Increased shortening reaction in Parkinson's disease reflects a difficulty in modulating long loop reflexes.

Short and medium latency electromyographic (EMG) responses to stretch of the triceps surae muscle and long latency EMG responses (LL) in the anterior tibial muscle (TA) were evoked by toe-up tilt of a movable platform while standing or sitting, in normal subjects and patients with Parkinson's disease. With the stimulus parameters used (amplitude 4 degrees, velocity 50 degrees/s), LL in TA were absent in normals while sitting, but were present in 11 of the 12 patients with Parkinson's disease. In patients, LL latencies were identical in both positions. The results indicate that patients with Parkinson's disease have difficulty modulating long latency responses according to functional demands. Long latency responses in TA in the sitting position may correspond to the shortening reaction observed by others when much stronger stimuli were used.