Muscle and Motoneuron Synergies in Biomimetic and Non-Biomimetic 3-DoF Tasks

Coordinated control of hand and wrist movements requires precise and simultaneous activation of multiple muscles and joints. To facilitate this task, the central nervous system is thought to organize the neural control to muscles into low-dimensional modules, also known as synergies. While motor adaptation research has primarily focused on muscle-level synergies, recent studies suggest that motoneurons may form distinct synergistic groups, pointing to a more fundamental layer of neural organization. This neural perspective on movement control is particularly relevant for myoelectric prostheses control, where effective interpretation of motor intentions is crucial for intuitive and adaptive operation. This work investigates differences in encoded information at the muscle and motoneuron level during real-time myoelectric control by analyzing both muscle and motoneuron synergies using high-density surface EMG. Participants performed a 3-DoF goal-reaching task under biomimetic and non-biomimetic control mapping to assess neuromuscular adaptation. Our findings showed that motoneuron synergies occupy a higher-dimensional manifold and exhibit more spatially and temporally distinct activation patterns than muscle synergies, potentially enabling a wider range of movement outputs in myoelectric control applications. Moreover, both synergies maintained high spatial and temporal consistency between control maps. This suggests that existing motor strategies can be adapted to variations in control paradigms without forming entirely new synergy patterns, supporting the development of personalized, adaptive controllers.