A powered orthosis, adapted to be secured to a corresponding body portion of the user for guiding motion of a user, the orthosis comprising a plurality of structural members and one or more joints adjoining adjacent structural members, each joint having one or more degrees of freedom and a range of joint angles. One or more of the joints each comprise at least one back-drivable actuator governed by a controller for controlling the joint angle. The plurality of joint controllers are synchronized to cause the corresponding actuators to generate forces for assisting the user to move the orthosis at least in part under the user's power along a desired trajectory within an allowed tolerance. One embodiment comprises force-field controllers that define a virtual tunnel for movement of the orthosis, in which the forces applied to the orthosis for assisting the user may be proportional to deviation from the desired trajectory.