Gait design for a biped robot is an intriguing problem. The objective is to replicate an efficient gait according to the jogging dynamics of a human in a biped robot. This paper aims to find an optimal gait for jogging dynamics of a biped robot on a continuous-time nonlinear mathematical model. The nonlinear model is approximated using the describing function method and requires the gait to be sinusoidal. It is revealed that the natural oscillation of an undamped biped robot is also an optimal gait. The optimal frequency reduces to compensate for damping. The characteristic of the optimal gait is further studied in extensive simulations.