Dopaminergic medication alters muscle synergy during sit-to-stand motion in Parkinson’s disease

Background: Parkinson's disease (PD) is a progressive neurodegenerative disorder that impairs motor function, thereby influencing daily activities, including sit-to-stand (STS) motion. Dopaminergic medication improves motor symptoms; however, its effects on neuromuscular control during STS motion remain unclear. This study investigated the effects of dopaminergic medication on muscle synergy and kinematic performance during STS motion in patients with PD. Methods: Fourteen patients with PD performed STS motion in the OFF and ON medication states. Surface EMG data from eight trunk and lower limb muscles and kinematic data of the center of mass (COM) trajectory were recorded. Muscle synergies were extracted using non-negative matrix factorization to assess temporal features and activation patterns. Kinematic features, including STS duration, time to seat-off, and COM displacement angle (initial to seat-off), were analyzed. Results: Dopaminergic medication significantly improved muscle synergy, achieving earlier initiation of the seat-off synergy and improved coordination between the propulsive and postural stabilization synergies. Neuromuscular improvement showed associations with changes in functional performance. Kinematic analysis revealed that the ON state was marked by shorter movement duration, reduced seat-off time, and a downward COM trajectory. These findings indicated that dopaminergic medication improves muscle synergy activation timing to enhance movement efficiency. Conclusion: These findings suggest that dopaminergic medication enhances the temporal precision of neuromuscular coordination and resolves the dysfunctional compensatory strategies during STS motion. These results provide novel insights into how dopamine modulates motor control in PD, with implications for clinical assessment and rehabilitation.