A unified view of quiet and perturbed stance: Simultaneous co-existing excitable modes

Robert Creath, Tim Kiemel, Fay Horak, Robert Peterka, John Jeka

Research output: Contribution to journalArticlepeer-review

222 Scopus citations


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 1 Hz. 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.

Original languageEnglish (US)
Pages (from-to)75-80
Number of pages6
JournalNeuroscience Letters
Issue number2
StatePublished - Mar 29 2005


  • Ankle strategy
  • Biomechanics
  • Hip strategy
  • Motor programs
  • Posture

ASJC Scopus subject areas

  • General Neuroscience


Dive into the research topics of 'A unified view of quiet and perturbed stance: Simultaneous co-existing excitable modes'. Together they form a unique fingerprint.

Cite this