THE CONCEPT OF METASTABILITY FOR ONE-LEGGED STANDING

• Published in: The Ohio journal of science Vol. 117; no. 1; p. A29
• Main Authors: Moore, Lauren, Carr, Neal, Zurcher, Ulrich, Sung, Paul
• Format: Journal Article
• Language: English
• Published: Columbus Ohio Academy of Science 01.04.2017
• Subjects: Back pain; Balance; Metastable state; Pain; Poisson density functions; Posture; Probability distribution functions; Random walk; Trajectory analysis; Trajectory measurement; Variation
• ISSN: 0030-0950; 2471-9390

Purpose and Goals: Standing on one foot is a standard test to examine the neurological function of postural control. However, there are few quantitative measures that can be used to distinguish healthy subjects from those with impaired control. We use the center-of-pressure (CoP) to measure the motion of the body and use mathematical methods from the theory of random walks to characterize the irregular behavior of the trajectory. Hypothesis: CoP trajectories exhibit a transition from random to piecewise ballistic motion associated with sways of the center-of-gravity of the body. Methods: We analyze CoP trajectories of 20 subjects (10 healthy control subjects and 10 subjects with low-back pain). The recruitment of subjects was approved by the Institutional Review Board. Trajectories are measured for times longer than 30 s. Results: The trajectories have stochastic character for times 0 < t< 20 ms and are ballistic for time t > 20 ms and is characterized by a constant velocity. The straightline trajectory is interrupted by sharp turns ("turning points"). Turning points are associated with minima of the short-time fluctuations of the trajectory. Turning points cluster in certain regions, which we associate with basins of metastability. The distance and times between turning points follow an exponential distribution, which suggests that the transition between clusters follows a Poisson process. Preliminary data suggests that the characteristic length- and time-scales derived from the distributions can be used to distinguish healthy subjects from those with impaired postural control.