PHYSICAL HEALTH

This section reviews what is known about the relationship between physical activity and (1) somatic growth, development, and function and (2) health- and performance-related fitness.

Somatic Growth, Development, and Function

Growth occurs through a complex, organized process characterized by predictable developmental stages and events. Although all individuals follow the same general course, growth and maturation rates vary widely among individuals. Just as it is unrealistic to expect all children at the same age to achieve the same academic level, it is unrealistic to expect children at the same age to have the same physical development, motor skills, and physical capacity. Regular physical activity does not alter the process of growth and development. Rather, developmental stage is a significant determinant of motor skills, physical capacity, and the adaptation to activity that is reasonable to expect (see Box 3-2).

BOX 3-2

Growth, Development, and Maturation. Growth is the normal process of increase in size as a result of accretion of tissues characteristic of the organism; growth is the dominant biological activity for most of the first two decades of life. Changes in (more...)

Developmental Stages

Postnatal growth is commonly divided into three or four age periods. Infancy spans the first year of life. Childhood extends from the end of infancy to the start of adolescence and is often divided into early childhood, which includes the preschool years, and middle childhood, which includes the elementary school years, into the 5th or 6th grade. Adolescence is more difficult to define because of variation in its onset and termination, although it is commonly defined as between 10 and 18 years of age (WHO, 1986). The rapid growth and development of infancy continue during early childhood, although at a decelerating rate, whereas middle childhood is a period of slower, steady growth and maturation. Differences between boys and girls are relatively small until adolescence, which is marked by accelerated growth and attainment of sexual maturity (Tanner, 1962).

Across developmental stages, neurological development and control of movement advance in cephalocaudal and proximodistal directions; that is, they advance “head to toe” (cephalocaudal) and “midline to periphery” (proximodistal), while predictable changes in body proportions also occur. For example, the head accounts for 25 percent of recumbent length in an infant and only 15 percent of adult height, while the legs account for 38 percent of recumbent length at birth and 50 percent of adult height. These changes in body proportions occur because body parts grow at different rates. From birth to adulthood, as the head doubles in size, the trunk triples in length, and arm and leg lengths quadruple.

Coincident with these changes in body proportions, and in part because of them, the capacity to perform various motor tasks develops in a predictable fashion. For example, running speed increases are consistent with the increase in leg length. Neurological development also determines skill progression. Young children, for example, when thrown a ball, catch it within the midline of the body and do not attempt to catch it outside the midline or to either side of the body. As proximodistal development proceeds, children are better able to perform tasks outside their midline, and by adolescence they are able to maneuver their bodies in a coordinated way to catch objects outside the midline with little effort.

Physically active and inactive children progress through identical stages. Providing opportunities for young children to be physically active is important not to affect the stages but to ensure adequate opportunity for skill development. Sound physical education curricula are based on an understanding of growth patterns and developmental stages and are critical to provide appropriate movement experiences that promote motor skill development (Clark, 2005). The mastery of fundamental motor skills is strongly related to physical activity in children and adolescents (Lubans et al., 2010) and in turn may contribute to physical, social, and cognitive development. Mastering fundamental motor skills also is critical to fostering physical activity because these skills serve as the foundation for more advanced and sport-specific movement (Clark and Metcalfe, 2002; Hands et al., 2009; Robinson and Goodway, 2009; Lubans et al., 2010). Physical activity programs, such as physical education, should be based on developmentally appropriate motor activities to foster self-efficacy and enjoyment and encourage ongoing participation in physical activity.

Biological Maturation

Maturation is the process of attaining the fully adult state. In growth studies, maturity is typically assessed as skeletal, somatic, or sexual. The same hormones regulate skeletal, somatic, and sexual maturation during adolescence, so it is reasonable to expect the effect of physical activity on these indicators of maturity to be similar. Skeletal maturity is typically assessed from radiographs of the bones in the hand and wrist; it is not influenced by habitual physical activity. Similarly, age at peak height velocity (the most rapid change in height), an indicator of somatic maturity, is not affected by physical activity, nor is the magnitude of peak height velocity, which is well within the usual range in both active and inactive youth. Discussions of the effects of physical activity on sexual maturation more often focus on females than males and, in particular, on age at menarche (first menses). While some data suggest an association between later menarche and habitual physical activity (Merzenich et al., 1993), most of these data come from retrospective studies of athletes (Clapp and Little, 1995). Whether regular sports training at young ages before menarche “delays” menarche (later average age of menarche) remains unclear. While menarche occurs later in females who participate in some sports, the available data do not support a causal relationship between habitual physical activity and later menarche.

