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For bipedal robots, how to realize natural, versatile and dynamic motions like human is still a challenging topic. In order to achieve such high performance by bipedal robots, one useful approach is to develop joint mechanism with high back-drivability, that makes the dynamic coupling forces such as inertia force, centrifugal force available for dynamic motions and results reduction of necessary capacity for joint actuators leading to the reduction of weight of the robot. For this purpose, in this paper, we propose a novel knee joint mechanism consisting of a crossed four-bar linkage with variable transmission (CFLVT) and a joint stop inspired from human structure. CFLVT behaves as a transmission with variable reduction ratio according the knee joint angle to smooth the static torque so that the knee joint can be driven by a small actuator with low reduction ratio. In addition, a joint stop at the maximum extending angle is introduced in the knee joint which is useful for force assist just like that of a human. The characteristic and design of CFLVT and joint stop are described first. And a prototype of the bipedal robot implemented with CFLVT and joint stop is developed. The feasibility of the proposed mechanism is shown by experimental results for walking and kicking motions.