Time-frequency analysis of the muscle sound of the human diaphragm.

The time-frequency characteristics of muscular sounds (phonomyogram) produced by the contraction of the human diaphragm under various contractile states is evaluated with the cone-kernel distribution. The results show that the instantaneous frequency of the phonomyogram of the diaphragm has a high cross-correlation (an average of 0.91 +/- 0.06) with the transdiaphragmatic pressure, with a delay varying between 25 and 35 ms. The instantaneous frequency response of the phonomyogram of the diaphragm shows a behaviour similar to that of the frog muscle; it rapidly rises and then fades out. However, the maximum of the instantaneous frequency of the phonomyogram of the diaphragm is not proportional to the maximum of the transdiaphragmatic pressure. This analysis also demonstrates the usefulness of the cone-kernel distribution for studying frequency-modulated signals like the muscular sound signals.