title: A unified view of quiet and perturbed stance: simultaneous co-existing excitable modes.
abstract: When standing quietly, human upright stance is typically approximated as a single segment inverted pendulum. In contrast, investigations which perturb upright stance with support surface translations or visual driving stimuli have shown that the body behaves like a two-segment pendulum, displaying both in-phase and anti-phase patterns between the upper and lower body. Here we present evidence that a single-segment characterization of quiet stance is inadequate. Similar to perturbed stance, quiet stance has simultaneously co-existing in-phase and anti-phase patterns. Subjects stood with eyes closed in three sensory conditions: a fixed surface, a foam surface, and a sway-referenced surface. Spectral analysis showed that the body behaved like a multi-link pendulum with two co-existing modes. The angles of the trunk and leg segments were in-phase for frequencies below 1 Hz and anti-phase for frequencies above 1Hz. The shift from in-phase to anti-phase sway showed an abrupt change for the fixed and foam surfaces, but a gradual change for the sway-referenced condition with the trunk showing a phase lead over the legs. The coexistence of in-phase and anti-phase patterns during quiet stance suggests that the ankle and hip strategies are not extremes along a behavioral continuum of mixed strategies. They are "simultaneously co-existing excitable modes", both always present, but one of which may predominate depending upon the characteristics of the available sensory information, task or perturbation.
authors.affiliation: Department of Kinesiology, University of Maryland, College Park, MD 20742, USA.
authors.forename: Robert
authors.initials: R
authors.lastname: Creath
authors.forename: Tim
authors.initials: T
authors.lastname: Kiemel
authors.forename: Fay
authors.initials: F
authors.lastname: Horak
authors.forename: Robert
authors.initials: R
authors.lastname: Peterka
authors.forename: John
authors.initials: J
authors.lastname: Jeka
mesh_terms: D000328:Adult; D005260:Female; D006801:Humans; D008297:Male; D010775:Photic Stimulation / Q000379:methods; D004856:Postural Balance / Q000502:physiology*; D011187:Posture / Q000502:physiology*; D011597:Psychomotor Performance / Q000502:physiology*