Usability studies are an essential and iterative component of technology development and ease its transfer from the laboratory to the clinic. Although such studies are standard methodology in today's graphical user-interface applications, it is not clear that current methods apply to new technologies such as virtual reality. Thus experimentation is needed to examine what existing methods can be viably transferred to the new user-interaction situations. In this paper, 5 integrated interfaces with 3 simultaneous users are evaluated via a set of usability studies, which adapt traditional methods for assessing the ease of use of the interface design. A single expert domain user was run in an intensive study that examined the therapist manual and interfaces of the Rutgers Ankle Rehabilitation System (RARS). The interface and manual were extensively modified based on this evaluation. A second study involving 5 therapists was then conducted to evaluate the telerehabilitation component of the RARS system. In both studies, the tester and developer's observations, along with the session videotapes and therapist-user questionnaires, were triangulated to identify user problems and suggest design changes expected to increase the usability of the system. Changes that resulted from the analysis with the domain expert are described and recommendations for how to conduct usability studies in such multiuser remote virtual reality situations are proposed. Results from the pilot usability telemonitoring studies are also presented. The validity of usability studies in the development and refinement of rehabilitation technology is highlighted.