System Identification of Evoked Mechanomyogram to Clarify Lower Limb Muscle Stiffness in Treadmill Walking

Abstract

The purpose of this study was to clarify the stiffness of the anterior tibial muscle in the stance and swing phases during walking. Electrical stimulation was applied to the common peroneal nerve once every two steps at heel strike, and in the stance and swing phases. Mechanomyograms (MMGs) of the anterior tibial muscle were measured with an acceleration sensor. The measured MMGs were divided into stimulated and non-stimulated MMGs, and each set of MMGs was synchronously averaged. A Kalman lter was constructed by approximating the non-stimulated MMG with an autoregressive model. The stimulated MMG was smoothed with the Kalman lter, and then the walking acceleration was obtained. Evoked MMG was extracted by subtracting the walking acceleration from the stimulated MMG. The transfer function from electrical stimulation to evoked MMG was identiied using a singular value decomposition method. The natural frequency as an index of muscle stiffness was calculated from the poles of the transfer function. The natural frequency of the evoked MMG during rest was also calculated. The evoked MMGs at heel strike, stance, and swing phases and during rest were approximated well with the sixth-, sixth-, eighth-, and sixth-order models, respectively. The higher order model in the swing phase may reeect the complex vibrations involving the foot and tendons. The natural frequency was highest at heel strike, followed by the stance phase, swing phase, and at rest. Stiffness of the anterior tibial muscle at the stance and swing phases in walking was clariied by the proposed method.