Stepping responses of young and old adults to postural disturbances: kinematics.

OBJECTIVES: When large disturbances of upright stance occur, balance must usually be restored by taking a step. We undertook this study to examine the biomechanics of stepping responses to sudden backward pulls at the waist. Primarily, response differences between young and old healthy adults were sought.

DESIGN: A controlled laboratory study.

SUBJECTS: Two groups of healthy and physically-fit adult females, 12 of mean age 22 (Young) and 12 of mean age 73 years (Old).

MEASUREMENTS: Response kinematics were measured. From them, the stepping strategies of the subjects were derived, including the timing, length, and height of the first step taken and the rotations of major body segments and at major body joints that occurred.

RESULTS: In response to sufficiently large backward pull forces, all subjects responded by taking one or more steps backwards. No significant age group difference appeared in the smallest disturbance for which subjects sometimes used a step response. A significant age group difference appeared in the smallest disturbance at which subjects began consistently to use step responses, and that disturbance was larger for the Old than for the Young. Distinct age group differences were found in stepping strategy. At large disturbance levels, the Young mostly responded by taking a single step, whereas the Old mostly responded by taking multiple steps. The steps taken by the Old, compared with those of the Young, were significantly shorter, had significantly smaller heights, and were taken significantly earlier in the responses. Body segment and joint rotations were generally modest, and few significant age group differences were found in these kinematics.

CONCLUSIONS: In restoration of perturbed balance by step-taking, the responses of the healthy, physically-fit young and old adults studied here were similar in many respects, but they differed in some important features. Joint range of motion (ROM) limitations are unlikely to explain age group differences in stepping responses to postural disturbances among healthy subjects because the ROM actually used in any of the responses observed were substantially smaller than the ROM available.