System identification of an evoked mechanomyogram of the erector spinae muscle

The purpose of this study is to establish a method of estimating erector spinae muscle stiffness using mechanomyogram (MMG). The participants were five healthy adult males. They kept contraction levels of 10, 20, …, 70% MVC. The electrical stimulation was percutaneously applied to the erector spinae muscle. The evoked MMG systems were identified using a singular value decomposition method, and the natural frequency of the transfer function was calculated. The relationship between the contraction level and the natural frequency was examined. The evoked MMGs were approximated by a transfer function of the third order. The natural frequencies were from 2 to 6 Hz. The natural frequency increased as the contraction level increased. However, the inter-individual difference was larger than the increase in the natural frequency. This might be caused by changes in muscle thickness and length dependent on the contraction level.