In order to design quadruped robot legs with characteristics of light weight, flexibility and high stiffness, the effect of muscle-stretching on the stiffness of the bone of a dog's foreleg in high speed movement is studied from the view of biology. Then two states, i.e. muscle-stretching and non-muscle-stretching, are investigated and compared by kinematics and dynamics simulation in ADAMS, which imitates the movement of a dog's foreleg in trotting gait. The results show that the bone with muscle stretching has higher stiffness, which is able to support the body's weight and realize stable movement without being damaged. Furthermore, it is proposed and verified that the spring elastic stretching force has the same effect as the muscle-stretching force, and a shank mechanism equipped with an elastic stretching actuator is designed based on this conclusion. The joint elastic force is converted into the stretching force acting on both ends of the shank through the mechanical design, and the correlation among the change of acting point, spring stiffness, and the maximal stress of this mechanism is investigated. The results suggest that this design not only effectively enhances the stiffness of the leg, but also remains the controlling characteristics of the joint elastic force.