Compensatory stepping in response to waist pulls in balance-impaired and unimpaired women

Abstract

An effective stepping response is often critical in avoiding a fall. Our objective was to study the effects of age and balance impairment on anterior and posterior compensatory stepping strategies in response to waist pull perturbations of 1–5% body weight (BW). Based on maximal unipedal stance time (UST), we tested 15 balance-impaired old (BI, UST < 10 s, mean age = 76 years), 12 healthy old (O, UST > 30 s, mean age = 71 years), and 13 healthy young women (Y, UST > 30 s, mean age = 23 years). Randomized anterior and posterior pulls of 1–5% body weight (BW) were applied to the waist while kinematic and kinetic recovery responses were recorded. Results show that O and BI required 0.5 more steps than Y to recover balance for posterior pulls of 4–5% BW (P < 0.01). For anterior pulls of 4–5% BW, only BI had a greater probability of step initiation (P ≤ 0.05 or <0.02) and mean number of steps required to recover balance (P < 0.03). The Y used 93% greater torso extension and 24% greater torso flexion in responding to anterior and posterior pulls, respectively. In the posterior, but not anterior direction, O employed smaller (P < 0.007), but more laterally-directed (P < 0.03) steps than the Y. The BI were less able to attenuate their momentum during the step landing in both directions. We conclude that the additional steps required by the BI in both directions occurred because their initial step failed to properly arrest their momentum. Controlling torso inclination before step liftoff and linear momentum after step landing are critical components of successful compensatory stepping.

Introduction

Falls are a significant problem in older adults. Following a perturbation that might cause a fall, one must either recover balance using a feet-in-place postural sway response, or use a protective movement such as a compensatory step. In studies of subject responses to translational platform or waist pull perturbations of stance, balance-impaired older adults more frequently choose to step rather than use the sway response strategy preferred by healthy older adults, and healthy older adults exhibit similar behavior when compared to healthy young adults [1], [2], [3], [4], [5], [6], [7]. Moreover, when taking a step, older balance-impaired subjects require more steps than do healthy older adults, and the healthy older subjects require more steps than do young to regain their balance [1], [6], [7], [8], [9], [10]. Why are there differences in the probability and number of steps between these three groups?

A number of possible biomechanical factors are thought to influence step initiation and number. One possible influence on step initiation is variability in step preparation. Anticipatory postural adjustments (APAs), defined as a preparatory loading of the stepping foot intended to move the center of mass over the stance foot in anticipation of a step, are less frequent in old versus young in response to perturbed stance [9]. The fact that these APAs are relatively brief and small in comparison to APAs for volitional steps makes their importance unclear [9]. A possible contributor to multiple steps is placement of the initial step. Luchies et al. [8] found that older adults did not step as far as young controls in the direction of the perturbation, and this could account for the multiple steps used by the old. Additionally, lateral step placement is important for the maintenance of lateral stability [11], [12]. Rogers et al. [13] found that anterior compensatory steps landed more laterally in fallers versus non-fallers. While McIlroy and Maki [9] found no differences between young and old in initial anterior and posterior compensatory step placement, subsequent steps were directed laterally. Thus, although lateral step placement may improve frontal plane stability, it may not decrease the number of steps needed. Finally, center of mass (COM) motion may also be critical in determining a stepping response. In Pai et al. [2], COM velocity and the distance between the COM position and step landing were good predictors of stepping and in Jensen et al. [5], COM linear momentum tended to be higher in older than young adults. It may well be that the ability to control COM velocity is a critical factor to limiting the number of steps. So, of all the biomechanical factors thought to influence step initiation and number, these three factors, APAs, step placement, and COM velocity, would seem to be the most important and are thus the focus of the present study.

The purpose of the present study is to analyze step initiation and execution in healthy young, healthy older, and balance-impaired older women. Anterior and posterior compensatory steps are evoked by rapid-onset waist pulls using a range of perturbation magnitudes elicit a similar number of feet-in-place and stepping responses. No instructions are given to encourage or discourage stepping in order to elicit natural responses rather than maximal capacities.

Based on the background above, we hypothesized that balance-impaired versus unimpaired older adults, and unimpaired older adults versus young controls would be more likely to initiate stepping and have a greater number of steps. We also expected to observe less frequent APAs, more lateral step placement, shorter steps, and diminished control of COM motion in balance-impaired versus unimpaired and in unimpaired versus young control.