1. A method for determining the tidal volume (V T ) of a subject, comprising the steps of: determining a first anatomical characteristic representing a first linear displacement of the subject's rib cage in a first orientation; determining a second anatomical characteristic representing a first linear displacement of the subject's abdomen in said first orientation; determining a third anatomical characteristic representing a first axial displacement of the subject's chest wall in said first orientation; determining a first rib cage volume-motion coefficient representing said first orientation of the subject; determining a first abdomen volume-motion coefficient representing said first orientation of the subject; determining a first chest wall volume-motion coefficient representing said first orientation of the subject; providing a first mathematical relationship that is adapted to determine V T of the subject as a function of said first, second and third anatomical characteristics, said first rib cage volume-motion coefficient, said first abdomen volume-motion coefficient and said first chest wall volume-motion coefficient; and determining V T of the subject with said first mathematical relationship and said first, second and third anatomical characteristics, and said first abdomen volume-motion coefficient, first abdomen volume-motion coefficient and first chest wall volume-motion coefficient, whereby said determined V T represents the subject's V T at said first orientation.