Lyon, Ian N.P.; (1997) The control of balance in human stepping. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis examines the medio-lateral body motion during single steps in an attempt to understand how balance is controlled in human stepping and gait. During a step the body's centre of mass (CoM) is not over the base of support. The body is unstable and falls sideways, but is 'held together' such that it moves approximately as a single unit. Over a range of step directions, there is a close relationship at the end of the step between stepping foot position and CoM position and velocity. This may be in order that the stepping limb can catch and redirect the fall of the body securely. A freely- falling model of the body closely predicts body motion during the step. This suggests that the position and velocity of the CoM at the end of the step are determined by the values at the start. Subjects are found to vary these starting values systematically with step direction and duration, and also to take into account initial posture. Together with evidence that the duration of the single-support phase is determined in advance, this suggests that the body motion during the step could be controlled ballistically. This strategy may be used because body motion is difficult to influence appreciably once the step is under way. In responding to a cue to change step direction 'midflight', subjects are able to alter body motion but 1) are more able to increase than to decrease the rate of the sideways fall, and 2) appear to have to resort to a multi-segment strategy. In contrast, responses appearing in the swing limb at short latency suggest that ordinarily swing limb motion may be subject to 'on-line' control. Thus inaccuracies in the ballistic control of the body mass may be compensated for by mid-step alterations in swing limb motion.

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