During exercise, with voluntary movement, or with disease inspiration may also call into play the accessory muscles which help to stabilize and elevate the rib cage. Expiration may become active with use of the abdominal and intercostal muscles to help force out gas. At the end of inspiration and at the end of expiration when there is no flow, the pressure due to resistance must be equal to zero. As inspiration proceeds, muscle force is needed to generate pressure to overcome the elastic recoil of the lung and to overcome the resistance of the respiratory system. An optimal or minimal work of breathing exists for any minute ventilation and any set of respiratory mechanics.
Minute ventilation is the total amount of gas moved into and out of the lung in a minute.
There is an optimum combination of rate and tidal volume to minimize work. The slowest units will be those which have high resistance and high compliance. The fastest units will be those which have low resistance and low compliance. Obstructed lungs have long time constants due to increased resistance. This means that they take a long time to exhale.