The stretch reflexes and shortening reactions evoked in six normal subjects during reinforcement were compared with those recorded from patients with Parkinson's disease. Although the responses to stretch and shortening of triceps were similar in both groups, they differed in the biceps muscle. In Parkinson's disease increasing muscle stretch of biceps was associated with increasing reflex activity, whereas the reverse relationship occurred in the activated normal subjects. The biceps EMG during sinusoidal stretching comprised two peaks in the activated normal group, a phenomenon not seen in Parkinson's disease. It is postulated that this response in the activated normal group is due to activation or reinforcement engaging long-loop reflexes. It is concluded that the rigidity of Parkinson's disease is not simply an exaggeration of the stretch reflexes found in normal man but differs in that the effects of flexor reflex afferent nerve fibres are submerged by group Ia afferent activity, and that some long-loop reflex pathways are no longer operative in Parkinson's disease.
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