From motor primitives to context-dependent coordination: a system neuroscience approach integrating cognitive, affective, and experiential factors into muscle synergy analysis.

Muscle synergies are traditionally viewed as stable, low-dimensional neuromuscular modules reflecting neural constraints. However, growing evidence suggests this mechanistic interpretation is incomplete. We propose a conceptual extension of the framework, arguing that coordination patterns reflected into multichannel EMG are emergent properties of an integrated cognitive-motor system rather than purely motor primitives. We identify three classes of non-motor factors that may shape motor output: (i) Task internalization and learning dynamics: synergy structure evolves with practice and the formation of internal models; (ii) affective and psychological states: emotional conditions (e.g., stress, anxiety) modulate muscle co-activation and stability; (iii) prior experience and sensorimotor memory: embodied history biases action selection and coordination. Consistent with predictive processing and embodied cognition, we argue these factors are constitutive dimensions of motor control rather than mere noise. This perspective implies that inter-subject variability in synergy structure reflects systematic differences in cognitive, affective, and experiential states. To enhance interpretability, we propose integrating minimal assessments into experimental designs: (i) task strategy evaluation, (ii) affective state characterization via physiological proxies or scales, (iii) documentation of prior motor experience, and (iv) analysis of learning trajectories. This integrative view complements existing models, providing a richer theoretical foundation to interpret variability, adaptability, and individual differences in human motor control.