In order to understand the human motion control strategies and to restore these functions, or to artificially generate limbs motion it is necessary to have an accurate understanding of the limb dynamics. The inertial parameters can be identify easily, however the joint dynamics is still difficult to model due to the time change with muscle contraction level, fatigue and non-linear dynamics. Using Functional Electrical Stimulation (FES) we propose to identify the joint active dynamics with the pendulum test and to establish a relationship between the level of muscle contraction induced by the stimulation and the visco-elasticity. We measure the data of 2 healthy subjects and propose a model for the knee joint visco-elasticity changes.