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Skeleton The skeleton is the permanent supportive framework of the body.
It provides protection for vital organs and is the main mineral reservoir.
Bone tissue constitutes most of the skeleton, accounting for 14-17 percent
of body weight across the life span (Trotter and Peterson, 1970; Trotter and
Hixon, 1974). Skeletal strength, which dictates fracture risk, is determined
by both the material and structural properties of bone, both of which are
dependent on mineral accrual. The relative mineral content of bone does
not differ much among infants, children, adolescents, and adults, making up
63-65 percent of the dry, fat-free weight of the skeleton (Malina, 1996). As
a fraction of weight, bone mineral (the ash weight of bone) represents about
2 percent of body weight in infants and about 4-5 percent of body weight
in adults (Malina, 1996). Bone mineral content increases fairly linearly with
age, with no sex difference during childhood. Girls have, on average, a
slightly greater bone mineral content than boys in early adolescence, reflect-
ing their earlier adolescent growth spurt. Boys have their growth spurt later
than girls, and their bone mineral content continues to increase through
late adolescence, ending with greater skeletal dimensions and bone mineral
content (Mølgaard et al., 1997). The increase in total body bone mineral is
explained by both increases in skeletal length and width and a small increase
in bone mineral density (Malina et al., 2004).
Many studies have shown a positive effect of physical activity on inter-
mediate markers of bone health, such as bone mineral content and density.
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Educating the Student Body: Taking Physical Activity and Physical Education to School
Relationship to Growth, Development, and Health
113
Active children and adolescents have greater bone mineral content and
density than their less active peers, even after controlling for differences
in height and muscle mass (Wang et al., 2004; Hind and Burrows, 2007;
Tobias et al., 2007). Exercise interventions support the findings from obser-
vational studies showing beneficial effects on bone mineral content and
density in exercise participants versus controls (Petit et al., 2002; Specker
and Binkley, 2003), although the benefit is less than is suggested by cross-
sectional studies comparing active versus inactive individuals (Bloomfield
et al., 2004). The relationship between greater bone mineral density and
bone strength is unclear, as bone strength cannot be measured directly in
humans. Thus, whether the effects of physical activity on bone mineral den-
sity translate into similar benefits for fracture risk is uncertain (Karlsson,
2007). Animal studies have shown that loading causes small changes in
bone mineral content and bone mineral density that result in large increases
in bone strength, supporting the notion that physical activity probably
affects the skeleton in a way that results in important gains in bone strength
(Umemura et al., 1997). The relatively recent application of peripheral
quantitative computed tomography for estimating bone strength in youth
has also provided some results suggesting an increase in bone strength with
greater than usual physical activity (Sardinha et al., 2008; Farr et al., 2011).
The intensity of exercise appears to be a key determinant of the osteo-
genic response (Turner and Robling, 2003). Bone tissue, like other tissues,
accommodates to usual daily activities. Thus, activities such as walking
have a modest effect at best, since even relatively inactive individuals take
many steps (>1,000) per day. Activities generating greater muscle force
on bone, such as resistance exercise, and “impact” activities with greater
than ordinary ground reaction forces (e.g., hopping, skipping, jumping,
gymnastics) promote increased mineralization and modeling (Bloomfield et
al., 2004; Farr et al., 2011). Far fewer randomized controlled trials (RCTs)
examining this relationship have been conducted in children than in adults,
and there is little evidence on dose response to show how the type of exer-
cise interacts with frequency, intensity, and duration. Taken together, how-
ever, the available evidence supports beneficial effects of physical activity
in promoting bone development (Bailey et al., 1996; Modlesky and Lewis,
2002).
Physical activity may reduce osteoporosis-related fracture risk by
increasing bone mineral accrual during development; by enhancing bone
strength; and by reducing the risk of falls by improving muscle strength,
flexibility, coordination, and balance (Bloomfield et al., 2004). Early
puberty is a key developmental period. Approximately 26 percent of the
mineral content in the adult skeleton is accrued during the 2 years around
the time of peak height velocity (Bailey et al., 2000). This amount of min-
eral accrual represents approximately the same amount of bone mineral
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Educating the Student Body: Taking Physical Activity and Physical Education to School
114
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