Aging and time to instability in posture

Two experiments are reported that were set up to examine the spatial-temporal boundaries of postural instability in up-right stance as a function of age (60-96 years) and postural conditions. Subjects stood on a force platform under dif-ferent experimental conditions (vision/no vision and arms up/down) so that the effect of age on key dynamic properties of postural stability could be determined. The findings showed that the ratio of the area of the motion of the center of pressure to the area within the stability boundary increased with age. Also, the virtual time-to-contact with the postural stability boundary decreased with age. Collectively, the findings show that the margins to the spatial-temporal bound-aries of postural stability decrease with advancing age in the elderly. These reduced margins of dynamic stability may be a factor contributing to the progressive instability of posture with aging in the elderly. T)OSTURE is a fundamental activity in human action. Even L in actions that are not typically labeled as a postural or a balance task, the role of the posture of the head and torso is central to the production of a given movement sequence, and the realization of an action goal. It is well established that cer-tain features of postural support change during the advancing years of life so that the stability of posture can be a problem with aging in the elderly (1,2). The particular aspects of bal-ance that decline with age and make older adults more prone to falling have not been identified specifically (3). This prob-lem persists because balance is a complex process involving the coordinated activity of many body segments to realize the goals of postural tasks. To balance with the feet in place, as in quiet upright bipedal stance, requires that the position of the body's center of gravity (COG) must be maintained vertically over the potential region of stability at the base of support (4). Tra-ditionally, postural stability has been measured by determin-ing the degree of motion of the center of pressure at the sur