There is still some misuse of the terminology spasticity and rigidity. Let’s clear things out a little. According to Dorland’s medical dictionary 2006 ed., spasticity means a state of being spasm which in turn mean a sudden, violent, involuntary muscular contraction; of hypertonicity. Rigidity meaning stiffness or inflexibility. These definitions has no true differentiating power.
According to dictionary of neurological sign 3rd ed., spasticity is an increased resistance to the passive movement of a joint due to abnormally high muscle tone (hypertonus) which varies with the amplitude and speed of displacement of a joint. While rigidity is an increased resistance to the passive movement of a joint which is constant throughout the range of joint displacement and not related to the speed of joint movement; resistance is present in both agonist and antagonist muscles. It is also stated in this book that stiffness is different from rigidity, in contrast of Dorland’s. Stiffness by this book is a principal symptom due to continuous motor unit activity within muscles itself.
This definition is consistent of what is provided by Medscape. In short spasticity is usually uni- directional, and is velocity and amplitude dependent. Rigidity is bi-directional, and is non velocity or amplitude dependent. The important of such differentiation is that spasticity is typically caused by damaged to the corticospinal tract, while rigidity is usually extra-pyramidal in origin.
A simple way to assess spasticity is by fast flexion or extension of selected joint, typically the elbow or knee, to elicit a sudden increase in tone. This demonstrate the velocity dependent nature of spasticity. Another phenomenon known as clasp-knife phenomenon, describe a sudden release of tone after an initial hypertonia of selected joint movement when the examiner passively move the joint to the extreme, this demonstrate the amplitude nature.
To test for rigidity, passively move the joint in both direction, a relatively uniform rigidity in both agonist and antagonist muscle group is known as lead-pipe rigidity; if there is tremor superimposed with background increase of tone – cogwheel rigidity. These rigidity is commonly seen in Parkinson’s disease.
1. Dorland’s Medical Dictionary 2006
2. Dictionary of Neurological sign 3rd ed.
4. Parkinson’s Disease and Movement disorder 5th ed.