The quantitative and objective evaluation of spasticity is desirable in rehabilitation and orthopedics where subjective evaluations are mostly used. In the present study, data from a simple pendulum test are used for the evaluation of the spasticity with the help of biomechanical modeling. The spasticity of a knee joint is modeled as nonlinear feedback of muscle lengthening velocity and the muscle length. Through the optimization of the modeling error, the feedback parameters are determined. The threshold of muscle lengthening velocity in the reflex system is suggested as a severity index of the spasticity.