Muscle synergy changes with cutaneous stimulation during resting tremor and reaching task in Parkinson's disease

Resting tremor affects voluntary reaching movements in patients with Parkinson's disease (PD). Previously, we found that cutaneous afferents evoked by electrically simulating the superficial radial nerve could inhibit tremor activity [3]. Yet, the mechanism that cutaneous stimulation affects tremor as well as voluntary movements remains unclear. In this paper, we used the method of muscle synergy analysis to further investigate the muscle activation patterns of tremor and reaching movements. Three PD patients with tremor dominant symptom participated in this preliminary study. The patients performed targeted reaching movements with no visual feedback of their hands, while transcutaneous electrical stimulation was delivered to inhibit tremor activity. Kinematics and EMGs of six muscles were recorded during resting and reaching tasks. Muscle synergy analysis was performed with data during resting and task stages with and without cutaneous stimulation. Results showed that during resting state the time profiles of muscle synergy in the tremor cycle displayed two main alternating components that were distributed to antagonistic muscle pairs in upper arm, forearm and wrist. Cutaneous stimulation affected the vector patterns and time profiles of muscle synergies for tremor and reaching movements differently. Our results indicate that muscle synergies are sensitive to central descending drives of muscles, as well as evoked peripheral afferents.