Puberty is the developmental period that represents the beginning of sexual maturation. It is marked by the appearance of secondary sex characteristics and their underlying hormonal changes, with accompanying sex differences in linear growth and body mass and composition. The timing of puberty varies, beginning as early as age 8 in girls and age 9 in boys in the United States and as late as ages 13-15 (NRC/IOM, 1999). Recent research suggests that the onset of puberty is occurring earlier in girls today compared with the previous generation, and there is speculation that increased adiposity may be a cause (Bau et al., 2009; Rosenfield et al., 2009). Conversely, some data suggest that excess adiposity in boys contributes to delayed sexual maturation (Lee et al., 2010). Pubescence, the earliest period of adolescence, generally occurs about 2 years in advance of sexual maturity. Typically, individuals are in the secondary school years during this period, which is a time of decline in habitual physical activity, especially in girls. Physical activity trends are influenced by the development of secondary sex characteristics and other physical changes that occur during the adolescent growth spurt, as well as by societal and cultural factors. Research suggests that physical inactivity during adolescence carries over into adulthood (Malina, 2001a,b; CDC, 2006).

It is critical that adolescents be offered appropriate physical activity programs that take into account the physical and sociocultural changes they are experiencing so they will be inspired to engage in physical activity for a lifetime. As discussed below, adequate physical activity during puberty may be especially important for optimal bone development and prevention of excess adiposity, as puberty is a critical developmental period for both the skeleton and the adipose organ.

Adolescence is the transitional period between childhood and adulthood. The adolescent growth spurt, roughly 3 years of rapid growth, occurs early in this period. An accelerated increase in stature is a hallmark, with about 20 percent of adult stature being attained during this period. Along with the rapid increase in height, other changes in body proportions occur that have important implications for sports and other types of activities offered in physical education and physical activity programs. As boys and girls advance through puberty, for example, biacromial breadth (shoulder width) increases more in boys than in girls, while increases in bicristal breadth (hip width) are quite similar. Consequently, hip-shoulder width ratio, which is similar in boys and girls during childhood, decreases in adolescent boys while remaining relatively constant in girls (Malina et al., 2004). Ratios among leg length, trunk length, and stature also change during this period. Prior to adolescence, boys have longer trunks and shorter legs than girls (Haubenstricker and Sapp, 1980). In contrast, adolescent and adult females have shorter legs for the same height than males of equal stature. Body proportions, particularly skeletal dimensions, are unlikely to be influenced by physical activity; rather, body proportions influence performance success, fitness evaluation, and the types of activities in which a person may wish to engage. For example, there is evidence that leg length influences upright balance and speed (Haubenstricker and Sapp, 1980). Individuals who have shorter legs and broader pelvises are better at balancing tasks than those with longer legs and narrower pelvises, and longer legs are associated with faster running times (Dintiman et al., 1997). Also, longer arms and wider shoulders are advantageous in throwing tasks (Haubenstricker and Sapp, 1980), as well as in other activities in which the arms are used as levers. According to Haubenstricker and Sapp (1980), approximately 25 percent of engagement in movement-related activities can be attributed to body size and structure.

