Because locomotor behavior is rhythmic motion, a steady gait of a legged robot indicates a stable limit cycle in the state space. Therefore, we need to control the robot to establish a limit cycle to produce a stable gait. Because different steady gaits have different stable limit cycles, a change of the gait pattern implies that the robot state moves from one limit cycle to another. Even if a robot obtains steady gaits, many difficulties remain for the gait transition.
For the gait transition, the following two issues are crucial: (1) because a robot has many Degrees Of Freedom (DOFs), it is difficult to determine how to produce robot movements to connect one gait pattern to another, in other words, how to construct adequate constraint conditions in motion planning; (2) even if the robot establishes stable gait patterns, it may fall over during the gait transition and it is difficult to establish stable gait transition without falling over.