We evaluated an index of diaphragm efficiency (Effdi), diaphragm power output (Wdi) relative to electrical activation, in 5 healthy adults during tidal breathing at usual endexpiratory lung volume (EELV) and diaphragm length (Ldi.ee) and at shorter Ldi.ee during hyperinflation with expiratory positive airway pressure (EPAP). Measurements were repeated with an inspiratory threshold (7.5cmH₂O) plus resistive (6.5 cmH₂O.l ^-1.s) load. Wdi was the product of mean inspiratory transdiaphragmatic pressure ([[DELTA}}Pdi.mean), diaphragm volume displacement measured fluoroscopically (Vdi) and 1/inspiratory duration (Ti^-1). Diaphragm activation, measured with esophageal electrodes, was quantified by computing root mean square values (RMSdi). With EPAP, a) EELV increased (mean r²=0.91(0.01)), b) in 4 subjects, Ldi.ee decreased (mean r²=0.85(0.07)) and mean Effdi decreased 34% per 10% decrease in Ldi.ee (p<0.001) and, c). in 1 subject, gastric pressure at EELV increased 2-3 fold, Ldi.ee was unchanged or increased and Effdi increased at 2 of 4 levels of EPAP (p0.006, ANOVA). Inspiratory loading increased Wdi (p=0.003) and RMSdi (p=0.004) with no change in Effdi (p=0.63) or its relationship with Ldi.ee. Effdi was more accurate in defining changes in Ldi.ee ((true positives + true negatives)/total=0.78(0.13)) than Pdi.mean.RMSdi^-1, RMSdi or Pdi.mean.Ti (all <0.7, p0.05, without load). Thus, Effdi was principally a function of Ldi.ee independent of inspiratory loading, behaviour consistent with muscle force-lengthvelocity properties. We conclude that Effdi, measured during tidal breathing and in the absence of expiratory muscle activity at EELV is a valid and accurate measure of diaphragm contractile function.