In this study, the cam-like passive mechanism, known as the engage–disengage mechanism (EDM) of the intertarsal joint of ratites, and its design principles are investigated. This mechanism operates through the interplay of a muscle and three ligaments located on the medial and lateral sides of the intertarsal joint and the articular surface morphology of the tibiotarsus. The interplay of the musculoskeletal ligamentous elements creates two stable equilibrium points when they are almost fully extended and flexed. To elucidate the EDM in the intertarsal joints of ratites, we dissected the hindlimb of an emu (Dromaius novaehollandiae) and examined anatomical features around the joint. Subsequently, we replicated the intertarsal joint of ratites using a physical model. This model consists of three-dimensional-printed ostrich bones, coil springs, and nylon strings simulating the muscle and ligaments. This model successfully replicated the EDM and facilitated the analysis of the interplay of musculoskeletal ligamentous elements. We demonstrated that the medial ligaments and the morphology of the tibiotarsal articular surface play significant roles in facilitating the execution of EDM. Furthermore, we observed that the articular surface morphology resembles a well-known cam system in engineering and is responsible for the EDM.
