The lung volumes from which inhalation and exhalation proceed change during exercise. There have been previous attempts to predict these lung volumes but none have been completely successful. Prediction of initial lung volume is important in the accurate calculation of respiratory power during exercise, especially when the subject is encumbered with respiratory protective apparatus and exercising above the anaerobic threshold. Although there is no direct work advantage to varying lung volumes at the beginning of inspiration or expiration, such changes appear to be dictated by the asymmetry of lung recoil pressure about the lung relaxation volume. Equalizing inspiratory and expiratory elastic pressures has been found to match available data on lung initial volume.