Simple SummaryAn essential stage in the maturation of the central nervous system and the self-organization of neural circuits occurs during the first year of life. Sensory feedback resulting from interactive and spontaneous movements is instrumental for developing sensorimotor circuits in early infancy, and shorter gestational periods may have an impact on muscle strength and functionality. We examined developmental changes in muscle responses to passive movements and muscle activities during spontaneous movements in preterm and full-term infants by analyzing polymyographic recordings in the lower and upper limb muscles. The major finding of the current study was that there were age-related changes in the manifestation of muscle responses in both preterm and full-term infants. The first six months were the major observational period for the variations in post-term development of muscle activity in preterm infants.AbstractManifestation of muscle reactions at an early developmental stage may reflect the processes underlying the generation of appropriate muscle tone, which is also an integral part of all movements. In preterm infants, some aspects of muscular development may occur differently than in infants born at term. Here we evaluated early manifestations of muscle tone by measuring muscle responses to passive stretching (StR) and shortening (ShR) in both upper and lower limbs in preterm infants (at the corrected age from 0 weeks to 12 months), and compared them to those reported in our previous study on full-term infants. In a subgroup of participants, we also assessed spontaneous muscle activity during episodes of relatively large limb movements. The results showed very frequent StR and ShR, and also responses in muscles not being primarily stretched/shortened, in both preterm and full-term infants. A reduction of sensorimotor responses to muscle lengthening and shortening with age suggests a reduction in excitability and/or the acquisition of functionally appropriate muscle tone during the first year of life. The alterations of responses during passive and active movements in preterm infants were primarily seen in the early months, perhaps reflecting temporal changes in the excitability of the sensorimotor networks.