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This Article Full Text Full Text (PDF) Supplemental Tables All Versions of this Article: 102/3/1411    most recent 00131.2009v1 Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Google Scholar Articles by O'Connor, S. M. Articles by Kuo, A. D. PubMed PubMed Citation Articles by O'Connor, S. M. Articles by Kuo, A. D.

Direction-Dependent Control of Balance During Walking and Standing

¹Departments of Biomedical Engineering and ²Mechanical Engineering, University of Michigan, Ann Arbor, Michigan

Submitted 13 February 2009; accepted in final form 17 June 2009

Abstract

Human walking has previously been described as "controlled falling." Some computational models, however, suggest that gait may also have self-stabilizing aspects requiring little CNS control. The fore–aft component of walking may even be passively stable from step to step, whereas lateral motion may be unstable and require motor control for balance, as through active foot placement. If this is the case, walking humans might rely less on integrative sensory feedback, such as vision, for anteroposterior (AP) than for mediolateral (ML) balance. We tested whether healthy humans (n = 10) exhibit such direction-dependent control, by applying low-frequency perturbations to the visual field (a projected virtual hallway) and measuring foot placement during treadmill walking. We found step variability to be nearly 10 times more sensitive to ML than to AP perturbations, as quantified by the increase in root-mean-square step variability per unit change in perturbation amplitude. This is not simply due to poorer physiological sensitivity of vision in the AP direction: similar perturbations applied to quiet standing produced reversed direction dependence, with an AP sensitivity 2.3-fold greater than that of ML. Tandem (heel-to-toe) standing yielded ML sensitivity threefold greater than that of AP, suggesting that the base of support influences the stability of standing. Postural balance nevertheless appears to require continuous, integrative motor control for balance in all directions. In contrast, walking balance requires step-by-step, integrative control for balance, but mainly in the lateral direction. In the fore–aft direction, balance may be maintained through an "uncontrolled," yet passively stabilized, series of falls.