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Exploring the nature of postural sway Murnaghan, Chantelle Dawn

Abstract

Humans are unable to stand still, but rather experience continuous oscillations of the body known as postural sway. While the origins of postural sway are largely unknown, theories suggest that postural sway originates from the interaction between movements of the body (centre of mass, COM) and forces beneath the feet (centre of pressure, COP). The COP is commonly assumed to control or correct for deviations of the body from equilibrium, and delays or errors in control result in postural sway. In a sequence of 5 studies, this thesis used a novel experimental paradigm to investigate how postural sway is controlled or used by the central nervous system. The first of five experiments tested whether COP displacements would be reduced when the body was externally stabilized, as traditional theories would predict. Contrary to our hypothesis, COP displacements actually increased, suggesting an exploratory role for postural sway. Using the same experimental protocol, Study 2 provided participants with visual feedback of the COM or COP to determine if increases in COP displacements could be the result of sensory illusions or motor drift. Study 3 provided participants with an explicit verbal cue indicating how and when COM stabilization would occur to determine whether increases in COP displacements reflect an attempt to adapt the internal model of the body during stance. Study 4 examined whether increases in COP displacements could be the result of increases in oscillatory cortical drive. Using an upper limb postural task, the fifth and final study extended the findings from Studies 1-4 to determine whether exploratory behaviour may be a more global phenomenon and observed in other postural tasks that do not involve whole body stability. Individually, the results of Studies 1-4 provide evidence which challenges traditional theories of postural control. In addition, they provide evidence against alternative explanations for increases in COP displacements and suggest that this behaviour may be a more global phenomenon and observed in any postural task (Study 5). Collectively, they provide evidence supporting a potential exploratory role of postural sway and question the basis of current clinical practices designed to deal with balance control deficits due to age or disease.

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Attribution-NonCommercial-NoDerivatives 4.0 International

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