Motor Development

Motor development depends on the interaction of experience (e.g., practice, instruction, appropriate equipment) with an individual's physical, cognitive, and psychosocial status and proceeds in a predictable fashion across developmental periods. Clark and Metcalfe (2002) provide an eloquent metaphor—“the mountain of motor development”—to aid in understanding the global changes seen in movement across the life span. Early movements, critical for an infant's survival, are reflexive and dominated by biology, although environment contributes and helps shape reflexes. This initial reflexive period is followed quickly by the preadapted period, which begins when an infant's movement behaviors are no longer reflexive and ends when the infant begins to apply basic movement skills (e.g., crawling, rolling, standing, and walking) that generally are accomplished before 12 months of age. The period of fundamental motor patterns occurs approximately between the ages of 1 and 7 years, when children begin to acquire basic fundamental movement skills (e.g., running, hopping, skipping, jumping, leaping, sliding, galloping, throwing, catching, kicking, dribbling, and striking). Practice and instruction are key to learning these skills, and a great deal of time in elementary school physical education is devoted to exploration of movement. Around age 7, during the so-called context-specific period of motor development, children begin to refine basic motor skills and combine them into more specific movement patterns, ultimately reaching what has been called skillfulness. Compensation, the final period of motor development, occurs at varying points across the life span when, as a result of aging, disease, injury, or other changes, it becomes necessary to modify movement.

While all children need not be “expert” in all movement skills, those who do not acquire the fundamental motor skills will likely experience difficulty in transitioning their movement repertoire into specific contexts and engagement in physical activity (Fisher et al., 2005; Barnett et al., 2009; Cliff et al., 2009; Robinson et al., 2012). A full movement repertoire is needed to engage in physical activities within and outside of the school setting. Thus, beyond contributing to levels of physical activity, physical education programs should aim to teach basic fundamental motor skills and their application to games, sports, and other physical activities, especially during the elementary years (i.e., the fundamental motor patterns and context-specific periods). At the same time, it is important to be mindful of the wide interindividual variation in the rate at which children develop motor skills, which is determined by their biological makeup, their rate of physical maturation, the extent and quality of their movement experiences, and their family and community environment.

An increasing amount of evidence suggests that people who feel competent in performing physical skills remain more active throughout their lives (Lubans et al., 2010). Conversely, those who are less skilled may be hesitant to display what they perceive as a shortcoming and so may opt out of activities requiring higher levels of motor competence (Stodden et al., 2008). Children who are less physically skillful tend to be less active than their skillful counterparts (Wrotniak et al., 2006; Williams et al., 2008; Robinson et al., 2012) and thus have a greater risk of overweight and obesity (Graf et al., 2004). Fundamental skills are the building blocks of more complex actions that are completed in sports, physical activities, and exercise settings. For example, throwing is a fundamental skill that is incorporated into the context-specific throw used in activities such as handball, softball, and water polo. Fundamental skills are of primary interest to both physical education teachers and coaches, and physical education classes should be designed to challenge learners to develop their motor skills.

In 1998 the Centers for Disease Control and Prevention's (CDC's) Division of Nutrition and Physical Activity organized a workshop to determine future directions for research on physical activity. The workshop convened 21 experts from a wide range of academic disciplines. One recommendation resulting from the proceedings was for future research to describe the temporal relationship between motor development and physical activity (Fulton et al., 2001), signifying the importance of better understanding of the nature of the relationship between motor competence and physical activity. The assumption of this relationship is implied in multiple models of motor development (Seefeldt, 1980; Clark and Metcalfe, 2002; Stodden et al., 2008), which emphasize the importance of motor competence as a prerequisite for engagement in physical activity throughout the life span.

Two models that are commonly used to examine this relationship are Seefeldt's (1980) hierarchical order of motor skills development and the dynamic association model of Stodden and colleagues (2008). Seefeldt proposed a hierarchical order of motor skills development that includes four levels: reflexes, fundamental motor skills, transitional motor skills (i.e., fundamental motor skills that are performed in various combinations and with variations and that are required to participate in entry-level organized sports, such as throwing for distance, throwing for accuracy, and/or catching a ball while in motion), and specific sports skills and dances. With improved transitional motor skills, children are able to master complex motor skills (e.g., those required for playing more complex sports such as football or basketball). At the end of this developmental period, children's vision is fully mature. The progression through each level occurs through developmental stages as a combined result of growth, maturation, and experience. Seefeldt hypothesized the existence of a “proficiency barrier” between the fundamental and transitional levels of motor skills development. If children are able to achieve a level of competence above the proficiency barrier, they are more likely to continue to engage in physical activity throughout the life span that requires the use of fundamental motor skills. Conversely, less skilled children who do not exceed the proficiency barrier will be less likely to continue to engage in physical activity. Thus, it is assumed that “a confident and competent mover will be an active mover” (Clark, 2005, p. 44). For example, to engage successfully in a game of handball, baseball, cricket, or basketball at any age, it is important to reach a minimum level of competence in running, throwing, catching, and striking. The assumption of the existence of a relationship between motor competence and physical activity is at the “heart of our physical education programs” (Clark, 2005, p. 44). A thorough understanding of how this relationship changes across developmental stages is crucial for curriculum development and delivery and teaching practices.

