System Identification of Evoked Mechanomyogram of Anterior Tibial Muscle —Analysis of the Transfer Function Depending on Muscle Contraction Level—

Abstract

The purpose of this study is to evaluate the viscoelastic characteristics of the anterior tibial muscle based on system identification of evoked mechanomyogram (MMG). Input data consisted of the electrical stimulation of common peroneal nerve, which made the anterior tibial muscle contract. The output data consisted of the evoked MMG. The MMG was detected with an accelerometer attached to the anterior tibial muscle. Seven healthy male subjects participated in the experiment. The electric pulses whose width were 500 μs were applied to common peroneal nerve five times with inter-pulse intervals of 300 ms, and the data were acquired for 1.5 s at sampling frequency of 2 kHz. The measurements were carried out at six contraction levels. The evoked MMG was well estimated with the tenth order transfer function. Then the transfer function was decomposed to five sets of the second order transfer function. The coefficients of the second order function provide the viscoelastic characteristics.There were significant correlation between contraction level and the elastic indices in many cases. On the other hand, the viscous indices often did not show significant correlation. The elastic indices were classified into two groups. One increased as the contraction level increased, but another remained unchanged. We supposed that the former indices corresponded to the muscle elasticity and the latter ones corresponded to the passive elasticity such as subcutaneous fat or skin.