Current interface design practices are based on user models and descriptions derived almost exclusively from studies of able-bodied users (Keates et al., 1999). However, such users are only one point on a wide and varied scale of physical capabilities. Users with a number of different physical impairment conditions have the same desire to use computers as able-bodied people (Busby, 1997), but cannot cope with most current computer access systems (Edwards, 1995). It is important to identify the differences in interaction for users of differing physical capability, because the border between the labels able-bodied and motion-impaired users is becoming increasingly blurred as the generation of computer users inexorably becomes older and physically less capable. If user models are to retain their relevance, then they have to be able to reflect users'' physical capabilities (Stary, 1997). Through empirical studies, this paper will show that there are very important differences between those with motion-impairments, whether elderly or disabled, and able-bodied users when they interact with computers. It attempts to quantify where those differences occur in the interaction cycle with the use of a very straightforward user model, the Model Human Processor (MHP) (Card, Moran and Newell, 1983), which describes interaction purely in terms of perception, cognition and motor component times. Although this model is simplistic compared to the more recent sophisticated models, it affords a simple and valuable insight into interaction cycles and offers a building block on which to base more comprehensive models. This work is predicated on the idea that the use of this model in detailed analysis of the basic interaction cycle will provide a means for studying motion impairment at both an individual and general level.