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Variable stiffness joints are a kind of compliant mechanism to improve the safety of physical human-robot interaction, which has attracted much attention in recent years. Elastic elements are used in the internal kinematic structures of variable stiffness joints. A new mechanically bionic variable stiffness joint using leaf springs was designed. The joint stiffness was adjusted by changing the effective length of leaf springs. By controlling the relative motion of the differential gear train, the position and stiffness of variable stiffness joint can be controlled respectively. Two structures were used in the new joint to exert external forces on the ends of the leaf springs. Based on the elliptic integral solution to large deflection problems of cantilever beams, the effects of two force exertion structures on the joint stiffness property were studied. The simulation results showed that the force exertion structure of the end had great effect on the stiffness during the joint deflection. The joint stiffness was decreased as the increase of joint deflection angle when the force exertion structure was that the external force point was fixed to the joint arm. The stiffness was increased with the increase of deflection angle when the force exertion structure was that the external force point moved relative to the joint arm. The physical experiments of the prototype were conducted to verify the validity of the analysis results. The experimental results showed that the changing range of the joint stiffness was larger. And two force exertion structures can meet the requirements of continuous high stiffness and collision safety respectively.