**3.2. Optimised Fuzzy Model as Viscoelasticity**

The vicoelasticity is represented by a fuzzy model. The GA optimization is used to optimise 48 parameters including 30 associated with the membership function, 15 weights associated with the minimized fuzzy rules and 3 scaling factors of fuzzy model. The scaling factor for the normalization and denormalization of two inputs and output were 0.0091247, 0.0053982 and 35 respectively. The fuzzy model takes into account the nonlinear component of passive viscoelasticity. A three-dimensional plot that represents the mapping from knee angle and knee angular velocity to viscoelastic moment is shown in Figure 9. This surface plot shows the normalised viscoelasticity (unit less) changes as a function of the normalised knee angle (unit less) and normalised velocity (unit less). The presence of the non-linearities in the viscoelasticity can be noted on this uneven surface shape from both angles. The valley for knee angle between 0.5 and 1 gives high viscoelasticity could be due to the high stiffness near extended knee joint.
