A system for monitoring the respiratory activity of a subject, which comprises one or more signal generating elements being kinematic or light emitting elements, applied to the thorax of a subject, for generating signals that are indicative of movement of the thorax of the subject； a receiver for receiving the generated signals during breathing motion of the subject； and one or more computing devices in data communication with the receiver, for analyzing the breathing motion. The more computing device is operable to calculate, in response to the received generated signals, the magnitude of a maximum displacement in 3D space of the one or more signal generating elements during a cycle of the breathing motion； and to calculate the magnitude of a current displacement in 3D space of the one or more signal generating elements during the breathing motion with respect to a reference tidal volume associated with the maximum displacement in 3D space.
