Characterization of the musculoskeletal system is essential for diagnosis providing the implications for therapy corresponding to causes of the diseases. This paper presents a characterization of an ankle neuromuscular system of patients with spasticity, to provide quantitative pathological level of the ankle spasticity with biomechanical and neurological disorders. Measurements from manual spasticity evaluation combined with a suggested neuromuscular model and parameter optimization process enabled a reliable characterization of the spastic ankle flexors. The model included two non-neural parameters representing the viscoelasticity of the muscle and four neural parameters showing the dynamics of muscle activation and corresponding force only using the measured joint angle and resistance torque. Torque contributions from non-neural parameters especially elastic properties of muscle was greater than 50% of the overall torque, common in both patients with spasticity and healthy controls. Among subgroups of the patients, subjects with short post diseases period less than 5 years, had higher torque contribution level from neural components more than 50% of the overall torque compared to the patients with longer post diseases period more than 10 years who had overall torque less than 30% of the total estimated torque. We concluded that proposed model based ankle flexor characterization served as a tools for diagnosing the patients with spasticity corresponding to their causes of diseases with both quantified neural and non-neural parameters.