Of the three mechanical components limiting maximal lung volume, Pmus-max is probably the most important. The rationale for this statement is based on the balance of the following three pressures that determine TLC: the passive inward expiratory recoil of the chest wall (about 15 cm H2O); the inward expiratory elastic recoil of the lung (about 25 cm H2O); and the inspiratory pressure of the respiratory muscles (40 cm H2O), which equals the sum of the recoil pressures of the lung and chest wall. It is the slope of these volume-pressure characteristics (called tangent compliance) at TLC that determines their importance in limiting maximal volume. For example, if the lung were infinitely stiff at TLC, increases in inspiratory muscle force at TLC would not be able to increase volume above TLC, and we would conclude that the stiffness of the lung, not Pmus-max, is the more important determinant of TLC. By contrast, if the lung and the passive chest wall were relatively compliant, small changes in Pmus-max at TLC could have a substantial effect on the volume achieved, and we would say that the passive characteristics of the lung and chest wall were not very important in determining TLC. In fact, the least compliant of the three pressure-volume characteristics is that of Pmus-max (Fig 1), which is relatively flat near TLC,9,11compared with those of the lung12and the relaxed chest wall. Thus, because Pmus-max decays so rapidly with increasing volume near TLC, small changes in the recoil pressures of the rib cage and lung would have a relatively small effect on TLC. The importance of Pmus-max in determining TLC is supported by experiments13 showing that partial curarization reduces TLC substantially at doses that produce only modest reductions in inspiratory muscle force at resting lung volume.