The phrenic nerves were stimulated bilaterally in human subjects with supermaximal shocks and the transdiaphragmatic pressure (Pdi) twitches recorded at end-expiratory lung volume with glottis closed. Stimulus maximality was monitored by the evoked muscle potentials. The highly reproducible twitch amplitudes elicited from the relaxed diaphragm were greater with bound (48 +/- 13 cm H2O) than unbound (34.6 +/- 10 cm H2O) abdomen in sitting position. With bound abdomen, the twitch amplitudes were similar in sitting and supine positions. When the same stimuli were applied during voluntary contractions the superimposed twitch amplitude declined almost linearly with the voluntary Pdi exerted, reflecting a progressively increasing activation by the CNS. From this relationship both the maximal Pdi (Pdimax) and the relative degree of diaphragm activation (% Pdimax) can be estimated for any type of breathing effort. Generally no twitch could be detected during maximal efforts (Pdimax: 218 +/- 34 cm H2O) indicating that all stimulated motor units were already fully activated. The Pditwitch/Pdimax ratio of 0.21 +/- 0.04 was similar to the twitch/tetanus ratios of isolated mammalian muscle, suggesting that bilateral stimulation can activate all phrenic motor neurons.