Body segment growth during infancy.

In order to better understand relationships between physical growth and motor development during infancy, changes in segment inertias were examined. A rationale based on dynamic systems theory and intersegmental moments was used to select the variables: the segment mass and principal moments of nine segments. The hypothesis was that there are differences in growth velocity between segments and these are consistent with the principles of cephalocaudal development and distal to proximal growth. A mathematical model of the segments was used to examine changes of the inertia parameters. Twenty-seven normal infants carried to full term and aged between 2 and 9 months at the beginning of the study were photographed once a month for 6 months. Outlines of segments from front and side 35 mm images were digitized and mass and principal moments of inertia estimates obtained. Polynomial regression was used to give the development curves regression coefficients for the 155 data points. Also, linear regressions were fitted to the six growth data points of each subject. Most of the relationships between chronological age and segment mass and principal moments were significant and linear. The differences in growth velocity between segments were significant and partially consistent with the growth principles. However, there were considerable differences, in particular for the lower trunk and the shank, where it appears that growth may be irregular until after the age of 15 months.