This article describes the dynamics, control, and stability of extenders, robotic systems worn by humans for material handling tasks. Extenders are defined as robot manipulators which extend (i.e., increase) the strength of the human arm in load maneuvering tasks, while the human maintains control of the task. Part of the extender motion is caused by physical power from the human; the rest of the extender motion results from force signals measured at the physical interfaces between the human and the extender, and the load and the extender. Therefore, the human wearing the extender exchanges both power and information signals with the extender. The control technique described here lets the designer define an arbitrary relationship between the human force and the load force. A set of experiments on a two-dimensional non-direct-drive extender were done to verify the control theory.