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 upright stance as a function of age (60-96 years) and postural conditions. Subjects stood on a force platform under different 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 boundaries 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 certain 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 balance that decline with age and make older adults more prone to falling have not been identified specifically (3). This problem 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). Traditionally, postural stability has been measured by determining the degree of motion of the center of pressure at the surface of support through force platform technology (4-6). The exact location of the center of pressure is generally assumed to be an accommodation to the location of the vertical projection of the center of gravity of the body in an upright bipedal stance (5,7). It is well established that postural sway increases with age (1,8-10). The dependent measures of the area and length of the center of pressure have been used to infer a correlate of postural sway and the degree of stability for a given stance. Although these measures of the center of pressure are most frequently cited in the postural stability literature (5,7) they are poor operational reflections of stability because they are not direct measures of the relation of the motion of the body to the stability boundary. Rather than concentrating on indirect measures of postural stability, this study examines the space-time evolutionary properties of the center of pressure trajectory in normal upright bipedal stance. Studying postural stability by way of a dynamic system approach allows the center of pressure trajectory to be considered in relation to the estimated stability boundary rather than as a measure independent of a stability frame of reference. In the experiments reported here we use virtual time-to-contact (VTC), which is a variable that determines the temporal proximity of the potential time to contact the center of pressure trajectory with the two-dimensional stability boundary (11). VTC is determined from the dynamics of the center of pressure over each point in time of the center of pressure time series and is based on the virtual trajectory of the center of pressure should it continue to move with those same dynamics (the formal method for calculating VTC is provided in Appendix B). VTC is a useful approach to the measurement of postural stability because the center of pressure moves in space-time, and all other approaches to the postural stability region have been based on an estimate of a single spatial dimension. It has been shown with young adults that VTC systematically decreases as the base of support in stance is reduced and as the speed of postural swaying movements is increased (11). A central measure for the determination of VTC is the boundary of stability support in posture. It should be noted that it is impossible to maintain the center of gravity motionless, and that a person who is attempting to maintain balance will spontaneously sway back and forth and side to side. There are limits to the spatial-temporal aspects of this sway, and they vary with the information available to the individual and the configuration of the base of support in action. The stability boundary of the base of support can be measured both geometrically and functionally. The geometric base of support is usually determined by the area contained within the spatial boundaries of the position of the feet on the surface of support (12-14). In contrast, the functional base of support has been defined previously (3) as the proportion of the anterior-posterior dimension of the base of support utilized during sustained maximal forward and backward leaning. For the purpose of this investigation, diagonal movements as well as anterior, posterior, and lateral movements are included to provide a two-dimensional spatial estimate of the boundaries of the functional base of support. Traditionally, investigations of the postural stability, including those of aging subjects, have been experimentally limited to a single spatial dimension and indirect measures of the postural stability boundary. In summary, the purpose of this study was to examine the margins of stability for both the geometric and functional