Accurate natural frequency estimation method for myotonometer using a system identification method

The purpose of this study is to propose a novel method to estimate natural frequency using a myotonometer with the aid of a system identification technique. A myotonometer is an instrument that measures muscle hardness based on an indentation method. We utilized a MyotonPRO (Myoton AS, Tallinn, Estonia) as a myotonometer. The myotonometer applies a mechanical rectangular pulse to an object and records the acceleration of the indenter. The built-in program of the myotonometer calculates the natural frequency of the object from the recorded acceleration with fast Fourier transformation. This calculation, however, showed the dependency of the natural frequency on the rectangular pulse width. To overcome the dependency, we propose a novel technique to estimate the natural frequency. Our proposed method extracted the acceleration that was not affected by the rectangular mechanical pulse. Then the extracted acceleration was regarded as an output of a system from the mechanical pulse to the acceleration. The transfer function of the system was identified, and then the natural frequency of the transfer function was calculated. We applied the proposed method to the estimation of the natural frequency of a gel-like object mimicking human soft tissue. The proposed method provided a smaller standard deviation of the natural frequency than the built-in program of the myotonometer.