This method for detecting activity (A) of a physical system carrying a motion sensor comprises the following steps: extracting (100) a sequence (M) of measurements provided by the motion sensor deducing (102) therefrom an observation sequence (O) calculated on the basis of the measurement sequence (M) determining (104) the activity (A) of the physical system in the form of a sequence of states corresponding to the observation sequence, using a statistical model of components of the observation sequence in view of a plurality of possible predetermined states of the physical system.At least one component of the observation sequence (O) is calculated on the basis of a reference value ({circumflex over (λ)}) of a vector representing a reference axis. In addition, the method comprises the following calibration steps: calculation (106) of a likelihood value of the observation sequence (O) in view of said statistical model comparison (106) of this likelihood value with a predetermined threshold on the basis of this comparison, activating an estimation (108) of a new reference value ({circumflex over (λ)}) of the vector representing the reference axis.
