Instability during Stepping and Distance between the Center of Mass and the Minimal Moment Axis: Effect of Age and Speed

Abstract

The goal of this study was to analyze the instability during stepping of young and older adults at fast and spontaneous speed. To this aim, the anteroposterior and the mediolateral distances between the body center of mass (COM) and the minimum moment axis (MMA) were computed. A total of 15 young adults (25y.o. [19-29]) and 15 older adults 68.7y.o. [63-77] volunteered for this study. For the computation of the distances, a complete biomechanical protocol combining two force platforms and a 3D motion capture analysis system was setup. The subjects were equipped with 47 reflective markers and were modeled as a frictionless multibody with 19 segments, 18 joints and 42 degrees of freedom and were asked to perform a stepping at both speeds. The stepping was divided in 5 phases with successive swing and double stance phases. The greater instability was observed during the swing phases. The distances indicate a significant higher instability at fast speed for both groups (p < 0.001) for all the phases. The anteroposterior distance also increases significantly for older adults highlighting greater instability while no differences were observed for the mediolateral distance all along the 5 phases suggesting higher risks of backward of forward falls during stepping.