A robotic leg of high-speed locomotion should be designed such that the mobility and the force producibility are maximized simultaneously. The mobility is crucial in a direction perpendicular to a line spanning from the head of upper limb to the tip of the lower limb, which is related to the rapid movement of the leg during swing. On the other hand, the force producibility is crucial in the same direction to the line, which is related to the thrust force of a leg in stance. In this paper, the tangential mobility and radial force producibility being defined mathematically, the mechanism design factors, i.e., an actuator configuration and a length ratio of upper limb and lower limb, are introduced, analysed, and determined for the leg to maximize both the tangential mobility and the radial force producibility. Applying the proposed design parameters, a robotic leg is fabricated, and the running motion is realized in experiment.