| Sumario: | The incidence of falls in the elderly as well as their social and medical impacts has led to a great deal of research about postural stability. Several clinical and laboratory methods have been developed to assess postural stability. The present work is aimed to increase the usefulness of a force platform to assess postural stability.The center of pressure (COP) is a common variable to quantify postural control. Like many biological measurements, the COP has an intrinsic variability that affects the reliability and the validity of postural control outcomes. The unreliability of the postural steadiness measures decreases the power of a study to detect differences. We estimate the intrasession reliability using a statistical design based on the measurement error on different time interval. In general, the reliability increases with the duration of the trial. It was found that the COP mean velocity is the most reliable COP variable. Only two trials of two minutes must be averaged to obtain an ICC over .90 in both A/P and M/L directions.Evaluation of postural control is often based on the interpretation of the mechanical relationship between COM and COP. We compare three methods to estimate the COM which are based on different biomechanical considerations: (1) the kinematic method; (2) the zero-point-to-zero-point double integration technique (GLP) and (3) the COP filter method (LPF). The validity of the kinematic method was increased by extending the adjustments Zatsiorsky-Seluyanov's mass inertia parameters. Our results show that the GLP method is comparable to the kinematic method. Both methods are independent of standing conditions. The COP moves continuously anteriorly and posteriorly with respect to the COM, and the displacement of the COP is greater than the COM. However, the COM estimated by the LPF method is not always within the COP trajectory.Common measurements of postural steadiness are based on the assumption that the COP displacement is a random process. We conduct a study based on the opposite idea in which specific patterns (shifting, drifting, fidgeting and peak-to peak) of COP displacement were quantified using computer automatic recognition algorithms. Elderly patients with neurological impairments show significant differences in COP migration patterns amplitude compared to healthy elderly patients. It is hypothesized that the 'conservative' and 'operative' level of the postural control system may be in relation to these COP migration patterns.
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