Muscle relaxation in Parkinson's disease: a reaction time study

We tested the hypothesis that the relaxation reaction time in Parkinson's disease (PD) is delayed, as a sign of disorder in the control of voluntary motoneuron derecruitment. We compared, in the triceps brachii muscle, the reaction times (RTs) of the onset (O-RT) of electromyographic (EMG) activity during initiation of a contraction with the RTs of the termination of EMG tonic activity during full relaxation (R-RTs). Fourteen patients with idiopathic PD and 10 normal controls were examined. Mean R-RTs for all controls were 30 ms shorter than mean O-RTs. Mean R-RTs for all patients were approximately 70 ms longer than mean O-RTs. In two untreated patients levodopa therapy improved both O-RT and R-RT, but the difference between the two was unchanged. There was no correlation between EMG level and R-RT or between peak force and O-RT in either controls or patients. O-RT and R-RT were correlated with the bradykinesia score. In some patients, bursts of late activity were recorded after the R-RT; the duration of this activity was correlated with the duration and staging of the disease and with bradykinesia and rigidity scores. The reversed latency of onset and termination of muscle contraction in PD suggests an abnormality in the inhibitory spinal mechanisms, possibly stemming from a defect in the pathways descending to the spinal cord.