1. A system for determining a posture of a person, comprising: at least one motion sensor configured to be affixed to the person, the at least one motion sensor configured to measure k measurement axes, k being greater than or equal to two; at least one filter coupled to the at least one motion sensor and configured to generate output signals that include first signals with frequencies higher than a first threshold selected for each axis of measurement, and second signals with frequencies below a second threshold lower than or equal to said first threshold; a calculation module configured to calculate a density of probability of a first variable representing said first signals, said first variable modeled by a Chi-2 law with a degree of freedom equal to k, and to calculate a density of probability of a second variable representing said second signals, said second variable being a one-dimensional variable representative of the second signals along the k axes of measurement and being modeled by a Gaussian law; and an analysis module configured to determine a posture of the person out of a plurality of postures by combining: joint densities of probabilities of said first and second variables, the joint densities of probabilities being defined for each posture of the plurality of postures; and probabilities of transition between two successive postures of the plurality of postures.