A single-link-plus-foot inverted pendulum model in the frontal plane and lateral stability boundaries. X indicates the COM position in the medio-lateral direction. m, l and M are whole body mass, pendulum length (distance from the ankle to the COM), and ankle joint moment. The lateral stability boundaries were defined in two ways: one with the BOS width (BOS stability boundary), and the other adjusted with the COP position at FSLO, considering it as a functional limit for COP movement (functional stability boundary). Stability margins were calculated as the shortest distances from the experimental data to those stability boundaries.