The response to muscle stretch and shortening in Parkinsonian rigidity. Brain 95, 795–812. doi: 10.1093/brain/95.4.795

IN parkinsonian rigidity, a reflex muscle contraction may be elicited during the act of stretching that muscle (dynamic component of the tonic stretch reflex) and also while stretch is maintained (static component). Careful observation will disclose that a brief contraction takes place in most muscles during shortening and that contraction may continue as long as the muscle remains in the shortened position. These shortening reactions, which may be classified as dynamic and static in the same manner as the stretch reflexes, were first described by Westphal (1880). Most electromyographic studies of parkinsonian rigidity have been concerned with the relative contributions of alpha and gamma motoneurons to the hyperactivity of stretch reflexes. Struppler and Fleischhauer (1955) found that the ankle-jerks of hemi-parkinsonian patients were of lower amplitude on the affected side, while the H reflexes were equal on each side, and concluded that fusimotor activity might be decreased in Parkinson's disease. A similar conclusion was drawn by Hassler (1955) from the observation that tendon reflexes were apparently not facilitated by the Jendrassik manoeuvre, but facilitation of tendon-jerks may be seen in parkinsonia