A system for determining activity of a mobile element, includes at least one motion sensor having a measurement axis, and which is securely connected to the mobile element. A filter for selects, for each measurement axis of the motion sensor, high frequencies above a first threshold, and processing equipment for determines a unidimensional high-frequency component (y(n)) equal to the square of the Euclidean norm of the high frequencies along the measurement axes of the motion sensor. A calculator calculates, for each state, probability density functions (Py,i) of the high-frequency component, the probability density function corresponding to each state according to a Chi-square law with a degree of freedom equal to the number of measurement axes of the motion sensor. An analyzer defines states of the mobile element, utilizing the probability density function of the high-frequency component for each state, together with the probabilities of transitions between two successive states.
