This study investigated the hypothesis that virtual time-to-contact, which specifies the time to reach the functional stability boundary, is a variable controlled in the maintenance of upright posture. Three different levels of support surface compliance were used on a force platform (no foam, 5 cm of foam, and 15 cm of foam). The participant’s task was to stand still under each surface support condition both with and without vision. The stability boundary was determined for each set of conditions where the participant was required to lean as far as possible in all directions of the horizontal plane without losing stability. The results showed that the no vision conditions had a significantly larger center of pressure displacement than the vision conditions. No vision and increasing support surface compliance also increased the velocity of the center of pressure trajectory. The distribution of the radial displacement of the center of pressure showed relatively equal frequency over spatial location with no central tendency. The virtual time-to-contact with the stability boundary decreased as platform surface support became more compliant. Furthermore, the distribution of virtual time over the effective scaling range was a power law with a larger exponent in the more unstable no vision and increasing surface foam conditions. The findings provide additional evidence for the hypothesis that virtual time-to-contact with stability boundaries is a postural control variable that is regulated rather than the preservation of minimal motion around the center of the stability region as proposed in pendulum models of posture
