Static versus dynamic predictions of protective stepping following waist-pull perturbations in young and older adults

The purposes of this study were: (1) to determine the frequency of protecti ve stepping for balance recovery in subjects of different ages and fall-sta tus, and (2) to compare predicted stepping based on a dynamic model (Pai an d Patron, 1997. Journal of Biomechanics 30, 347-354) involving displacement and velocity combinations of the center of mass (COM) versus a static mode l based on displacement alone against experimentally induced stepping. Resp onses to three different magnitudes of forward waist pulls were recorded fo r 13 young, 18 older-non-fallers and 18 older-fallers, The COM phase plane trajectories derived from motion analysis were compared with the model-pred icted threshold values for stepping. We found that the older fallers had th e highest percentage of stepping trials (52%), followed by older-non-faller s (17.3%), and young (2.7%) at the lowest perturbation level. Younger subje cts stepped less often than the elderly at the middle level. Everyone consi stently stepped at the highest level of perturbation. Overall, the dynamic model showed better predictive capacity (65%) than the static model (5%) fo r estimating the initiation of stepping. Furthermore, the threshold for ste p initiation derived from the dynamic model could consistently predict when a step must occur. However, it was limited, especially among older fallers at the low perturbation level, in that it considered some steps 'unnecessa ry' that were presumably triggered by fear of falling or other factors. (C) 1998 Elsevier Science Ltd. All rights reserved.