A polygon model of the functional base-of-support improves the accuracy of balance analysis

Abstract

Mathematical balance models have the potential to identify people at risk of falling before an injury occurs. However, most balance models depend on a model of the base-of-support (BOS) of the feet to calculate how well someone is balancing. Here we evaluate the functional base-of-support (fBOS): the convex polygon on the bottom of the foot that can support a large fraction of the body's weight. First, we develop a geometric model of the fBOS by measuring the center-of-pressure (COP) and kinematic data of the feet of 27 younger adults instructed to move their body mass in large loops without taking a step. We extract a planar convex polygon that contains the COP data. Finally, we compare the area of this fBOS model to a marker-based BOS model before evaluating if the fBOS differs across four everyday conditions: footwear, stance-width, foot dominance, and during single and double stance. We found that the fBOS is much smaller (23% the size) than a marker-based BOS model. Our analysis suggests that using the fBOS, rather than a marker-based BOS, can improve the accuracy of the margin-of-stability by 20% of foot width and 16% of the length. In addition, we found that the fBOS area does not differ across footwear (p=0.88), stance-width (p=0.88), and foot dominance (p=0.68), but during single stance the fBOS is 17% (p=0.0003) larger than during double stance. So that others can use and extend our work, we have made the models, example data and code publicly available.