System Identification of Mechanomyogram Evoked by Nerve Stimulation and Its Application to Analysis of Muscle Mechanical Properties Depending on the Stimulation Rate

Abstract

The purpose of this study is to explain the mechanical characteristics of muscle depending on the electrical nerve stimulation rate using the transfer function of the evoked mechanomyogram (MMG). Seven healthy male subjects participated in the experiment. Transcutaneous electrical stimulation was applied to common fibular nerve, and isometric contraction of the anterior tibialis muscle was evoked. The pulse width of the electrical stimulation was 500 [μs], and the intensity was a supramaximal stimulus referring to the MMG peak-to-peak (MMGpp) value. The stimulation rates were adjusted to between 1 [pulses/s] and 10 [pulses/s]. Twelve evoked MMGs were measured at various stimulation rates. The system in which the input data consisted of the electrical stimulation and the output data consisted of the evoked MMG was identified by the singular value decomposition method. The transfer function of the system was compared with the second-order mechanical model, and the coefficients of the denominator of the transfer function were evaluated as the indices of the muscle vicoelasticity. The system was well estimated with the sixth-order model. In addition, MMGs up to 50 [pulses/s] of the stimulation rate were measured in two subjects, and they were also well estimated with a sixth-order model. Some of the coefficients reflecting the muscle elasticity increased as the stimulation rate increased, but others did not. In conclusion, the system of the evoked MMG could be approximated with the sixth-order model, and it was suggested that the transfer function of MMGs in twitch and tetanus reflected the characteristics of the active and passive elements of the muscle and subcutaneous tissue.