A system for preventing discomfort to a user of a robotic exoskeleton (200) determines the existence of an exoskeleton operating condition which has the potential to cause at least one of a discomfort or an injury to a user (204) when the exoskeleton is being worn by the user. Responsive to the determining, an exoskeleton control system (224) selectively controls at least one viscous coupling (208, 210) disposed at an interface location (201, 203) of the exoskeleton where a physical interaction occurs between a portion of the user and a portion of the exoskeleton when the exoskeleton is in use. The control system selectively varies a viscosity of a fluid (216) comprising the viscous coupling to control the stiffness of the interface.