The great virtue of VC as an index of respiratory muscle
strength is that it is easy to perform, but its disadvantage
is that it is nonspecific. Vital capacity is reduced
by both obstructive and restrictive pulmonary disease, as
well as by extrapulmonary factors, such as obesity or
ascites. Thus, although a normal supine VC excludes
moderate to severe respiratory muscle weakness, a reduced
VC can be due to weakness, or respiratory disease, or
both.