Body Fat Levels and Muscle Mass in Middle Childhood

Body Fat Levels.

The amount of body fat, or adipose tissue, can be measured in skinfolds, a pinch of skin and fat measured to the nearest millimeter (mm) by a skinfold caliper. Skinfold measurements are usually taken at several different sites on the body. There is a slight decrease in adipose tissue after age 4. At approximately age 7 for girls and age 8 for boys, there is a gradual increase in fat tissue, referred to as the adiposity rebound, which continues at a slightly higher rate throughout adolescence.

Another method used to measure body fat is the body mass index or BMI. BMI is a number that shows body weight adjusted for height. BMI is both gender and age specific and shows a pattern of development similar to skinfolds. BMI decreases during the preschool years and then increases into adulthood.

Before puberty, fat content is highly correlated with weight in both girls and boys. After puberty, muscle mass is more highly correlated with weight in boys. Girls consistently have greater skinfold totals than boys, and after age 12 this difference becomes more pronounced. By the end of the growth spurt in adolescence, fat accounts for 16% of total body weight in boys and 27% of total body weight in girls. As a result of the increase in adipose tissue after age 7, we find that physical activity and a balanced diet are important in maintaining body weight for both sexes, but particularly for girls. The number of children in middle childhood who are overweight or obese has increased in the United States over the past several decades (National Center for Health Statistics, 2004).

Muscle Mass.

We are born with most of the muscle fibers we are ever going to have. During the course of normal development, there is a large increase in muscle length and breadth with age. An increase in muscle size, called hypertrophy, results from both genetic and environmental factors. Muscle development appears to proceed along a normal maturational course consistent with bone growth and is a prerequisite to certain motor abilities. Systematic physical activity, however, enhances muscle composition. Physically active children have a higher proportion of muscle mass to body fat.

Boys and girls appear to be equal in tasks that require muscle movement until puberty. As boys enter puberty, the release of the hormone testosterone promotes the growth of muscle and subsequently widens the gender difference in muscle mass between males and females. Since muscle mass is harder to measure than body fat levels, measures of strength are often used to reflect muscle growth.

A popular myth is that weight training should not begin until puberty because testosterone production is responsible for the increase in muscle mass in boys. This myth includes the warning that not only will premature weight lifting fail to produce any muscle gain but also it may interfere with normal growth. This myth is only partially supported by research. Lifting maximum weights in middle childhood will not produce the significant muscle gain that is typically sought. In addition, using incorrect lifting techniques to lift maximum weights can cause serious and sometimes permanent injury to immature tendons, ligaments, and bones (Mazur, Ketman, & Risser, 1993). However, other research shows that resistance training (i.e., low weight at high repetitions) in prepubescent boys and girls can produce increases in muscle mass and strength (Falk & Eliakim, 2003; Guy & Michell, 2001).