Levodopa-induced alterations in vertical center of mass and muscle synergy structure during Parkinsonian gait.

Parkinson's disease (PD) is a neurodegenerative disorder characterized by gait impairments that affect the regulation of whole-body movement, which can be quantified through metrics such as vertical center of mass (vCoM). In healthy gait, vCoM oscillations contribute to metabolic efficiency by facilitating the exchange between kinetic and potential energy; however, this mechanism is often disrupted in PD. Although levodopa improves motor impairments, its impact on vCoM and neuromuscular control remains poorly understood. This study investigated levodopa effects on vCoM and neuromuscular coordination in fifteen patients with PD (Hoehn & Yahr 2-3) during PD OFF (before levodopa) and PD ON (after levodopa) and compared to nine healthy older adults (HOA). Kinematic and electromyography data were collected during an overground walk to quantify vCoM and muscle synergies. Here we show that levodopa significantly increased maximum vCoM displacement (p = 0.002), with PD OFF showing significantly reduced displacement compared to HOA (p < 0.001) while PD ON did not differ significantly, indicating a shift in vCoM displacement towards healthy patterns. Temporal patterns of muscle synergies showed that PD OFF exhibited prolonged activation duration and slower slope-to-peak in Synergy 2 (push-off) compared to HOA while spatial patterns of Synergy 2 showed an increase in gastrocnemius muscle contribution in PD ON compared to PD OFF (p = 0.002). Together these findings demonstrate that levodopa might partially restore whole-body gait mechanics through a unique reorganization of muscle synergy structure, pointing to a possible functional link between neuromuscular coordination and gait dynamics in PD.