Muscle responses to limb block during spontaneous movements in infants.

Young infants manifest prominent neuromuscular responses to changes in muscle length, along with a variety of spontaneous movements. The first months of life are an important period during which sensorimotor integration and muscle tone gradually mature. In adults, muscle responses may also be observed when coordinated limb movements are transiently blocked. Given that infants normally exhibit spontaneous limb activity, here we examined whether a transient upper or lower limb block evoked consistent muscle responses while the infants were allowed to perform spontaneous movements with the other limbs. We examined polymyographic recordings in 12 bilateral arm and leg muscles in full-term and preterm infants (1-7 months old). Overall, muscle activity, its spectral characteristics, and agonist-antagonist coactivation were comparable before and after the block in both full-term and preterm infants, suggesting that the transient limb immobilization effect is not evident or consistent, as opposed to previously reported prominent muscle responses to muscle lengthening or shortening. The lack of consistent muscular responses to limb block supports the idea that individual limb motion during spontaneous movements is relatively independent of the control of other limbs, and that sensory input during changes in muscle length is more effective in revealing sensorimotor connections than its non-appearance.