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Optimization of modularity during development to simplify walking control across strides

Walking in adults seems to rely on a small number of modules allowing to reduce the number of degrees of freedom effectively regulated by the central nervous system (CNS). However, the extent to which modularity evolves during development remains unknown, particularly regarding the ability to generate several strides in an optimized manner. Here we compared the modular organization of toddlers and adults during several strides of walking. We recorded the electromyographic activity of 10 bilateral (lower limbs) muscles in adults (n=12) and toddlers (n=12) during 8 gait cycles, and used non-negative matrix factorization to model the underlying modular command. While the muscular activity of all strides could be factorized into a consistent low-dimensional modular organization in adults, significantly more computational modules were needed in toddlers to account for their greater stride-by-stride variability. Activations of these modules varied more across strides and was less parsimonious in toddlers than in adults, even when balances constrained were diminished. These findings suggest that the modular control of locomotion of adults evolves as the organism develops and practices. They also suggest that new walker can flexibly activate a higher number of modules and benefit from a higher space of possible action, which could serve motor exploration.