Shortening reaction in patients with cerebellar ataxia.

During passive extension of the elbow, the triceps muscle normally shows a burst of electromyographic activity. This shortening reaction (SR) is known to be exaggerated in extrapyramidal disease states, but the effects of cerebellar disease are unknown. The SR was measured in both arms of a patient with hemiataxia, the unaffected arm serving as a matched control. In the ataxic arm, the SR was significantly larger and did not show the normal increase during "reinforcement." In a patient with bilateral ataxia, the SR was grossly exaggerated as compared with the SRs in a group of normal subjects. The present findings indicate an unexpected point of similarity between cerebellar and extrapyramidal disease states, which is discussed in terms of the anatomical connections of the cerebellum and basal ganglia. The techniques used in this work provide a quantitative approach to disorders of movement and muscle tone.