Analysis of general movements of infants towards understanding of developmental principle for motor control

We focus on the spontaneous movements of young infants who have not yet acquired voluntary movements. Dynamical complexity and variability are characteristic points of the movements in the spine position, which are called general movements (GMs). We studied the GMs of 8 normal and 2 abnormal infants from 1 to 6 months old of age. Kinematic analysis of the trajectories of hands and feet using a nonlinear prediction method revealed that a u-shape change occurs around 2 months of age. The trajectories were very complex with no clear coordination among the limbs at the age of 1 and 3 months, while rhythmic movements were conspicuous at the age of 2 months. GMs of 2 abnormal infants with brain impairments showed a simply rhythmic pattern and a random pattern, respectively. The u-shape change in the GM of normal development and the loss of complexity of movements of brain damaged infants support the hypothesis that the neuromuscular system of newborns has nonlinear dynamics and that freezing and freeing degrees of freedom play an important role in motor development.