Paired SLIPs model.

On the left one SLIP is in the single support phase (the swing leg is not represented), while the SLIP on the right is in the double support phase. In the middle a mass is connected to the CoM of each pendulum through a spring and damper system representing the arms’ impedance. We indicate the center of mass of each SLIP by m_i for i = 1, 2, while l_i, and α_i represent the SLIPs’ initial leg length, the leg stiffness, and the angles of attack, respectively; k_i, b_i and are the impedance parameters and the equilibrium lengths of each connection between the SLIP’s CoM and the CoM of the object, which has a total length of L_t and its relative position with respect to the first SLIP is indicated as x_t. Finally, x_i and z_i indicate each SLIP’s CoM horizontal and vertical position while d is the distance between the foot point of the first pendulum’s stance spring and that of the second pendulum.