A system for monitoring the respiratory activity of a subject, which comprises one or more movement sensors, applied to the thorax of a subject, for generating first signals that are indicative of movement of the thorax of the subject； a receiver for receiving the first 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 computing device is operable to generate a first breathing pattern from the first signals； divide each respiratory cycle experienced by the subject and defined by the first pattern into a plurality of portions, each of the portions delimited by two different time points and calculate, for each of the plurality of portions of a given respiratory cycle of the first pattern, a slope representing a thorax velocity； derive, from the given respiratory cycle of the first pattern, a pulmonary air flow rate of the subject during predetermined portions of the respiratory cycle； compare between corresponding portions of the first pattern and average flow rates during different phases of the breathing cycle, to calibrate a thorax velocities of the subject with pulmonary air flow rates； and determine respiratory characteristics of the subject for subsequent respiratory cycles experienced by the subject, based on a calculated thorax velocity and the calibration.
