The growth of children's moment of inertia.

As children grow, their moments of inertia increase. The magnitude and timing of these changes can affect the rotations of the body. The present study evaluated inter-individual differences in the transverse centroidal moment of inertia for 12 boys between 5 and 16 yr, using intra-individual data from three successive years. Segmental masses and moments of inertia were estimated using the elliptical zone model and the model then repositioned into two configurations: a layout position from a back handspring and a tuck position from a back somersault. In each case, the mass centroid and the moment of inertia about the transverse axis were calculated. With growth indexed by age, it was shown that the rate of change increases with age. For the children 10 yr and younger, the rate of change of moment of inertia was approximately 30% of the rate for the older children. Also, at each age level, there was a wide range of moments of inertia. In order to improve the prediction of moment of inertia, height and mass were tried as predictors with a noticeable improvement in correlation and linearity. The best predictor, however, was found to be the product of mass and height squared (M X H2) with correlations of 0.99 and 0.97. It is suggested that, because of the effects of growth on the moment of inertia. M X H2 could be used in conjunction with age in order to better appreciate the potential effects of change of moment of inertia.