Measuring the kinematics of the knee during gait or other activities is an important component of many biomechanical studies. There are a limited number of tools available with which to perform such measurements. These include video based tracking systems (Vicon Motion Systems, Peak Performance Tech.), electromagnetic tracking systems (Ascension Tech., Polhemus Inc.), spatial linkages (Acufex, Enduratec), and radiographic techniques (not commercialized). With the possible exception of spatial linkages, all of these tracking systems require a substantial and costly infrastructure which limits their use to relatively small spaces in laboratory-like settings. There is a need then, for an in-expensive, easily portable tracking system which can be used in a wide variety of settings.