Development of postural adjustments in sitting position during the first half year of life.

Little is known about the development of postural adjustments during early ontogeny. We examined postural adjustments due to sudden perturbations during sitting in 40 healthy term infants (28 males, 12 females) assessed in groups of eight at 1, 2, 3, 4, and 5 months of age. Surface electromyograms of neck, trunk, and leg muscles were recorded while the infants were exposed to a random series of horizontal forward and backward displacements of the surface of support. Video recordings of spontaneous motor behaviour were analyzed. For part of the analyses, previously collected data on 26 infants aged 6 to 10 months were included. In general, postural adjustments at all ages were direction specific and showed large variation. Within the variation developmental changes could be observed, revealing a transient decrease in postural activity at 3 months of age. After this transition, direction-specific postural activity was correlated with spontaneous motor behaviour. This was true, in particular, for dorsal postural activity. The clinical relevance of these findings is discussed.