The main objective of this thesis work was to analyse the consequences of different unilateral deficiencies on the postural behaviour, and more precisely on the role of the legs on upright stance maintenance. In order to complete this objective, patients who had suffered a first hemisphere stroke and patients victims of a collateral lateral ankle ligament sprain have been included. Patients were evaluated when standing upright on a system composed by two rectangular force platforms collaterally disposed, permitting to distinctly analyse the role of each leg on the balance control. Similitudes have been observed between stroke and ankle sprain patients: -i) the organisation of the reaction forces measured under the injured leg is perturbed, revealing a support inaptitude for controlling standing. - ii) consequently, patients adopt an asymmetrical body weight distribution on the supports. - iii) a deterioration of the spatio-temporal relationship between the two supports has been emphasised. In comparison to healthy subjects, the control of the two supports would be more independent. - iv) consequently, the injured support influences on postural regulation are decreased; in contrast to the healthy where the support role is enhanced. - v) their balance is characterised by an increase of the means called into play and/or by a postural instability. These features are partly issued from the body weight asymmetry. On the other hand, the asymmetry is insufficient to explain the totality of these impairments. The postural behaviour of patients constrained by stroke or an ankle sprain would be strongly influenced by the sensory deficiencies. Sensory process integrity seems to be determinant in the construction of a symmetrical postural scheme, which should be preferentially used in stance construction.
