Sexual dimorphism of positioning of the body centre of mass in Wrocław schoolchildren aged 9-14 years

Abstract

The changes of a position of body centre of mass (bcm) were analysed on 198 Wrocław school children (115 boys and 83 girls). The aim of this study was twofold: first — to establish the characteristics of ,,a growth curve” of bcm during the considered period of ontogeny; second — to check the differences caused by the sexual dimorphism as well as the relations between changes of bcm height and individual adolescence rate. The girls were measured during the subsequent six years, beginning from their 9-th year of age; the boys were one year older, and measured during the subsequent five years. Together with the height of bcm the other 10 morphological traits were taken into account. The biological age of children was established by the use of Tanner's 5-grade scale [Tanner 1963]. It made possible to divide the sample into three subgroups differing in degree of development of the secondary sexual features. The so called Perkal's natural indices [Perkal 1953] were used in the sectional analysis of a growth curve. In terms of these indices „the developmental promotion” of a bcm was established on the ground of the morphological traits under consideration. In the longitudinal approach the method of canonical decomposition [Szczotka 1981] was used. This method enables to decompose each individual growth curve into a weighted sum of, naturally ordered, component functions which are independent of an individual and characterize some aspects of growth of the trait under study. The two function approximation was used with sufficient accuracy (over 90% of the total variance explained). The first function was assumed to represent the general growth of a trait, while the second was interpreted as ,,irregularity” of growth (involving velocity, acceleration and other particular growth parameters). Such a two-dimensional pattern could be easily presented graphically (Figs. 9 and 10). The main obtained results are following: 1. The height of bcm is highly positively correlated with the traits describing linear dimension of a body, mainly in the vertical direction. Nevertheless for girls near the end of adolescence period (13 - 14 years of age) there appears a distinct influence of the hip breadth on the position of a bcm. 2. The extent of ,,a developmental promotion” of both the index and height of bcm, as estimated on grounds of somatical features with the help of Perkal's natural indices, remains similar: for both sexes with one-year preceding by girls. The distinct differentiation does occur at the end of an adolescence period and is connected with differences in the reciprocal transition of masses of different parts of body in both sexes. : 3. The maturation rate is clearly connected with a general size of a body. The early maturing individuals are in general greater during the investigated period of ontogeny and have, in connection with it, the higher position of bcm. The late maturing individuals are relatively smaller. 4. The girls present the higher level of „a regularity of growth” in comparison with boys during the investigated period of ontogeny. The more distinct influence of a factor of „irregularity” occurs at the end of adolescence period for girls.