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Abstract
The central goal of this thesis is to improve the understanding of the control of balance during walking in a complex environment. It strives to provide unique and novel insights into the recovery responses to medial/lateral (M/L) trunk perturbations during over-ground locomotion. How are recovery strategies selected, modified, and combined according to perturbation size and the phase of gait? How are recovery goals priorised in the context of a narrow walking path? How is prior knowledge of a pending perturbation exploited? What role does the lower field of vision play?
Each experiment employed six healthy young adult participants: Studies 1 and 3 involved males; Study 2 involved females. The analyses focused on the rightward trunk perturbations delivered to the left shoulder. Study 1 manipulated perturbation magnitude, and the phase of gait of perturbation onset (left single support: LSS or right single support: RSS). Following LSS perturbations, step widening moved the right side of the base of support (BOS) further to the right of the centre of mass (COM). Larger magnitude LSS perturbations were loosely met with increased step widening. Following RSS perturbations, step narrowing decreased the distance that the left foot was to the right of the COM. An apparent trend of more step narrowing following larger magnitude RSS perturbations was not significant (a risk of collision of the left swing foot with the right stance foot). RSS perturbation trials demonstrated substantial feet-in-place responses, where as LSS ones showed dependence on change in base of support strategies.
Study 2 imposed M/L walkway constraints to foot placement (explicit: instructions to stay inside coloured boundaries, or implicit: cautioning of physical wooden kick-plates to the sides of the walkway). As opposed to Study 1, step narrowing was increased following larger RSS perturbations. In the narrow walkways, there was substantial contribution of feet-in-place responses following both LSS and RSS perturbations.
Study 3 imposed explicit MIL walkway constraints, and manipulated availability of the lower field of vision, and advanced knowledge of pending perturbation. With the benefit of prior knowledge, participants stepped away from the perturbation bar but still within the MIL walkway constraints, and shifted their COM toward the perturbation bar. These modifications served to increase the body's resistance to being pushed sideways, resulting in less disruption of the path of the body, and requiring smaller reactive gait modifications in subsequent steps. Study 3 demonstrated pronounced exploitation of feet-in-place responses, with the global maximum M/L COM velocity occurring before the heel contact of the perturbed step.
The findings of these three studies demonstrate that recovery responses to mechanical M/L trunk perturbations are not simply hardwired reflexes, but can be modulated according to perturbation magnitude and its direction with respect to the phase of gait, M/L walkway constraints, the lower field of vision and walking surface, and prior warning.
