RESPIRATORY muscle contraction during inhalation. The work is accomplished in three phases: lung compliance work, that required to expand the LUNGS against its elastic forces; tissue resistance work, that required to overcome the viscosity of the lung and chest wall structures; and airway resistance work, that required to overcome airway resistance during the movement of air into the lungs. work of breathing does not refer to expiration, which is entirely a passive process caused by elastic recoil of the lung and chest cage. (Guyton, Textbook of Medical physiology, 8th ed, p406)