ABSTRACT
Exercise machines are a billion-dollar industry worldwide, and are particularly popular in wealthy countries such as the United States (Li & Horowitz, 1997a,b). Most of the exercise machines currently available allow users to exercise in a single degree of freedom motion and exercises are repetitive in nature. Exercise is achieved by providing resistance to the user and there are three main categories of resistance: the weight-based , friction based, and damper-based machines (Li & Horowitz, 1997a,b).
