Reliability of COP-based postural sway measures and age-related differences

Abstract

The objectives of this study were to assess the within-day and between-day reliability of several center of pressure (COP)-based measures of postural sway and identify whether there were age-related differences in reliability. Thirty-two healthy individuals (16 younger and 16 older) participated. COP was recorded during quiet upright stance on 4 different days, and a variety of measures determined: mean velocity, median power frequency, RMS distance, sway area, and two fractal measures derived from Hurst rescaled range analysis (H_R/S) and detrended fluctuation analysis (DFA). Intraclass correlation coefficient (ICC) and standard error of measurement (SEM) were used to quantify reliability. Mean velocity was the most reliable measure. DFA exponents had relatively better reliability than H_R/S exponents. In general, within-day reliability was better than between-day. In comparison with younger participants, older participants exhibited better relative reliability (ICC) for all COP measures and comparable absolute reliability (SEM) except for mean velocity and sway area. These results may be useful in guiding the future selection and interpretation of COP-based measures.

Introduction

Assessment of human postural (or balance) control is of frequent interest to researchers and clinicians. Decrements in balance control have been considered an important factor in the probability of a fall [1], [2], and falls are often a proximal cause of injury and disability in the general population as well as in contemporary industry [3], [4]. Performance of the balance control system is commonly assessed in the laboratory by quantifying sway during quiet upright stance. The most commonly used measures of sway are various parameters derived from temporal patterns of center of pressure (COP). Traditional COP-based measures (e.g. ellipse area, mean COP velocity, etc.) have demonstrated differences with age [5], [6], sensory conditions [7], and pathology [8], [9], and have been related to risk of falls [1], [2]. However, it has also been suggested that these measures ignore the dynamic characteristics of COP movement and do not represent more subtle aspects of postural control [10]. To overcome these limitations, some modern approaches have been described, including Hurst rescaled range analysis (H_R/S) and detrended fluctuation analysis (DFA), which are intended to reveal the underlying fractal properties of the COP time series. Some of these modern methods have been reported as being more sensitive to factors thought to affect balance (e.g. fractal COP measures are more sensitive to age-related changes [11]).

In general, selection of a sway measure for research or clinical use should be motivated by several factors, including the measure reliability. COP mean velocity, for example, has been reported as the most reliable among traditional measures [12], [13], [14], and within-day results appear to have better reliability than between-day results [15]. Despite these valuable studies, important questions remain regarding the reliability of COP-based measures of postural control. No study to our knowledge has investigated the reliability of the H_R/S exponent. Only one study assessed within-day reliability of Hurst exponent derived from DFA method and age-related differences [16]. Whether age-related differences in reliability are present for other COP measures remains unknown. In the present study, we assessed within- and between-day reliability of both traditional COP measures and two more recent measures. Results from this work were intended to facilitate better selection of COP-based sway measures in future studies.