Biomechanical analysis of body mass transfer during stair ascent and descent of healthy subjects

The purposes of this study were to: 1) assess whole body center of mass (CM) motion in the frontal, sagittal, and transverse planes; 2) compare CM displacement with center of pressure (CP); and, 3) further define the stance and swing subphases of stair ascent (SA) and stair descent (SD) based on critical CM, CP, and ground reaction force (GRF) events. SA and SD were analyzed on a convenience sample of 11 subjects. Unpaced data were collected from 28 SA trials and 24 SD trials utilizing a bilateral SELSPOT II/TRACK data acquisition system and two Kistler force plates at a sampling frequency of 153 Hz. Twenty-six discrete data points were chosen from each trial for analysis. Each identified point detailed the intersection, separation, maximum or minimum value of CM, CP, or GRF in all three planes. Specific phases of SA and SD are presented and described. The actions of CM, CP, and GRF are presented during each phase. Results further refine the phases originally described by McFayden and Winter. Subtle differences in phases and duration of single and double support are demonstrated between SA and SD. Based on these results, it is apparent that SD is a more dynamic process with greater inherent instability. Knowledge of SA and SD phases and CM/CP dynamics in healthy, normal subjects will permit comparison with patients exhibiting various pathologies. Such comparison should facilitate the development of appropriate intervention strategies.