| Body composition research is to find out constitution, the state of health and the degree of aging. Basing on this, people can exploit the effective methods to control weight. By doing this, people have a proper ratio between the body fat and the lean body mass.The incidence of obesity-related diseases are reduced. People are more healthy and have a high quality life. The most important period of life is puberty which is not only a transition period from children to adults, but a second peak of body growth. The level of constitution and health in this period directly influences the body situation in their adulthood and has a certain relationship with their aging, life expectancy.This research selects the students of Beijing No.71 middle school as its study object. They are 12 to 15 years old. The biological age for boys is 13 to 15, while for girls 12 to 14. The students are divided into four groups on the basis of Body Mass Index (BMI) . They are underweight group, normal group, overweight group and obese group. The research is to discuss the growth characteristics of body composition, the factors affecting the body composition to establish a body composition assessment formula suitable for the youth. By using the questionnaire and measurements (body shape method, skinfold thickness, bioelectrical impedance method and dual energy X-ray absorptiometry), this paper tests on every student and regard the result of dual energy X ray absorptiometry as the criterion.Results1 The variation of body composition of adolescent of Boys (biological age 13-15 years) and girls (biological age 12-14 years) is slightly different. Their body composition is proportional to BIM. The amount of body fat and the muscle and the variation of bone mineral content are positively related to BMI.2 With the increase ofBMI, the whole body fat percentage and body part fat percentage are on the rise. Body fat percentage between underweight group, overweight and obesity is significantly different (P <0.01). The rate of increase between boys and girl is not the same. However, accumulation of fat distribution in the boys and girls is the same.3 Effect factors influencing the rate of body fat between boys and girls is different. The variation of the rate of body fat of all students has relationship with living habits, dietary behaviour, nutrition intake, physical activity, daily physical exercise factors. It is shown by the equity of drinking water, snacks, fast food, meat. The longer boys sit and lie, the higher the rate of body fat becomes in them. Girls' rate of body fat are shown by the amount of drinking water, increasing of staple food, food gorging, more meat. Like boys, The longer girls sit and lie, the higher their rate of body fat becomes.4 The variation of the rate of body fat of all students has relationship with genetic factors:Fathers' body index have a great influence on boys. On the contrary, fathers' and mothers'body index have not a great influence on grils.5 The result of BIA is correlated well not only with the skinfold thickness studied by our forefathers but with the rate of body fat measured by DEXA. At the same time, BIA and DEXA measurement of body fat rate are well consistent with each other. But the rate of body fat tested by BIA and the previous skinfold thickness is different from the rate of body fat measured by DEXA to some degree.ConclusionIn this study, body mass index (BMI) as the basis for derivation of the formula. The formula used to find out the rate of the whole body fat is based on the skinfold thickness method. This formula is reliable and effective.The formula is as follow.Biological age 13-15 years old for boysBody fat rate =3.221+0.816×abdominal skinfold +0.936×department of triceps skinfold -0.691×scapular skinfoldBiological age 12-14 years old for girlsBody fat rate =-1.017+0.474×thigh skinfold +0.533×department of triceps skinfold +0.335×abdominal skinfold +0.254×calf skinfold...
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