A modular exoskeletal device adapted to fit the lower extremities of a patient during rehabilitation. The device has only two actuators during the standing stage of rehabilitation. Two additional actuators can be added, as modules, during the walking stage of rehabilitation. The actuators are affixed to the patient and provide controlled motion to at least one of the joints of the patient. A stationary control unit is separated from the patient. The control unit communicates with and directs the actuators, and has a hybrid control algorithm, such that the actuator forces are adjusted as the patient regains control of some joint motions, which is based upon the sliding-mode control theory. A back brace is affixed to the patient and helps to keep the torso of the patient in a stable, substantially vertical position.