Although the term “spasticity” refers to one of the most important concepts in physical medicine and rehabilitation, it is inconsistently defined, particularly in the Japanese literature.
Here, I have presented several definitions widely used throughout Japan and discussed differences among those proposed by Lance, Shimamura, and Tanaka. The use of the phrase “phasic and tonic stretch reflexes” and the meaning of “rigidity” were indicated to cause confusion regarding the definition.
Exaggerated tendon jerks without a reflex response during a continuous muscle stretch should not be included under spasticity. However, if tendon jerks are exaggerated, the increased stretch reflexes without muscle (or joint) movement should be termed “spasticity” rather than “rigidity.” Although some researchers use the term “rigidospasticity” to express severe spasticity, it differs from the original meaning.
According to the discussion, I agree with the definition proposed by Lance：“Spasticity is a motor disorder characterized by a velocity-dependent increase in tonic stretch reflexes (muscle tone) with exaggerated tendon jerks, resulting from hyperexcitability of the stretch reflex, as one component of the upper motor neuron syndrome.” However, it is a mysterious phenomenon that depends not only on velocity but also on muscle length. Further review of its definition is required.