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Abstract-A recent model to analyze the Center of Pressure trajectories is based on the fractional Brownian motion. By doing so, one note that standing still is describe by different mechanisms following the frequency. Previous studies exhibit the existence of a control mechanism which stabilize the upright position at a large enough time scales (from 0.3 s to 1.2 s depending on the method and on the authors) or equivalently at low frequencies. The different mechanisms are separated by a critical time scale or equivalently a critical frequency. This critical frequency is fundamental to understand the control mechanism of upright position : only physiological phenomenon at frequencies larger than this critical frequency could contribute to the task of maintaining equilibrium. A new statistical method is introduced based onto the recent progress in signal processing : the wavelets analysis. The algorithm is entirely automatic. Seventeen healthy young subjects were studied under quiet-standing conditions, the mean value of critical frequency is 1.8 Hz corresponding to a mean critical time scale 0.68 s. The algorithm is entirely automatic. Keywords-Postural Control, Stabilogram, Fractional Brownian motion, Wavelet analysis. I. INTRODUCTIONA good model of control mechanism of undisturbed stance control in human upright posture is useful in applications. Several statistical studies have been proposed in the past. All are based on the center of pressure (COP), which is the resultant point of the reaction forces measured by a force platform. In the seventies, these data were analyzed as a set of points without any order, although COP is time varying and should be considered as time series or as the observation of the trajectory of a stochastic process at discrete times. In this framework the questions are : i) which class of stochastic processes provide a good model? ii) how determining with reliability the parameters of the process? iii) what is the biological meaning of these parameters ? Different models have been proposed (Ornstein-Uhlenbeck processes in [6], roughly speaking a spring with a random excitation), but the most convincing seems the model of "socalled" fractional Brownian motion (f.B.m.) introduced by Collins and De Luca. Using the stabilogram diffusion analysis, they detect two different zones on the log/log plot of the stabilogram with two different slopes : these two zones are interpreted as corresponding to two different mechanisms at high frequencies (an erratic comportment) and at low frequencies (regulation).Moreover they determined by a graphical method a critical time corresponding to the change between the two regulation mechanisms. However the method proposed by Collins & De Luca is heuristic and depends on the subjectivity of the user, see for e.g. [7]. For studying the influence of experimental conditions on sufficiently large samples, an automatic procedure of determination of the change time is needed. A second drawback of the pioneer work of Collins & De Luca and of the use statistical met...

Résumé L’objectif de notre étude était de compléter et d’approfondir les travaux déjà réalisés concernant les effets de la position des pieds sur l’équilibre orthostatique. Nous avons testé une population de 25 sujets masculins sur 16 positions de pieds (combinaisons de 4 écartements et 4 angles d’ouverture) et suivant les conditions expérimentales de l’Association Française de Posturologie. L’augmentation de la surface du polygone de sustentation a entraîné une amélioration de l’équilibre orthostatique. Cette amélioration a été principalement observée suivant l’axe médio-latéral du sujet. L’effet stabilisateur de l’augmentation de l’écartement entre les pieds était plus important que celui de l’angle. Nous avons également noté que le centre de pression moyen était généralement situé sur le côté droit des sujets. Ce résultat peut être lié à leur latéralité car tous étaient droitiers.

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