title: Smooth enlargement of human standing sway by instability due to weak reaction floor and noise.
abstract: Human quiet standing is accompanied by body sway. The amplitude of this body sway is known to be larger than would be predicted from simple noise effects, and sway characteristics are changed by neurological disorders. This large sway is thought to arise from nonlinear control with prolonged periods of no control (intermittent control), and a nonlinear control system of this kind has been predicted to exhibit bifurcation. The presence of stability-dependent transition enables dynamic reaction that depends on the stability of the environment, and can explain the change in sway characteristics that accompanies some neurological disorders. This research analyses the characteristics of a system model that induces transition, and discusses whether human standing reflects such a mechanism. In mathematical analysis of system models, (intermittent control-like) nonlinear control with integral control is shown to exhibit Hopf bifurcation. Moreover, from the analytical solution of the system model with noise, noise is shown to work to smooth the enlargement of sway around the bifurcation point. This solution is compared with measured human standing sway on floors with different stabilities. By quantitatively comparing the control parameters between human observation and model prediction, enlargement of sway is shown to appear as predicted by the model analysis.
authors.affiliation: Department of Mechanical Engineering and Intelligent Systems, The University of Electro-communications, 1-5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japan; JST, CREST, 5 Sanbancho, Chiyoda-ku, Tokyo 102-0075, Japan.
authors.forename: Tetsuro
authors.initials: T
authors.lastname: Funato
authors.affiliation: Department of Aeronautics and Astronautics, Kyoto University, Kyoto daigaku-Katsura, Nishikyo-ku, Kyoto 615-8540, Japan; JST, CREST, 5 Sanbancho, Chiyoda-ku, Tokyo 102-0075, Japan.
authors.forename: Shinya
authors.initials: S
authors.lastname: Aoi
authors.affiliation: Department of Mathematics, Kyoto University, Kitashirakawa Oiwakecho, Sakyo-ku, Kyoto 606-8502, Japan; JST, CREST, 5 Sanbancho, Chiyoda-ku, Tokyo 102-0075, Japan.
authors.forename: Nozomi
authors.initials: N
authors.lastname: Tomita
authors.affiliation: Department of Aeronautics and Astronautics, Kyoto University, Kyoto daigaku-Katsura, Nishikyo-ku, Kyoto 615-8540, Japan; JST, CREST, 5 Sanbancho, Chiyoda-ku, Tokyo 102-0075, Japan.
authors.forename: Kazuo
authors.initials: K
authors.lastname: Tsuchiya
keywords: Hopf bifurcation; dynamical model; noise; posture control