Influences of Head Orientation on the Unperturbed Postural Sway in Human Upright Stance

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  • Tanaka Hideyuki
    Department of Human-Computer Interaction Science, Tokyo University of Agriculture and Technology
  • Uetake Teruo
    Department of Eco-design, Tokyo University of Agriculture and Technology

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We examined the effects of head orientation on postural sway during quiet upright standing. Twenty healthy male subjects with diverse balance abilities were included in this study. Postural sway was estimated using an instrumented force platform to record the center-of-pressure (COP) under a subject’s feet during standing. The subjects were tested with the ‘head straight’ (normal) and with the ‘head back’ (I.e., neck extension), both conditions with eyes-open. The COP trajectories were analyzed using a technique of statistical mechanics, better known as stabilogram-diffusion analysis, proposed by Collins and De Luca (1993). The extracted parameters were the effective diffusion coefficients (D) for the short-term (less than approximately 1.0 s) and the long-term (longer than 1.0 s) intervals, respectively, as well as the Hurst exponents (H) for each of the short- and long-term intervals, and some critical point coordinates (I.e., critical mean square displacements and critical time intervals). The short-term D, short-term H, and critical mean square displacements were significantly larger with the head back condition than with the head straight condition. No significant effects of head orientation were found in the long-term D and H or in the critical time intervals. That is, head back or neck extension caused an increase in the stochastic activity and positively correlated (persistent) behavior of the COP during shorter timescales. These results suggest that head backward orientation influences the open-loop (I.e., short-term) control mechanisms, but not the corrective feedback (I.e., long-term) mechanisms used to maintain postural stability.

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