Distinct locomotor precursors in newborn babies

Significance It is commonly thought that human locomotor development stems from a single precursor behavior, consisting of alternating flexor–extensor movements, such as kicking or stepping on ground. According to this view, kicking and stepping are identical movement patterns generated by the same neural mechanisms. Here we show that the neuromuscular modules of neonatal kicking and stepping are different, presumably related to different neural mechanisms. Kicking involves an adult-like number of temporal activation patterns, whose association with specific sets of muscles varies across movements. Ground-stepping involves a limited number of activation patterns, each associated with a stable muscle synergy. Since neonatal kicking and ground-stepping seem to anticipate subsequent developmental changes of locomotion in human babies, they might represent distinct locomotor antecedents.