Discharge frequencies of single motor units in human diaphragm and parasternal muscles in lying and standing

Single motor unit discharge was measured directly in diaphragm and paraster nal intercostal muscles to determine whether neural drive to human inspirat ory muscles changes between lying and standing. The final discharge frequen cy of diaphragmatic motor units increased slightly, by 1 Hz (12%; P < 0.01) , when subjects were standing [182 units, median 9.1 Hz (interquartile rang e 7.6-11.3 Hz)] compared with lying supine [159 units, 8.1 Hz (6.6-10.3 Hz) ]. However, this increase with standing occurred in only two of six subject s, in one of wham tidal volume increased significantly during standing. Par asternal intercostal motor unit final discharge frequencies did not differ between standing [116 units, 8.0 Hz (6.6-9.6 Hz)] and lying [124 units, 8.4 Hz (7.0-10.3 Hz)]. The discharge frequencies at the onset of inspiration d id not differ between lying and standing for either muscle. A larger propor tion of motor units in both inspiratory muscles had postinspiratory or toni c expiratory activity for lying compared with standing (15 vs. 4%; P < 0.05 ). We conclude that there is no major difference in the phasic inspiratory drive to the diaphragm with the change in posture.