At present during clinical examination, physical therapists mainly use the Modified Ashworth Scale (MAS) to evaluate the severity of spasticity. However, MAS is a qualitative evaluation and the results depend on the therapist. The purpose of this study is to propose a modeling method for quantitatively evaluating the severity of spasticity. The proposed method uses the Independent Component Analysis technique and data from the pendulum test of the patients leg. The method estimates a model of spasticity system and its input signals.