A system for monitoring the respiratory activity of a subject, which comprises two or more signal generating elements being inertial sensors, or light emitting elements, applied to the thorax of a subject, for generating signals that are indicative of displacement of the thorax of the subject throughout a predetermined time period a receiver for receiving the generated signals during breathing motions of the subject and one or more computing devices in data communication with the receiver, for analyzing the breathing motions. The one or more computing devices is operable to generate, by the two or more signal generating elements, signals that are indicative of displacement of the thorax of the subject, throughout the predetermined time period calculate the current displacements and relative phases of the signal generating elements throughout the predetermined time period and calculate, throughout the predetermined time period, the breathing volume from the displacements and the relative phases.
