Analysis of Postural Control During Voluntary Sway of the Anteroposterior Axis in Young and Elderly Humans

Abstract

Postural stability was evaluated in 20 young and 9 elderly subjects by phase coupling analysis while performing voluntary shifts of center of foot pressure (CoP) to target in the anteroposterior axis. The coupling index and the amplitude of CoP decreased significantly in the elderly compared to young subjects, indicating a reduced ability to follow the target in the elderly group. I. INTRODUCTION Increased postural sway during quiet standing is associated with a loss of balance in humans in unstable conditions (1). Since human activities involve voluntary migration of center of foot pressure (CoP), dynamic stability of CoP may be important to show quality of postural control. In this study, we compared CoP dynamics in young and elderly subjects while performing voluntary CoP movement to target, with special attention to the phase coupling between CoP and target trajectories. We hypothesized that in comparison to young, elderly people would display an uncoupling as reflecting deterioration of postural control. II. MATERIALS AND METHODS Twenty young (22.5¶ 1.1 yrs) and 9 elderly subjects (62.1 ¶ 3.3 yrs) participated in this study. During the experiments, the subject stood with socks on a wooden board placed on top of force platform (ALPS electric Co. Ltd.), which has 48 Y 48 piezoelectric sensor matrix. The width and height of the sensor matrix are 33 cm each. The subjects viewed the monitor which provided visual feedback on the actual CoP displacements. The monitor also showed the subject a target which moves cyclically at a period of 6 s in the anteroposterior directions with amplitude of 20% of the sensor height. The subjects were instructed to adjust their CoP to follow the trajectory of the target as accurately as possible for 3-min. The position of CoP was recorded as a function of time using PC via A/D interface with a sampling frequency of 20 Hz. Figure 1. Typical examples of analysis for young and elderly subjects. From the top, CoP displacement (solid line) and target (dotted line), instantaneous phases of CoP (solid line) and target (dotted line), andO�(solid line) and ACoP (dotted line). III. DATA ANALYSES