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Clonus is repetitive, rhythmic contractions of a muscle when attempting to hold it in a stretched state. It is a strong, deep tendon reflex that occurs when the central nervous system fails to inhibit it. Clonus is not the same thing as myoclonus, which is irregular and uncontrollable jerks of a muscle or group of muscles.

Neurologists often test for clonus in the ankle, the wrist and the kneecap (patella). A movie that demonstrates ankle clonus rather well can be found here (it's rather large and takes a long time to load):

Clonus in initiated in the spinal cord and is usually a sign of damage to the nerve tracts above the place where it is initiated. It is a common sign of multiple sclerosis, spinal cord injury, spastic paraparesis and other diseases. It is considered normal in new-born babies. In MS, clonus is often associated with spasticity and hypotonia and will often occur unilaterally (on one side of the body only).

It is rare for a person to be aware of clonus before their neurologist.

Clonus links:
Clonus articles, support groups, and resources
Hypertonia, clonus and spasticity

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