Spectral analysis of human inspiratory diaphragmatic electromyograms.

The inspiratory diaphragmatic EMG was recorded via esophageal electrodes in six normal subjects. The EMG and ECG signals were analyzed by power density spectral analysis, before and after band-pass filtering (20--1,600 Hz). The EMG spectrum was concentrated in the bandwidth 25--250 Hz. Electrode motion introduced a significant artifact only at low frequencies. The ECG spectrum was also concentrated at lower frequencies, but substantial power from the cardiac signal spilled over across most of the EMG spectrum. Band-pass filtering was therefore effective in minimizing the former but not the latter. Of the various power and frequency parameters used to quantitate the EMG spectrum, the most stable was the centroid frequency. This was reproducible within and between subjects, and was not affected by changing tidal volume or inspiratory flow rate.