1. A method for detecting activity of a physical system carrying a motion sensor, comprising the following steps: extracting a sequence of measurements provided by the motion sensor, deducing therefrom an observation sequence calculated using the sequence of measurements, using a statistical model of components of the observation sequence in view of a plurality of possible predetermined states of the physical system, determining the activity of the physical system in the form of a sequence of states corresponding to the observation sequence, wherein at least one component of the observation sequence is calculated on the basis of a reference value ({circumflex over (λ)}) of a vector representing a reference axis, and in that the method comprises the following calibration steps: calculating a likelihood value for the observation sequence in view of said statistical model, comparing said likelihood value with a predetermined threshold (δ), on the basis of this comparison, activating an estimation of a new reference value ({circumflex over (λ)}) for the vector representing the reference axis.