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Sitting is one of the first developmental milestones that an infant achieves. Thus measurements of sitting posture present an opportunity to assess sensorimotor development at a young age, in order to identify infants who might benefit from therapeutic intervention, and to monitor the efficacy of the intervention. Sitting postural sway data was collected using a force plate from infants with typical development, and from infants with delayed development, where the delay in development was due to cerebral palsy in most of the infants in the study. The center of pressure time series from the infant sitting was subjected to a number of different analyses, both traditional linear analyses, and a number of nonlinear analyses based on information theory, nonlinear dynamics, and artificial intelligence. The traditional linear measures of postural sway did not detect a difference between the two groups, but several of the nonlinear measures did detect differences. Postural sway of infants with delayed development was found to have more repeated patterns in their postural sway, and to control posture on a slower time scale than infants with typical development. Additionally, spectral analysis was performed, and high frequency (20 -30 Hz) features were observed in the postural sway of infants with typical development that were not apparent in the postural sway of infants with delayed development, and these high frequency features were particularly prominent in the posterior sway in the anterior-posterior axis in early sitting. The origins of the features are not certain, but the fastest control is from stretch reflexes, and stretch reflexes may be contributing to the postural sway control in infant sitting. Dynamic systems theory, as applied in developmental psychology, suggests that infants need to explore a wide range of postural sway control muscle synergies, in order that the upright sitting behavior emerge. Infants with cerebral palsy often have muscle spasticity associated with altered stretch reflexes, and this may limit the exploration of a wide a range of postural control strategies, as compared to infants with typical development.