Developmental Changes of Muscle Synergies in an Infant's Walking

Primitive stepping is one of the primitive reflexes in newborns in response to external stimuli. It is known that primitive stepping disappears about two months after birth, but its role and the relationship with voluntary gait acquired later are still unknown. In this study, we extracted muscle synergies, spatiotemporal coordination patterns of muscle activities, from EMG measured in one infant during growth (4-18 weeks of age) using non-negative matrix factorization (NMF). We found that a synergy changed before and after the disappearance of the primitive stepping, and others maintained the recruited muscles but changed the onset timing of the activations.