TIME AND FREQUENCY-DOMAIN ANALYSIS OF ACOUSTIC-SIGNALS FROM A HUMAN MUSCLE

We wished to determine whether evoked force output from a human muscle could be inferred from the amplitude and the frequency of evoked musc ular sounds. Sounds from adductor pollicis were recorded with a microp hone and compared to evoked force changes produced by varying the inte nsity of nerve stimulation or of voluntary effort as well as by fatigu e and twitch potentiation. In all instances, high correlations were ob tained between evoked acoustic signals amplitude and evoked twitch for ce changes. Instantaneous time-frequency distributions of evoked acous tic signals also showed a high degree of cross correlation with evoked instantaneous twitch force, but no relationship was found for the gro up between the maximal frequency of sound and force changes. We conclu de that evoked force changes from human muscles cannot be reliably inf erred from frequency domain analysis of evoked sound signals and that, for this purpose, analysis of sound signals in time domain can be emp loyed, an analysis potentially useful for muscles whose force output c annot be directly measured. (C) 1997 John Wiley & Sons, Inc.