The concept of metastability for one-legged standing

Standing on one foot has been characterized by a continuum between static equilibrium (standing) and dynamic equilibrium (walking). This suggests that sways of the body are important for a person to maintain the upright position and prevent a fall. We examine the center of pressure (COP) changes with visual input, and find that the character of COP dynamics is different on different time scales: it is random (stochastic) on short time scales $0 < t < 20\, \mbox{ms}$, ballistic (deterministic) on intermediate time scales $20\, \mbox{ms} < t < \mbox{200 ms}$, and random on long time scales $200 \, \mbox{ms} < t < 25 \, \mbox{s}$.