Longitudinal tracking of muscle synergies of health infants during the critical months of learning to walk

![CDATA[Rationale: Infants learn to walk from 6 months of age with adults’ support. However, the course of maturation of muscle synergies over the critical months of “learning to walk” has not been thoroughly characterized. We sought to characterize how muscle synergies develop to support bipedal walking by comparing muscle synergies at different time points, and against those in healthy adults. Methods: Surface electromyography activities of 14 lower-limb muscles were measured in two groups of subjects in overground walking: 1) 16 infants were longitudinally recorded at three time points: supported, assistive, and independent walking; 2) 16 adults. Muscle synergies were extracted by the non-negative matrix factorization algorithm. The similarity of muscle synergies between groups were evaluated by the principal angles. Repeated measures ANOVA was applied to evaluate their similarities. Results: Comparison of the average cosine of principal angles of all synergy vectors showed that the muscle synergies of the infants across stages were similar [F(2,510) = 2.32, p = 0.0994]. Three muscle synergies were identified as particularly consistent across all stages. There were two adult-specific muscle synergies that were absent in infants. Conclusions: Three muscle synergies in adults were also found in infants across all three stages, suggesting that they might be genetically hardwired for walking. The synergies identified in the supported stage of walking development were suppressed in the subsequent assisted and independent stages, followed by the emergence of the two adult-specific synergies. These adaptations might be the result of changing body compositions and the biomechanical requirements of walking.]]