The present study is aimed at ascertaining if muscle sound might be used as a detector of the contractile properties of individual human muscles "in vivo". In order to test this hypothesis, Soleus muscle (slow) and Vastus Lateralis Femoris muscle (fast) were investigated in three healthy subjects during electrically elicited contractions. Evoked phonomyograms were obtained from isometric single twitch contractions using a microphonic apparatus. Time and frequency domain analysis were performed. Evoked phonomyogram rising time values obtained from the two muscles are significantly different (p less than .01) and this difference is clearly due to their different mechanical properties. The power spectrum of all signals was obtained by means of harmonic analysis routine and mean frequency thus obtained was taken into account. Power spectrum values are approximately 1.5 times greater in fast muscle than in slow muscle (p less than .01). These findings lead us to the conclusion that evoked phonomyography can be considered a useful technique for the assessment of mechanical properties of individual human muscles.