Postural Control Mechanisms in Young and Older Adults

Abstract

Recent findings have suggested that the type of postural control seems to change, with certain conditions promoting a more conscious control and others, a more automatic control. It has been proposed that by withdrawing attention from postural control, external focus and cognitive task conditions allow automatic mechanisms to control sway more efficiently. The present dissertation aimed to investigate whether two types of postural control exist and to provide evidence for these two types of postural control in healthy young and older adults. In experiment 1, the effect of attentional focus and cognitive tasks was examined in healthy older adults. It was found that the cognitive task yielded improvements in stability compared to focus conditions, which highlighted the fact that automaticity is possible in this group. In experiment 2, the effect of attentional focus and cognitive tasks was once again observed in older adults, this time by including electromyographic recordings of lower leg muscles to examine if changes were due to stiffening. Although improvements were observed in external focus and cognitive task conditions compared to baseline and internal focus, no change occurred in muscle activity which lends further support to the idea that changes are due to automaticity. Finally, dynamic measures of sway were used to help interpret the changes that occurred in attentional focus and cognitive task conditions in both young and older adults. The wavelet transform revealed a change in sensory contributions to postural control in cognitive task conditions. There was a shift to increased contributions from the cerebellum and the vestibular system, and a decrease in visual contributions, compared to other conditions. Sample entropy revealed changes in complexity of sway, with cognitive tasks presenting more complex, irregular and efficient sway in both groups compared to baseline standing and attentional focus conditions. Finally, the rambling-trembling decomposition highlighted increases in the spinal reflex contributions to sway in external focus and difficult cognitive task conditions in young adults, while no change occurred in older adults. Results of these experiments provide the evidence of two types of postural control; a more automatic type in cognitive task conditions and a more conscious type in baseline and internal focus conditions. The external focus elicited some changes that could have indicated automaticity, but clear differences were still present between this condition and the cognitive tasks, which suggest automaticity of sway should be viewed as a continuum.