La relación músculo-grasa en las extremidades durante la adolescencia

ln order to determine body composition during adolescence, mid-arm and maximum calf circumferences, as well as triceps and calf skinfolds were measured in a mixed semilongitudinal study of a group of children belonging to the social middle class which lives in three southern suburbs of Mexico City. These measurements were taken during the period which extended from two weeks before to two weeks after their birthday and half a year later, at the ages from 10 or 11 to 15.5 years ingirls and boys respectively. No statistically significant differences were found in the circumferences of both sexes, but boys and girls diverge in skinfold thickness, which is bigger in girls and decreases considerably in boys as they grow older. For imaginary cilindrical cuts in arm and calf, the muscular circumference, the total muscular and fat areas were calculated.The results showed that the similar total area has in boys a bigger bone-muscle area than in girls, but that both sexes have in the latter positive increments which are at all ages higher in boys. The fat area, on the contrary, increases with age rapidly in girls and deceases in boys, who have from 12.5 years on close to cero o negative increments The age of menarache for the girls of this study is 12.3 years (Faulhaber, 1984). Possibly related to it, exists an ¡ncrease in their relative muscle area at 12.5 years, combined with a loss in their relative fat area in arm and calf. The respective increments during the following year. from 12.5 to 13.5 years, show a big increase in the percentage of fat tissue, combined with negative increments in the muscle area, thus establishing again the general femenine proportions. As for boys, between the age of 13.5 to 14.5 years, high increments in the proportion of muscle is found, combined with a loss of fat, which does not reflect itself as clearly in the calf, might possibly be related to sexual maturation. The differences which exist between the results of the present study and those of other studies made in Mexico City are interpreted as due to distinctions in the social extraction of the samples taken