Functional symmetrization of neuromotor modules during locomotor development in human infants

During human locomotor development, neonatal neuromotor control modules known as muscle synergies are continuously modified into their mature forms to enable independent walking. How the early muscle synergies, developing neuromusculoskeletal systems, and sensorimotor plasticity interact to regulate this process remains unknown. To address this, we investigated concurrent changes in muscle synergies, kinematic synergies, and lower-limb biomechanical properties across 4 stages of supported and independent walking through longitudinal bilateral multi-muscle recording, kinematic tracking, and personalized neuro-musculoskeletal modelling in 11 human infants, while incorporating additional data from adults and elders for a whole-lifespan analysis. Our results argue that the initially bilaterally asymmetrical muscle synergies and limb biomechanical properties co-evolve to ultimately result in symmetrical kinematic synergies that may stabilize gait. Functional symmetrization of neuromotor modules may be a reflection of the co-development of muscle synergies, their associated kinematic functions, and limb biomechanical properties for achieving gait stability and control efficiency throughout the lifespan.