A body-weight support system that allows individuals with severe gait impairments to practice over-ground walking in a safe, controlled manner is disclosed. The system includes a body-weight support system that rides along a driven trolley and can be controlled in response to the movement of the subject using the system. They system is also configured to apply strong, yet brief perturbations to a subject as they are stationary or performing a dynamic task, such as walking, side stepping, etc., via the trolley of a body weight support system.