Dynamic balance control during sit-to-stand movement: an examination with the center of mass acceleration.

The purpose of this study was to establish the region of stability of balance control using the center of mass (COM) acceleration and to characterize age-related differences during sit-to-stand (STS) movement. Whole body motion data were collected from 10 young and 10 elderly subjects while performing STS at their self-selected manners. In addition, young subjects were asked to perform another block of trials with their trunk purposely bent forward prior to seat-off. With the use of a single-link-plus-foot inverted pendulum model, boundaries for the region of stability were determined based on the COM position at seat-off and its instantaneous velocity or its peak acceleration (ROSv or ROSa, respectively). No significant group differences were detected in COM velocities at seat-off. However, peak COM accelerations differed significantly between groups and conditions. This suggested that even though a similar COM momentum was observed at seat-off, this momentum was controlled differently prior to seat-off. Young and elderly subjects utilized similar strategies but with different COM acceleration profiles to perform STS. Furthermore, data from an elderly subject who complained of difficulty in STS during the experiment were located outside the forward boundary of the ROSa, demonstrating a potential use of ROSa to differentiate individuals with declined balance control ability. The ROSa could provide insights into how the COM is controlled prior to seat-off, which may allow us to better identify elderly individuals who are most likely at a risk for imbalance or falls.