of a compliant tendon without corresponding changes in muscle length. Thus, compensation of postural performance by neural modulation is limited by the properties of muscle and tendon.
| Identifer | oai:union.ndltd.org:ADTP/272625 |
| Date | January 2009 |
| Creators | Chew, John Zong Zheng, Prince of Wales Medical Research Institute, Faculty of Medicine, UNSW |
| Publisher | Awarded by:University of New South Wales. Prince of Wales Medical Research Institute |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | Copyright Chew John Zong Zheng., http://unsworks.unsw.edu.au/copyright |
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