Poster P16 - DOES ANTICIPATION OF WAIST-PULL PERTURBATIONS AFFECT PROACTIVE AND REACTIVE BALANCE RESPONSES DURING WALKING?

Introduction: Our older adult population is at an exceptional risk of falls, with injuries that can be devastating, contributing to reduced independence and quality of life at significant personal and financial costs. Compared to younger adults, older adults have worse reactive balance responses and are more vulnerable to balance challenges. Conversely, anticipatory proactive adjustments, such as changing step kinematics, may be used to prepare for and better respond to balance challenges. The purpose of this study was to investigate the effects of age on preparing for and responding to an impulsive waist-pull perturbation during walking. We hypothesized that: (1) independent of age, unanticipated perturbations would elicit larger margins of stability (MoS) than anticipated perturbations and (2) older adults would exhibit greater vulnerability to perturbations than young adults, with disproportionate effects when unanticipated.

Materials and

Methods: 20 young adults (8 M/12 F; mean ± standard deviation; age: 22.8±3.3 years; preferred walking speed: 1.39±0.14 m/s) and 18 older adults (9 M/9 F; age: 73.7±5.2 years; preferred walking speed: 1.31±0.19 m/s) participated. Participants walked unperturbed on the treadmill at their preferred walking speed for 2 minutes before completing a block-randomized series of walking trials during which participants responded to 6% body weight, 200 ms waist-pull perturbations delivered at the instant of heel strike in either the anterior or posterior direction via a motor-driven cable system [5]. We delivered perturbations either unexpectedly (i.e., unanticipated) or at the end of a three-second verbal countdown (i.e., anticipated). We recorded 3D motion capture from the arms, trunk, pelvis, and legs. We calculated anterior-posterior (MoSAP) and mediolateral (MoSML) margins of stability using standard procedures [6]. Values were extracted at the instant of heel strike on the leg contralateral to the perturbation: the step preceding (Preceding), the step immediately following (Perturbed), and two steps after the perturbation (Recovery). We used multifactorial ANOVAs with age (i.e., younger vs. older) and condition (i.e., unperturbed, anticipated, and unanticipated) as factors for each perturbation direction.

Results, Conclusions, and Discussions: Preceding step – The main effect of anticipation on MoSAP was significant for backwards and forward waist-pulls (p-values≤0.008). Here, independent of age and direction, only unanticipated backward waist-pulls elicited increased MoSAP compared to unperturbed walking, while both anticipated and unanticipated forward waist-pulls elicited increased MoSAP compared to unperturbed walking. We found a significant main effect of anticipation on MoSML in preparation for forward (p < 0.001), but not backward waist-pulls. Perturbed step – Independent of age and anticipation, backwards waist-pulls elicited increased MoSAP compared to unperturbed walking (p-values≤0.027). We found a significant main effect of anticipation (p < 0.001) and an age×condition interaction effect (p=0.028) on MoSAP in response to forward waist-pulls. Independent of anticipation, young adults exhibited greater MoSAP in response to forward waist-pulls compared to unperturbed walking (p-values < 0.001) – an effect absent in older adults. Independent of anticipation, age, or direction, waist-pull perturbations elicited greater MoSML than unperturbed walking (p-values < 0.001). Recovery step – We found a significant main effect of anticipation on MoSML and MoSAP in response to forward waist-pulls (p-values < 0.001). Unanticipated forward waist-pulls elicited greater MoSAP than anticipated (p=0.014). In addition, independent of anticipation, both anticipated and unanticipated forward waist-pulls elicited greater MoSAP and MoSML than unperturbed walking (p-values < 0.001); We also found an age×condition interaction effect (p=0.044) on MoSML in response to forward waist-pulls. Pairwise comparisons first revealed that younger and older adults exhibited greater MoSML in response to perturbations compared to unperturbed walking. In addition, compared to that when anticipated, only older adults exhibited greater MoSML in response to unanticipated forward waist-pulls. Cumulatively, we have identified unique effects of anticipation in older adults, particularly when recovering from a perturbation that would elicit a forward fall.
Given the complexity of environmental interactions that can precipitate falls, this is an important step toward understanding predisposing factors for and the ecological impact of age-related instability.

Acknowledgements (Optional): Supported by NIH (R21AG067388).