Progress and Prospects in Parkinson's Research/Symptoms/Clonus

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Clonus
Vibrato-like reaction to forced muscle stretching.

The following references are relevant:-

Hofmann (1962) Observations on peripheral servo mechanisms in Parkinsonian rigidity 

Burke et al (1972) Tonic vibration reflex in spasticity, Parkinson's disease, and normal subjects

Findley et al (1981) Tremor, the cogwheel phenomenon and clonus in Parkinson's disease