TABLE 1.

Definition of physical terms

Parameter	Definition	Comment
Strain	Dimensionless parameter describing   deformation	Strain can be thought of as an extension ratio, e.g., the fractional   length change of a spring under a stress. Ideally, the reference   state is the unstressed state of the spring/material. The lung   in situ is always prestressed, i.e., transpulmonary pressure is not   zero. Therefore, unless stated otherwise we reference lung strain   to the lung dimension (volume) at end expiration (e.g., lung   strain is tidal volume divided by the volume at end-expiration)
Stress	Force per unit area	Note that pressure and stress have identical units and definitions
Stiffness	Quantity that relates stress and   strain according to Hook's law	Engineers may refer to stiffness as elastic modulus or shear modulus.   It has the units of stress
Yield Stress	The maximal pressure which a   substance is capable of supporting   without fracturing	Yield Stress is typically 10−2 to 10−3 times the shear modulus
Plastic Deformation	A deformation of a body caused by   an applied stress which remains   after the stress is removed	Think mashed potato! All biologic materials including cells undergo   plastic deformations, i.e., they do not behave like ideal springs