The relationship between the mean transdiaphragmatic pressure swing developed with each inspiration (Pdi) and the fraction of the breathing cycle time spent in inspiration (TI/Ttot) (Pdi X TI/Ttot) was related to the maximal time that such a run could be sustained (Tlim). Four normal subjects breathed with a constant breathing pattern for 45 min or until Pdi could no longer be sustained, whichever came first. The breathing patterns included Pdi of 0.15-0.90 of Pdimax and TI/Ttot of 0.15-1.0. Pdi was obtained by adjusting an inspiratory resistance, and the timing by monitoring tidal volume with time base from an oscilloscope. The Tlim of a run was found to be inversely related to both Pdi and TI/Ttot and hence inversely related to their product, following a quadratic hyperbole function. Pdi X TI/Ttot represents an index of the tension time of the diaphragm (TTdi). The breathing pattern that could be sustained more than 45 min was found to have a TTdi of about 0.15, which was termed critical TTdi. Above that value Tlim decreased as a function of TTdi. The results are consistent with Tlim being related to diaphragmatic blood flow limitation.