Muscle mechanomyographic and electromyographic signals compared with reference to action potential average propagation velocity

The mechanomyogram (MMG) is the signal detected from the muscle due to the mechanical vibrations that accompany muscle contraction. It is being increasingly used for investigating muscle activities. Many models have been proposed for explaining the generation of this signal. In this paper a system for the simultaneous measurement of both the surface EMG and MMG is used to investigate and compare the mechanisms responsible for the generation of these signals. Two piezoelectric transducers placed a known distance apart on the muscle along the path of propagation of action potential were used to detect two MMG signals. At the same time, stainless steel wire electrodes are used to detect two surface EMG signals from the same areas of muscle monitored by the piezoelectric transducers. The measurements were carried out on the Biceps Brachii under a sustained voluntary light contraction. It was found that the two MMG signals are delayed with respect to each other by the same value of time delay that exists between the EMG signals. The time delay between the signals was estimated using the cross-correlation function. This shows that a similar mechanism to that of the EMG exists for the MMG, and that the MMG is the summation of twitches that seem to propagate along the motor units at the same rate as motor unit action potentials