The present study examined the stiffness in the medial gastrocnemius muscle at various angles between the rearfoot and the floor. Six healthy young men participated in the study. A triangle support was attached to the force plate. Each participant placed their forefoot and rearfoot onto the force plate and the triangle support, respectively. Electrical stimulation was applied to the medial gastrocnemius muscle using Ag-AgCl surface electrodes. The center of pressure was measured with the force plate, and electrically-induced fluctuation of the center of pressure was extracted using a Kalman filter. Measurements were performed at 10°, 20°, and 30°angles of the triangle support. The transfer function from the stimulation to the fluctuation was identified, and the poles of the transfer function were used to estimate the medial gastrocnemius muscle and ankle stiffness. The muscle stiffness increased as the angle of the triangle support increased.