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Abstract: Background and purpose: Regaining control of sitting posture is one of the first goals in the rehabilitation of patients with stroke. So, it requires a precise quantification of this postural behaviour. The purpose of the present investigation was thus to assess postural control during sitting in people with hemiparesis through a biomechanical analysis. Methods: Centre-of-pressure displacements were recorded by means of a force platform on which 10 patients with stroke and 10 age-matched healthy subjects were sitting. Centre-of-pressure trajectories were processed through space-time and frequency analyses. Results: These centre-of-pressure displacements of the patients with stroke were characterised by an increased control for maintaining sit position and by reduced postural performance, as enlightened by the larger surfaces covered by the centre-of-pressure displacements (P <.05) and increased velocities (P <.001), respectively. As shown from the frequency analysis, the impairment have affected predominantly the displacements occurring along the antero-posterior axis (P <.05). Interpretations: The analysis of centre-of-pressure displacements during sitting posture indicates an increased postural disturbance in patients with stroke. A platform device, because of the non-invasive, easy and fast measures carried out, should thus be viewed as an attractive tool for assessing the postural dysfunctioning encountered in sat patients with stroke. This tool could also be used for evaluating, the rehabilitation process following stroke. [Copyright &y& Elsevier]

Abstract: Postural asymmetry in humans is generally associated with different pathologies. However, its specific influence on undisturbed upright stance is poorly understood. To evaluate its separate effects on each support, the centre of pressure (CP) displacements were recorded through two force platforms. In a second step, the complex resultant centre of pressure trajectories (CPRes) were computed and decomposed into two elementary components: the horizontal displacements of the centre of gravity (CGh) and the difference in the plane of support between the vertical projection of CGh and CPRes (CP–CGv). These motions were then processed through a frequency analysis and modelled as fractional Brownian motion to gain some additional insight into their spatio-temporal organisation. Ten healthy adults were tested in three conditions consisting of various weight distributions. The quality of the mechanism involved in the control of the unloaded support CP motions appears to decrease as the asymmetry becomes more pronounced. To be precise, larger increases of the CP displacements are observed for the unloaded support compared to the loaded one. As a result, the CPRes motions are themselves augmented in the ML direction, inducing in turn larger CGh and CP–CG v motions. Postural asymmetry thus constitutes an important constraint on the control of upright undisturbed stance by generating changes in the control of both supports and by reducing the efficiency of the hip load/unload mechanisms. On the other hand, by inducing larger body sways, postural asymmetry necessitates higher energy expenditure and the setting of particular control mechanisms. [Copyright &y& Elsevier]

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]