Age-related differences in lower extremity power after support surface perturbations

OBJECTIVES: The purpose of this study was to examine a comprehensive measure relating to the ability to generate, absorb, and transfer mechanical energy introduced by a perturbation. It was hypothesized that this measure would reveal age-related differences leading to different balance recovery responses (i.e., feet-in-place and compensatory step). DESIGN: Cross-sectional, descriptive study. SETTING: The Motor Control Laboratory at the University of Oregon. PARTICIPANTS: Eighteen younger (aged 18-35) and 21 older (aged 65-85) women received forward and backward support surface translations of varying amplitudes and velocities. MEASUREMENTS: Lower extremity peak positive joint power and peak negative muscle power were examined at the largest perturbation in which a feet-in-place response was used and the subsequent perturbation where a step occurred. Peak positive joint power depicts the maximum rate of passive transfer of energy into a segment and is indicative of the maximum destabilization of that segment. Peak negative muscle power is the maximum rate of energy absorption by muscle, which reduces the effect of the perturbation on the segment. RESULTS: After a backward perturbation, there was a significant main effect of age for muscle power and a significant main effect of response type for joint power. For the forward condition, there was a significant main effect of response type for muscle power only. CONCLUSIONS: The mechanical reason for the emergence of the compensatory step after support-surface perturbations is clear; the feet-in-place response is energetically more demanding than the compensatory step. The energy analysis suggests a mechanical basis as one reason for older adults' reliance on the compensatory step: older adult muscles absorb less energy. The results of this study also highlight the importance of the proximal knee and hip musculature versus the distal ankle musculature.