Three different levels of analysis were conducted in a parallel fashion in order to understand spasticity. First we observed the behavior of the patient's limb muscles as the examination was conducted, both in normal volunteers and in patients with different degrees of spasticity. Second we analyzed the mathematical behavior of the curves depicted during the examination. Third we created a computer model that could mimic the behavior of the patients test. The data showed a progressive displacement of the final resting position from the normal vertical, as the spasticity severity progressed, to a more horizontal position near the starting position. Also, a diminished oscillating time as a consequence of transition from an under-damped to a critically damped and finally over-damped type of behavior, as spasticity was incremented. The computer model was constructed based on the second order differential equation that describes an oscillator. Subsequently, extra forces, identified by the EMG and clinical observation of the patients, were added to the model in order to reproduce the extra torques. Finally, two different components of the system are identified as needed, in order to adequately conduct the modeling. As a result, a biomechanical definition of spasticity is proposed.