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In the present study, spontaneous postural behavior has been analyzed in freely standing multiple sclerosis (MS) patients, exhibiting no clinically assessable abnormalities of postural control. This population has been compared with two other groups, healthy people and hemiparetic patients. This latter group represents a situation where the central nervous system (CNS) lesion is precisely localized in one anatomical site and no signal-conduction disorders are present; i.e., it has an opposite anatomical character with respect to the MS at a preclinical stage. The hypothesis underlying the modeling study is the presence of a controller block working in a feedback posture control system. This controller block receives the body sway as input, and produces the corresponding ankle torque stabilizing the body, the latter being modeled as an inverted pendulum. The CNS damage, caused by MS, is supposed to be reflected in some detectable change in the structure of the controller of the posture control system. The identification of the controller has been performed by means of a parametric estimation procedure which employed as input sequences, data recorded by means of a movement-analysis (MA) system. Reported findings show a structural change of the model of the controller block in the posture control system. This result may suggest the presence of an MS-specific reorganization of the posture control system. Some speculation is finally made on the black-box approach in comparison with traditional posturography, to arrive at hypothesizing a progression path for postural disorders.