GRAVITY CONSTRAINTS IN INFANT MOTOR DEVELOPMENT

The effect of gravity in determining if and when during infancy moveme nts eventuate and the rate at which they develop is unknown. In accord ance with intersegmental relationships (Hof, 1992), a muscle moment du ring infancy would have to develop more rapidly than the gravitational moment before movement could occur. In this investigation, the effect of growth through the influence of gravity on the joint moments in th e axial region when infants were in a prone or supine posture was exam ined. A mathematical model that considers the body to be composed of t ransverse elliptical cylinders, 1 cm deep and of known density, was us ed in estimating the mass of the 16 segments of the body. The gravitat ional moments about 3 joints within the axial region (C7-T1, T12-L1, a nd the hip) were determined by summation, using the radii from the joi nt transverse axis to the center of mass of the segments. Infants (N = 27) aged between 9 and 36 weeks at the beginning of the study were te sted monthly 6 times, and the effect of growth on the gravitational mo ments was represented by first-order polynomials. Age x Joint analysis of variance (ANOVA) of the mean slopes of the regressions for the gra vitational moments revealed significant main effects for age and joint . The means increased monotonically with the number of segments and de creased as infants aged. The mean slopes of the neck and trunk joints were significantly smaller than that of the the hip joint. With increa sing age, the gravitational slope was significantly smaller. The chang es in the gravitational moments during infancy are seen as likely cont rol parameters effecting phase shifts in motor patterns during develop ment.