A simple model of the feasible limits to postural stability

In standing balanced activities, the center of pressure (COP) must always reside under the foot. We investigated how this restriction can be used with a pendulum model to predict balance dynamics by comparing two different empirical measures to model predictions: (1) we compared the “COP safety margin” (SM), defined as the minimum distance between the COP and the edge of the foot, to model estimates, and (2) we compared the state trajectories of the center of mass to model-predicted “feasible states”, which indicate whether the subject has a chance of recovering balance. Ten standing subjects were asked to make brief horizontal pulls on a handle to a range of target forces. Subjects developed posterior momentum, generated a pull that reversed the center-of-mass motion (a bounce), and then recovered their balance. Model-predicted and COP SMs agreed well (mean r=0.92), and 99.8% of all trials agreed with feasible state predictions. These results support the validity of the pendulum model, support the clinical use of COP SMs as convenient measures of dynamic stability, and provide preliminary evidence that the feasible state measures can characterize motor planning