Temporal Optimality of a Via-Posture on Trajectory during Sit-to-Stand and Back-to-Sit Movements

Abstract

The purpose of this study was to examine the hypothesis that the minimum angle-jerk with via-point model could account for the temporal consistency of kinematics in sit-to-stand (STS) and back-to-sit (BTS) movements. The positions of bony landmarks on the subjects during the tasks were recorded using a Vicon motion analysis system to yield the angular displacement and position of the center of mass (COM) of a three-link rigid body model in the sagittal plane. Minimum angle-jerk and minimum jerk trajectories with a via-point were computed for joint angle and COM, respectively. Five to six candidate points were selected as the via-point from the measured trajectory based on the separate kinematic events. The results show the optimal angular trajectories using the via-points (via-angles) before the seat-off for STS, and at around the seat-contact for BTS resembles the measured angular trajectories well, indicating that the posture at the via-point was temporally optimal to produce the minimum angle-jerk trajectory for both movements. It is hypothesized that the multi-joint angular patterns during STS and BTS movements were organized to pass through the via-point, namely the via-posture along the minimum angle-jerk trajectory.