**4. Discussion**

In this approach the passive properties have been divided into two parts; First, mathematical model is used to represent the combination of knee joint inertial ( *M*<sup>i</sup> ) and gravitational ( *M*<sup>g</sup> ) moments. Second, fuzzy model is applied to represent the combination of the elastic moment ( *M*<sup>s</sup> ) and the viscous moment ( *M*<sup>d</sup> ) as viscoelastic moment of the knee joint. Mathematical model has been used because of the availability of this model and accessibility to estimate anthropometric inertia parameters. This could be an easy method to estimate these parameters without go through complicated clinical experiment since these parameters vary for each subject. The system such as viscoelasticity could be difficult to model due to complexity and nonlinerity. Therefore, fuzzy model is used to eliminate the development of complex mathematical model and helps to simplify the modelling task. These passive properties model then need to be integrated with active properties of the knee joint model to have a complete model of the knee joint. Finally, these models can be utilized as platform for the simulation purpose of the control system development.
