T Treitz | PubMed Health

PubMed1997

Gravity constraints in infant motor development.

The effect of gravity in determining if and when during infancy movements eventuate and the rate at which they develop is unknown. In accordance with intersegmental relationships (Hof, 1992), a muscle moment during 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 the axial region when infants were in a prone or supine posture was examined. A mathematical model that considers the body to be composed of transverse elliptical cylinders, 1 cm deep and of known density, was used in estimating the mass of the 16 segments of the body. The gravitational moments about 3 joints within the axial region (C7-T1, T12-L1, and the hip) were determined by summation, using the radii from the joint 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 tested monthly 6 times, and the effect of growth on the gravitational moments was represented by first-order polynomials. Age x Joint analysis of variance (ANOVA) of the mean slopes of the regressions for the gravitational moments revealed significant main effects for age and joint. The means increased monotonically with the number of segments and decreased as infants aged. The mean slopes of the neck and trunk joints were significantly smaller than that of the hip joint. With increasing age, the gravitational slope was significantly smaller. The changes in the gravitational moments during infancy are seen as likely control parameters effecting phase shifts in motor patterns during development.

Aging↗

PubMed1996

Changes in segment mass and mass distribution during pregnancy.

The purpose of the study was to evaluate the rates of change in the estimated masses and principal moments of inertia of the body segments during the second and third trimesters of pregnancy. Fifteen subjects, on average 15.1 weeks pregnant, were tested monthly until term. The body was modelled as 16 segments consisting of 2 cm stacked elliptical cylinders sectioned in the transverse plane. Linear regressions were fitted to the segment inertias and regression coefficients determined for the individual growth curves. The mean rate of increase for the lower trunk mass was 0.29 kg per week and 0.0069 kg m2 per week for the transverse axis principal moment. Differences between the means of the segments for rates of change of mass and the three principal moments were then tested. All four ANOVAs used to test the differences were significant. The post hoc analysis established that over the second and third trimesters the lower trunk inertias increased at a greater rate than all other segments and there were no significant differences between the remaining segments. The effect of such changes in the trunk segment masses and principal moments was illustrated by an analysis of intersegmental dynamics during a sit to stand of one pregnant subject whose lower trunk mass increased from 21.6 kg to 28.4 kg. A knowledge of the differences in an individual's segment inertias should lead to a better understanding of how movements are affected during pregnancy.

Abdomen↗