This paper describes a detailed model (pendulum test model) to be able to utilize for simulations of knee joint motions obtained by pendulum test. First, we construct a simple pendulum test model based on existing organ models and clarify from the simulation results by the use of the model that there exist two difficulties impairing its accuracy. Next, in order to overcome them, we add two functions to some of the organ models in the simple pendulum test model. One is a function that the muscle viscosity in the muscle model changes nonlinearly according to dynamic state transition of actin/myosin. The other is a function that the threshold value of the firing in the α-motoneuron model slightly decreases in the early stage of the pendulum test by the amount proportional to the total of the first burst firing. Finally, we confirm from simulations of knee joint motions measured experimentally that the improved pendulum test model has an excellent performance of accuracy. The model will be very useful for estimation of spasticity.