Lubans and colleagues (2010) recently examined the relationship between motor competence and health outcomes. They reviewed 21 studies identifying relationships between fundamental motor skills and self-worth, perceived physical competence, muscular and cardiorespiratory fitness, weight status, flexibility, physical activity, and sedentary behavior. Overall, the studies found a positive association between fundamental motor skills and physical activity in children and adolescents, as well as a positive relationship between fundamental motor skills and cardiorespiratory fitness. Other research findings support the hypothesis that the most physically active preschool-age (Fisher et al., 2005; Williams et al., 2008; Robinson et al., 2012), elementary school–age (Bouffard et al., 1996; Graf et al., 2004; Wrotniak et al., 2006; Hume et al., 2008; Lopes et al., 2011), and adolescent (Okely et al., 2001) youth are also the most skilled.

An advantage of the “proficiency barrier” hypothesis proposed by Seefeldt (1980) is its recognition that the relationship between motor competence and physical activity may not be linear. Rather, the hypothesis suggests that physical activity is influenced when a certain level of motor competence is not achieved and acknowledges that below the proficiency barrier, there is bound to be substantial variation in children's motor competence and participation in physical activity. The proficiency barrier is located between the fundamental and transitional motor skills periods. The transition between these two levels of motor competence is expected to occur between the early and middle childhood years. Stodden and colleagues (2008) suggest that the relationship between motor competence and physical activity is dynamic and changes across time. In their model the “development of motor skill competence is a primary underlying mechanism that promotes engagement in physical activity” (p. 290).

The relationship between skills and physical activity is considered reciprocal. It is expected that as motor skills competence increases, physical activity participation also increases and that the increased participation feeds back into motor skills competence. The reciprocal relationship between motor skills competence and physical activity is weak during the early childhood years (ages 2-8) because of a variety of factors, including environmental conditions, parental influences, and previous experience in physical education programs (Stodden et al., 2008). Also, children at this age are less able to distinguish accurately between perceived physical competence and actual motor skills competence (Harter and Pike, 1984; Goodway and Rudisill, 1997; Robinson and Goodway, 2009; Robinson, 2011), and thus motor skills are not expected to strongly influence physical activity. The literature supports this hypothesis, as indicated by low to moderate correlations between motor skills competence and physical activity in preschool (Sääkslahti et al., 1999; Williams et al., 2008; Cliff et al., 2009; Robinson and Goodway, 2009; Robinson, 2011) and early elementary school–age (Raudsepp and Päll, 2006; Hume et al., 2008; Morgan et al., 2008; Houwen et al., 2009; Ziviani et al., 2009; Lopes et al., 2011) children.

In older children, perceived competence is more closely related to actual motor skills competence. Older, low-skilled children are aware of their skills level and are more likely to perceive physical activity as difficult and challenging. Older children who are not equipped with the necessary skills to engage in physical activity that requires high levels of motor skills competence may not want to display their low competence publicly. As children transition into adolescence and early adulthood, the relationship between motor skills competence and physical activity may strengthen (Stodden et al., 2008). Investigators report moderate correlations between motor skills competence and physical activity in middle school–age children (Reed et al., 2004; Jaakkola et al., 2009). Okely and colleagues (2001) found that motor skills competence was significantly associated with participation in organized physical activity (i.e., regular and structured experiences related to physical activity) as measured by self-reports. A strength of the model of Stodden and colleagues (2008) is the inclusion of factors related to psychosocial health and development that may influence the relationship between motor skills competence and physical activity, contributing to the development and maintenance of obesity. Other studies have found that perceived competence plays a role in engagement in physical activity (Ferrer-Caja and Weiss, 2000; Sollerhed et al., 2008).

