A simple model of stability limits applied to sidestepping in young, elderly and elderly fallers.

Impaired lateral balance involving the frontal plane is particularly relevant to the problem of falls with aging. Protective stepping is critical to avoiding falling, and medio-lateral (M-L) stepping involves two quite complicated action choices -- lateral side step and crossover stepping. The aims of this study were to identify differences in movement patterns between young healthy subjects and elderly fallers and non-fallers (determined prospectively over a year), and to identify performance differences for the two types of stepping response. Our tool for these evaluations was a computational model of the center of mass as a pendulum, which identifies the limits of stability beyond which additional steps are required. In response to multi-directional stepper-motor induced waist-pull perturbations of standing balance, the older groups took multiple steps more often than the young (55% compared to 9% of the trials), and the largest differences were seen in the pulls to the side. On these side pulls, crossover stepping and limb collisions increased with age and prospectively determined fall risk. Consequently the model analysis focused only on the most problematic lateral pulls, and only on pulls to the right. In both stepping off and landing, the young most closely approached the stability limits predicted by the model, followed by the older non-fallers and then fallers. In crossover stepping, all groups landed closer to their limits when multiple steps occurred, though older fallers were closest to instability. These findings revealed distinctive age differences related to fall risk and shed light on such modeling approaches for understanding the reasons why older fallers may select stepping responses and the effectiveness of such responses in recovering balance.