Growth of segment principal moments of inertia between four and twenty years.

Although there have been numerous studies of the moments of inertia of the body segments using a variety of the techniques, few have reported the principal moments and none has considered the changes in the principal moments during childhood and adolescence. The purpose of this study was to use a mathematical model to determine the intraindividual changes and interindividual differences in the segment principal moments during growth. The body was modelled as 15 segments and composed of transverse elliptical zones of known density. Moments and products of inertia about the segment mass centroid were calculated and the principal moments and axes determined from the ellipsoid of inertia. A mixed longitudinal study of 12 boys over 9 yr for a total of 88 annual recordings covering the age range 4 to 20 yr was conducted. Polynomial regressions fitted to the intraindividual changes showed a similarity in the curves to 10 yr followed by a wide divergence of growth patterns. The changes in the principal moments across age follow the principles of cephalocaudad and distal to proximal development for all three axes. These changes are more accentuated than the changes for segment length, volume, or mass. The results were consistent with principal moments reported for cadavers and young male adult gamma radiation scans as well as estimates based on simplified models of the segments. The magnitude of the changes in principal moments with age makes it essential that appropriate parameters be used in the analysis or simulation of the movements of children and adolescents.