This paper describes a method of assisting muscle functions at gait initiation using a human leg model. We analyzed a human motion of walking mechanism at gait initiation by measuring 3D position data and force plate data and surface EMG. It is generally accepted that the backward shift of the COP at the center of pressure of the sole of the foot is very significant at gait initiation starting from the standing upright position. The mechanism generating the backward shift of the COP is discussed according to measured data. The human leg model with the same size and the approximation weight as those of a subject was constructed to simulate the condition beginning to walk from standing upright posture. Air cylinder actuators, serving as muscle functions, were installed in the part of muscle on the leg model. Further, a small actuator is equipped at the part of the heel to produce the backward shift of the COP at beginning of the walk. Gait initiation is well simulated when actuators were operated synchronizing with the surface EMG. Based on these results, a simple method for initiating the walk was discussed in relation to walking mechanisms analyzed.