Automatic Determination of the Transition Between Successive Control Mechanisms in Upright Stance Assessed by Modelling of the Centre of Pressure

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Abstract
A recently introduced concept models the trajectory of the centre of pressure as a fractional Brownian motion and reveals that two successive scaling regimes, acting hypothetically as open and closed loop mechanisms, are implicated in posture control. Objectivity is obviously required in the determination of the transition point, i.e. the point at which an open-loop control mechanism would switch to a closed-loop one, in order to provide reproducibility and automatism in the processing of data. In the method proposed herein, the transition point corresponds to the maximal distance separating a diffusion curve in a double logarithmic plot (mean square distances MSD calculated on each axis versus increasing time intervals ?t) from a straight line characterising a pure stochastic behaviour. In closed eye conditions, the switch appears medio-laterally in a 0.26–0.52 s range for ?t, the corresponding MSD being in the range of 1.86–10.50 mm 2. In the forward-backward direction, the transition is in a 0.28–0.42 s range and the corresponding MSD is between 3.60 and 15.17 mm 2. Finally, these co-ordinates induce scaling exponents over 0.50 for the shortest ?t, thus suggesting open-loop control, whereas those of longest ?t, ranged between 0 and 0.20, give evidence of close-loop control. This data is compared to previous data based upon empirical methods.