Muscle behavior during these two phases could be estimated from the data on F-L relations.
After the expiration, PA remains constant as the airway closes. The value of the model lies in the prediction
of the time course of muscle shortening during this phase of the maneuver. Eq. 1 is applicable during muscle shortening, and this equation together with the airway equations are solved for the two unknowns, f and l, as functions of time. The rate of recovery of f is affected both by muscle dynamics and by the stiffness