Dependency of the medial gastrocnemius muscle and ankle joint stiffness on the dorsiflexion angle

This study estimated the stiffness of the medial gastrocnemius muscle and ankle joint by varying the ankle dorsiflexion angle. Six healthy young males participated in the experiment. An inclined plate was attached to the force plate. Each participant stood on the inclined plate, and his feet were dorsiflexed during quiet standing. Electrical stimulation was applied to the medial gastrocnemius muscle. The center of pressure (COP) was measured by the force plate. Measurement was performed at 0, 6, 10, and 15 degrees of the dorsiflexion angle. The COP fluctuation due to electrical stimulation was extracted using a Kalman filter. The transfer function from the stimulation to the fluctuation was calculated using a system identification method. The poles of the transfer function were used to estimate the muscle and joint stiffness. The muscle and joint stiffness decreased and then increased as the dorsiflexion angle increased.