It has been argued that these constraints assume a modular architecture that facilitates the coordination and execution of diverse motor behaviors 6. Such flexibility demanded of the CNS is seemingly at odds with the fact that any motor command produced must be permissible by the structures of the underlying neuronal networks – or the neural constraints on movement generation 3– 5. The above considerations suggest that the motor-output generating neuronal networks must be versatile enough to satisfy the everchanging biomechanical needs of motor control.

The CNS must be equipped with mechanisms that can efficiently adjust motor commands, over multiple time scales, to accommodate both the changing neuro-musculoskeletal properties during development 2 and the changing task demands during learning, while ensuring that the high-dimensional commands are spatiotemporally coordinated and robustly generated. The learning of a new, difficult skill with novel kinematic, kinetic, or energetic requirements, for instance, would likely necessitate the acquisition of new muscle coordination patterns. Another obvious source of biomechanical demands comes from constraints imposed by the motor task itself. Thus, the muscle commands required for a preschooler to accomplish a movement, for instance, can be very different from those required for an adult because of differences in the biomechanical design of their limbs. One source of constraints comes from the current properties of the neuro-musculoskeletal system, including muscle strength, muscle stiffness, gains of neuromotor reflexes etc. For the movement to occur as intended, not only does the CNS need to specify a tremendous number of output variables, it must also take into account, during this specification, various biomechanical demands or constraints imposed on movement execution. To generate a movement, the central nervous system (CNS) must compute coordinated commands for thousands of motor units within hundreds of skeletal muscles. Fractionation and merging of muscle synergies may be a mechanism for modifying early motor modules (Nature) to accommodate the changing limb biomechanics and influences from sensorimotor training (Nurture).

Presences of specific synergy-merging patterns correlate with enhanced or reduced running efficiency. As adults train to run, specific synergies coalesce to become merged synergies. During development, synergies are fractionated into units with fewer muscles. We seek to understand how locomotor synergies change during development and training by studying the synergies for running in preschoolers and diverse adults from sedentary subjects to elite marathoners, totaling 63 subjects assessed over 100 sessions. Recent data have argued that muscle synergies are inborn or determined early in life, but development of the neuro-musculoskeletal system and acquisition of new skills may demand fine-tuning or reshaping of the early synergies. Complex motor commands for human locomotion are generated through the combination of motor modules representable as muscle synergies.

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