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 muscular sounds. Sounds from adductor pollicis were recorded with a microphone and compared to evoked force changes produced by varying the intensity of nerve stimulation or of voluntary effort as well as by fatigue and twitch potentiation. In all instances, high correlations were obtained between evoked acoustic signals amplitude and evoked twitch force changes. Instantaneous time-frequency distributions of evoked acoustic signals also showed a high degree of cross correlation with evoked instantaneous twitch force, but no relationship was found for the group between the maximal frequency of sound and force changes. We conclude that evoked force changes from human muscles cannot be reliably inferred from frequency domain analysis of evoked sound signals and that, for this purpose, analysis of sound signals in time domain can be employed, an analysis potentially useful for muscles whose force output cannot be directly measured.