A Lagrangian approach is used to perform an energy analysis on a nonlinear model of the human lung. Energy functions associated with the mechanics of breathing are obtained. Analytical expressions are derived for the work of breathing (WOB) and are subsequently evaluated for a typical pulmonary function laboratory test that includes tidal breathing, panting and the forced vital capacity maneuvers. The analysis successfully mimics information presented in the conventional Campbell diagram, which is often used to graphically estimate the work of breathing. In addition, it reveals dynamic aspects of muscular effort during a breathing cycle that are not immediately apparent in the Campbell diagram. Additional simulations, based on an expanded model that accounts for gas compressibility, reveal that WOB is underestimated during forceful breathing maneuvers when compressibility effects are ignored. The energy analysis provides valuable insight into the mechanics of respiration, and, in particular, the significance of individual model components.