Conceptual overview of human-robot cooperative, assist-as-needed gait training. The goal of this type of training is to allow the human to learn to compensate for the gait impairment, but to reduce the kinematic errors experienced during this adaptation process. The addition of an assist-as-needed robot results in two adaptive interacting subsystems: the robot and the human. Each subsystem is composed of an optimizing controller and a next-step estimator. The assist-as-needed robotic controller mimics the structure of the adapting human, and it is configured such that it relaxes its assistance faster than the human learns. This allows it to challenge the human yet limit performance errors in an assist-as-needed manner.