Ankle-subtalar joint model with eight muscles and two DOF joint angles. (a) There are eight muscles around ankle-subtalar joints: MG, LG, Sol, TA, TP, PB, PL and PT. The knee joint is fixed and the gravitational direction is parallel to the shank direction. (b) System inputs are muscle excitation levels between 0 and 1. System outputs are angular displacements of ankle joint (dorsiflexion/plantar flexion) and subtalar joint (inversion/eversion).