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This study was aimed at assessing the possible relationship between upright and sitting postures in healthy adults. The center of pressure trajectories from the force platform on which the subjects stood or sat were analyzed through a frequency analysis and modeled through the fractional Brownian motion framework. The same type of control process was involved during sitting and upright posture maintenance. Both upright and sitting posture would be controlled by the same mechanical law and/or by the same type of central process. Conversely, these two postures presented different characteristics and a relative independence. Both postures displayed specific biomechanical constraints and involved specific effectors, in particular along the anterior-posterior axis. Thus, performances in these two postures are completely independent in the anterior-posterior axis, whereas they are slightly linked in the medio-lateral axis. Improved trunk functions to improve postural stability have an interest solely to improve lateral stabilization. [ABSTRACT FROM AUTHOR]

Upright undisturbed stance constitutes a fundamental attitude involving very cunning sensori-motor strategies. Until now, few analyses on postural control have been undertaken to explain the influence of both supports in this particular task. With the aim of analysing asymmetrical postures resulting from neurological (hemiplegia) or traumatic (amputation) origins, a new analysis and a reference framework are proposed, describing the influence of each support in upright stance maintenance. Ten healthy adults (eight men and two women ranging in age from 20 to 42 years) volunteered for this study. The subjects were required to remain as still as possible in upright stance. The control mechanisms for equilibrium maintenance were estimated by separate measures of the elementary and resultant centres of pressure through two force platforms. The analysis of the relationship between the elementary and resultant centres of pressure on the one hand and the centre of gravity on the other hand emphasises that both took place similarly and at the same time in the control of upright undisturbed stance. This control is done through two strategies that come into play at the level of the ankle and/or hip in the medio-lateral and antero-posterior directions, respectively. Furthermore, the various trajectories (elementary and resultant centres of pressure, centre of gravity) and analytical methods (frequential, correlations and cross-correlations) are compared. This methodology should be very interesting in the analysis of pathologies inferring postural asymmetry in weight distribution, since one could suppose that asymetric patients could be characterised by specific control of both supports. [Copyright &y& Elsevier]

To assess whether prior stretching of a muscle can induce improved postural control, 15 healthy adults stood still upright with their eyes closed before and after a series of bilateral stretches of the triceps surae muscles. The analysis focused on the center of pressure (CP) and the vertical projection of the center of gravity (CGv) trajectories and their difference (CP - CGv). The prolonged stretching induced a forward shift of the mean position of the CGv. The frequency analysis showed a constancy of the amplitudes of both basic movements whereas an increased mean power frequency was seen for the CP - CGv movements. A fractional Brownian motion modeling of the trajectories indicates shortest time intervals and lower covered distances by the CGv before a change in its control occurs along the antero-posterior axis. This reorganization is thought to be a result of improved body movement detection, which allows postural control over the longest time intervals to be triggered more rapidly.