Comparison of three methods to estimate the center of mass during balance assessment
Introduction
The most common model used to characterise the postural control during quiet standing is the inverted pendulum. In this model, the postural control is defined by the relation between the center of pressure (COP) and the center of mass (COM). The COP oscillates on either side of the COM where the COP displacement always exceeds the COM. The COP is the integrated control variable whereas the COM is the controlled variable (Winter, 1995). The variable COP–COM, which is defined as the time course arithmetic difference of the COP and COM position, is highly correlated to the horizontal acceleration of the COM (Winter et al., 1996). The variable COP–COM is reported as the ‘error’ of the postural control system and provides important insight into the postural control mechanism. It was recently shown that the root mean square (RMS) error of the COP–COM is greater in elderly with neurological impairments compared with healthy elderly (Corriveau et al., 2000a). Metrological studies also demonstrated that the COP–COM variable has a high reliability in elderly subjects (Corriveau et al (2000b), Corriveau et al (2001); 2001).
The COP is defined as the point of application of the ground reaction forces under the feet measured by one or two force platforms. It is the outcome of the inertial forces of the body and the restoring equilibrium forces of the postural control system. The COM is an imaginary point at which the total body mass can be assumed to be concentrated. The position of the COM is hypothesised to be subject to body postural control. For convenience of certain calculations, it can be computed as the weighted average position of the segments. Several methods have been suggested to estimate the COM. The kinematic method (also known as ‘segmental method’) is based on the definition of the COM and has been frequently used in quiet standing (Hasan et al (1996a), Hasan et al (1996b); Winter et al., 1998; Corriveau et al (2000a), Corriveau et al (2000b), Corriveau et al (2001)). Recently, the mechanical relationships between COM and COP during quiet standing allowed researchers to define the COM based on Newtonian mechanics (King and Zatsiorsky, 1997; Levin and Mizrahi, 1996; Morasso et al., 1999; Shimba, 1984; Zatsiorky and King, 1998). Since the frequency content of the COP is higher than the COM, other estimation methods have been proposed using a low-pass filter (LPF) on the COP time series (Benda et al., 1994; Caron et al., 1997).
The COM estimation methods based on Newtonian mechanics are attractive in the clinical perspective because they require only a force platform to calculate the COP–COM. The purpose of this study is to compare three different methods to estimate the COM displacement during different standing tasks. Although earlier comparisons have been reported (Eng and Winter, 1993), to our knowledge this is the first time that techniques from three different methods to estimate the COM location are directly compared under different standing conditions. This comparison may help potential users to choose a method for estimating the COM location.
Section snippets
Protocol and data collection
Six young male adult subjects participated in this study. Informed consent was obtained from each subject before the experimentation. Subjects were instructed to stand barefoot in a side-by-side position on a force platform (model OR5-6, Advance Mechanical Technology Inc, Watertown, USA). They were asked to perform four different standing tasks with eyes open: (a) 30 s of quiet standing; (b) 30 s of one leg standing; (c) 30 s of voluntary oscillation about the ankles and (d) 30 s of voluntary
Results
The COP trajectory and the COM trajectories estimated for each task of one representative subject are presented (Fig. 2). Our results show that the kinematic–GLP RMS difference is significantly smaller than the kinematic–LPF and GLP–LPF RMS differences in the A/P direction during quiet stance (Fig. 3). We obtained similar results during one-legged stance (p<0.001) and during voluntary oscillation tasks (p<0.02). However, there is no difference between all comparisons of kinematic–LPF and
Discussion
The purpose of this study was to compare three different methods to estimate the COM during different standing tasks. The RMS difference between each COM trajectory has been used to determine the effects of these methods on the COM estimation. According to the unified theory of balance during quiet standing, the COP moves anteriorly and posteriorly with respect to the COM (Winter, 1995; Winter et al., 1996). It means the COM trajectory must be within the COP trajectory amplitude to maintain
Acknowledgements
Scholarships from the Fonds pour chercheur et aide à la recherche (FCAR) and Fond de recherche en santé du Québec (FRSQ) awarded to DL and FP, respectively, are acknowledged. DL thanks the office Québec-Amérique Jeunesse (OQAJ). Supported (FP) by the Natural Science Research Council of Canada and Canadian Institute of Health Research (227745–00). Dr. Duarte thanks to FAPESP/Brazil for his grant 00/03624-5.
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