On the kinematics of forelimb landing of frog Rana rugulosus

A frog can jump several times higher than its own height and then land smoothly on the ground. During the buffering phase, both forelimbs touch the ground and compact quickly to absorb most of the impact energy. However, the adjustment of the joint angles of the forelimb and the induced cushioning effect during the landing process have not been thoroughly investigated. In this study, we statistically summarized the angular displacements of forelimb joints with respect to landing velocities by using a high-speed motion capture system. It is found many joint angles were linearly influenced by landing velocity at both ground touching moment and maximum compression moment. Moreover, the double-peak pattern of ground reactive force was measured, which attributes to the forelimb landing and the followed abdomen/hindlimb landing. Before the appearance of the first peak, the compression of the forelimb and the reactive force revealed a linear relationship regardless of velocity, implying that the forelimbs act as a constant stiffness spring in landing. Accordingly, a simple spring-mass model was proposed and verified by simulation for forelimb cushioning of the frog. We anticipate our achievements to inspire the design of future landing mechanisms.