Growth and Development

In general, in children maximal oxygen consumption is higher per unit of body weight and higher in boys than girls, although the difference is small until the pubertal growth. The growth spurt usually comes earlier in girls than boys, so maximal oxygen consumption in 12- to 13-year-old girls may match or surpass that of age-matched boys. However, in boys, puberty results in much larger increments in total muscle mass, blood volume, and lung and heart size than girls. Girls acquire more fat mass than do boys and boys frequently lose body fat during the pubertal growth spurt. Consequently, puberty results in a large increment in Vo2max whether expressed in absolute or relative terms in boys. In girls, the relative rise in Vo2max during the pubertal growth spurt is smaller, since the absolute increase in muscle mass is less and the relative rise in fat mass (FM) is

PHYSICAL ACTIVITY 923

greater than in boys. Regular endurance exercise can result in a significant increment in the Vo2max of boys and girls (Brown et al., 1972; Mahon and Vaccaro, 1989, 1994; Vaccaro and Clarke, 1978) as well as in adults (Gallo et al., 1989; Maciel et al., 1985; Tabata et al., 1996).

It is generally assumed that the pattern of substrate utilization in children during rest and exercise is similar to that in adults. However, the data on effect of exercises of graded intensities and duration on the balance of substrate utilization in children are scarce. Compared to adults, the capacity of glycogenolysis in non-fully differentiated skeletal muscle is less in children, and they are generally less capable of speed and power-related activities (Krahenbuhl and Williams, 1992).

Physical activity levels in children vary widely, as they are capable of large amounts of spontaneous, self-directed physical activity (Blaak et al., 1992). The effects of exercise on body composition in children are likely greater than in adults, because of the much greater levels of growth hormone in children (Borer, 1995). Because growth hormone has both anabolic (tissue-building) and lipolytic (fat-mobilizing) effects (Bengtsson et al., 1990), it is not surprising that physically active children are stronger and leaner than their obese counterparts (Owens et al., 1999).

Results from the 1999 Youth Risk Behavior Study (CDC, 2000) indicate that only 29 percent of high school students attend physical education classes daily, and participation declines to 20 percent by grade 12 (Table 12-10). Furthermore, not only is there a decline in the frequency of physical education participation by high school students, but there is also a steady decline in the vigor of participation, as estimated by length of time engaging in physical activity/exercise during class.

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