Abstract: A mobile robot system and method for determining the stiffness of a human arm while moving with a user during overground interaction as the user holds the robot's handle and exchanges forces with it. A mobile base moves with the user, a robot arm interacts with the user, and a controller determines the stiffness. The robot arm includes servomotors driving a linkage mechanism, an end effector including the handle supported by the linkage mechanism, and a force transducer measuring a force applied by the user to the handle. The controller causes the robot arm to generate a force perturbation at the handle, measure a peak velocity achieved by the human arm, determine the stiffness of the human arm as a function of force and displacement, and control operation of the system based on the determined stiffness. A robot body may allow for adjusting the height of the robot arm.