Time-to-contact (TtC) is an alternative measure of postural stability to center of pressure (CoP) velocity. TtC is based on both spatial and temporal aspects of CoP displacement, definition of the boundary shape, and quantity of minima analyzed. Three boundary shapes and three minima selection methods were used to compute TtC during bipedal quiet standing. The results suggest that there is a strong correlation between TtC values obtained using each of the calculation methods (r ≥.73) and mean CoP velocity (r ≥ −.70). TtC was significantly affected by boundary shape and minima selection method. This limits the ability to compare absolute values, but relative levels of stability computed using TtC can be compared due to strong correlations. Given the task parameters studied, mean CoP velocity may even be adequate to assess levels of stability. Future studies are needed to examine the generalizability of these findings for different groups and task parameters.
The authors are with Liberty Mutual Research Institute for Safety, Hopkinton, MA.