The muscle sound properties of different muscle fiber types during voluntary and electrically induced contractions

Soundmyogram (SMG) and electromyogram signals were recorded simultaneously from the relatively fast medial gastrocnemius (MG) and slow soleus (SOL) du ring voluntary and electrically induced contractions. Using a spike-trigger ed averaging technique, the averaged elementary sound and corresponding MU spikes were also obtained from about 35 different MUs identified. The rms-S MG of MG increased as a function of force (P < 0.01). On the contrary, thes e values for SOL increased up to 60% MVC (P < 0.01), but decreased at 80% M VC. The relationship between the peak to peak amplitude of SMG and MU spike indicated significant positive correlations (r = 0.631 similar to 0.657, P < 0.01). During electrical stimulation at 5 Hz, the SMG power spectral pea k frequency (PF) was matched with stimulation frequency in both muscles. At higher stimulation frequencies, e.g., > 15 Hz, only in the MG was SMG-PF s ynchronized with stimulation frequency; the slow SOL did not show such sync hronization. Our data suggest that the SMG frequency components might refle ct active motor unit firing rates, and that the SMG amplitude depends upon mechanical properties of contraction, muscle fiber composition, and firing rate during voluntary and electrically induced contractions. (C) 1999 Elsev ier Science Ltd. All rights reserved.