1. A method for determining the tidal volume (V T ) of a subject, comprising: determining a first anatomical characteristic representing a first linear displacement of the subject's rib cage in a first orientation using a first sensor system; determining a second anatomical characteristic representing a first linear displacement of the subject's abdomen in said first orientation using a second sensor system; determining a third anatomical characteristic representing a first axial displacement of the subject's chest wall in said first orientation using said first sensor system and said second sensor system; determining a first rib cage volume-motion coefficient representing said first orientation of the subject; determining a corrected abdominal volume-motion coefficient based on an upright abdominal volume-motion coefficient and a linear slope of an abdominal volume-motion coefficient as a function of an axial displacement of the subject's chest wall; determining a first chest well 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 corrected abdominal 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 rib cage volume-motion coefficient, said corrected 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.