Analysis of Motor Module Transition from Tremor to Voluntary Reaching Movement in Patients with Parkinson's Disease

Voluntary movement could be initiated in spite of involuntary resting tremor in patients with Parkinson's disease (PD). It is unknown how the central nervous system (CNS) transits from the ongoing involuntary motor program of tremor to the execution of voluntary reaching movement. In this paper, muscle synergy analysis was used to unveil the motor module transitions from resting tremor to compound movement. Three tremor symptom dominant PD patients performed tasks of targeted forward reaching movements with background tremor. Kinematics and surface electromyography (EMG) of six muscles were recorded. Synergy vectors and time profiles were analyzed using EMG data during resting tremor and forward reaching. Results revealed that the representative vector of tremor remained unchanged during performance of voluntary movement. Other synergy vectors of tremor were modified to accommodate vectors of movement synergies. The merge of tremor module with movement module could be explained by a procedure of linear combination. The distribution of timing of movement initiation with respect to tremor cycle displayed a weak sign of clusters. The analysis suggested that the CNS combined two motor modules in order to initiate a voluntary movement with background tremor. More extensive analysis with a larger number of patients needs to be carried out to confirm these findings.