(A) Renderings of an initial joint and its folding (upper row), with their corresponding spring-mass systems shown in the lower row. The lateral black springs represent the rigid bars and disks. The red springs represented the links that cause the joint to fold. (B) Variables in this schematic are used for calculation of stiffness coefficients. (C) Illustration of computing the joint length. Each joint is modeled using two disks and the length of each inner limb is calculated according to its distance from the center of rotation. The center link (marked as purple) remains constant in time. (D) Illustrations of the folding angle and axis. (E) Illustration of computing the bar length for a folding and stretching element. We evaluate the angles between connected bars α, β and the bending radius r. The length of the centered section (marked as purple) remains constant in time, while the length of the remaining elements (e.g. a) are approximated accordingly.