The objective of this study is to propose a new indices to evaluate spasticity in the ankle joint of hemiplegic patients. Each subject sat on a bed with one foot supported with a jig, which was used to measure the response of the ankle joint angle. The subject was instructed to relax and not to generate voluntary force. A step-like load was applied to dorsiflex the ankle joint. The ankle joint angle and electromyograms of the soleus and tibialis anterior muscle were recorded. First, the step-like response was approximated with a mathematical model, which is based on musculoskeletal and physiological characteristics using the least squares method in order to estimate net inertia and the elastic and viscous coefficients of the foot. The torque generated by the elastic component was then estimated. The normalized elastic torque was approximated with a dumped sinusoid using the least squares method. The time constant and frequency of the normalized elastic torque were calculated. We propose two indices estimated from the relationships between the time constant and the frequency. One of the indices reflected the step-like load dependency. The other reflected the difference from healthy subjects. Both indices increased as the Ashworth scale increased.