Muscle stiffness estimation using center of pressure fluctuations induced by electrical stimulation

We proposed a novel technique to estimate muscle stiffness using center of pressure (COP) fluctuations induced by electrical stimulation. Six participants stood on a force plate and the COP displacement was measured with and without electrical stimulation applied to the gastrocnemius muscle. The displacement measured in the stimulated condition involved both intrinsic and electrically induced fluctuations. To obtain the intrinsic fluctuation, the measured displacement was smoothed using a Kalman filter constructed by approximating the displacement measured in the non-stimulated condition with an autoregressive model. The intrinsic fluctuation was subtracted from the measured displacement to obtain the electrically induced fluctuation. The fourth-order transfer function from the electrical stimulation to induced fluctuation was identified using a singular value decomposition method. The transfer function had natural frequencies of approximately 2 and 0.3 Hz. The former was close to the natural frequency of the gastrocnemius muscle, and the latter was close to the resonance frequency around the ankle joint. The proposed method can provide muscle and ankle joint stiffness in a standing posture.