Loss of reaction time specificity for movement direction in Parkinson's disease

Electromyographic (EMG) reaction times (RTs) of the right biceps brachii muscle and its integrated EMG (iEMG) from the onset until 50 msec after the start of activities for elbow flexion and forearm supination in the condition with or without warning signal were examined in seven patients with Parkinson's disease and seven age- and sex-matched normal subjects. In the control group RT of forearm supination with warning tended to be faster than that of elbow flexion and the reverse occurred without warning, and iEMG of forearm supination were significantly small compared to those of elbow flexion in each condition. In the Parkinson group both RT and iEMG of elbow flexion were nearly the same as that of forearm supination regardless the presence or absence of warning signal, indicating the loss of RT and iEMG specificity for the two direction of movement in Parkinson's disease.