Motor skills competence is an important factor; however, it is only one of many factors that contribute to physical activity. For instance, three studies have reported negative correlations between girls' motor competence and physical activity (Reed et al., 2004; Cliff et al., 2009; Ziviani et al., 2009), suggesting that sex may be another determining factor. A possible explanation for these findings is that since girls tend to be less active than boys, it may be more difficult to detect differences in physical activity levels between high- and low-skilled girls. It is also possible that out-of-school opportunities for physical activity are more likely to meet the interests of boys, which may at least partially explain sex differences in physical activity levels (Le Masurier et al., 2005). Previous research suggests that in general boys are more motor competent than girls (Graf et al., 2004; Barnett et al., 2009; Lopes et al., 2011) and that this trend, which is less apparent in early childhood, increases through adolescence (Thomas and French, 1985; Thomas and Thomas, 1988; Thomas, 1994), although one study reports that girls are more motor competent than boys (Cliff et al., 2009).

One component of motor competence is the performance of gross motor skills, which are typically classified into object control and locomotor skills. Consistent evidence suggests that boys are more competent in object control skills, while girls are more competent in locomotor skills (McKenzie et al., 2004; Morgan et al., 2008; Barnett et al., 2009). In light of these sex differences, it is important to examine the relationships of object control and locomotor skills with physical activity separately for boys and girls. For boys, object control skills are more related to physical activity than are locomotor skills (Hume et al., 2008; Morgan et al., 2008; Williams et al., 2008; Cliff et al., 2009), whereas evidence suggests that the reverse is true for girls (McKenzie et al., 2002; Hume et al., 2008; Cliff et al., 2009; Jaakkola et al., 2009). Three studies report a significant relationship between balance and physical activity for girls but not boys (Reed et al., 2004; Ziviani et al., 2009). Cliff and colleagues (2009) suggest that object control and locomotor skills may be more related to boys' and girls' physical activity, respectively, because of the activity type in which each sex typically engages.

The relationship between motor competence and physical activity clearly is complex. It is quite likely that the relationship is dynamic and that motor competence increases the likelihood of participating in physical activity while at the same time engaging in physical activity provides opportunities to develop motor competence (Stodden et al., 2008). Despite some uncertainty, the literature does reinforce the important role of physical education in providing developmentally appropriate movement opportunities in the school environment. These opportunities are the only means of engaging a large population of children and youth and providing them with the tools and opportunities that foster health, development, and future physical activity.

Stature

Regular physical activity has no established effect on linear growth rate or ultimate height (Malina, 1994). Although some studies suggest small differences, factors other than physical activity, especially maturity, often are not well controlled. It is important to note that regular physical activity does not have a negative effect on stature, as has sometimes been suggested. Differences in height among children and adolescents participating in various sports are more likely due to the requirements of the sport, selection criteria, and interindividual variation in biological maturity than the effects of participation per se (Malina et al., 2004).

Body Weight

Although physical activity is inversely related to weight, correlations are generally low (~r–0.15), and differences in body weight between active and inactive boys and girls tend to be small (Mirwald and Bailey, 1986; Saris et al., 1986; Beunen et al., 1992; Lohman et al., 2006;), except in very obese children and adolescents. Similarly, physique, as represented in somatotypes, does not appear to be significantly affected by physical activity during growth (Malina et al., 2004). In contrast, components of weight can be influenced by regular physical activity, especially when the mode and intensity of the activity are tailored to the desired outcome. Much of the available data in children and adolescents is based on BMI, a surrogate for composition, and indirect methods based on the two-compartment model of body composition in which body weight is divided into its fat-free and fat components (Going et al., 2012). While studies generally support that physical activity is associated with greater fat-free mass and lower body fat, distinguishing the effects of physical activity on fat-free mass from expected changes associated with growth and maturation is difficult, especially during adolescence, when both sexes have significant growth in fat-free mass. The application of methods based on the two-compartment model is fraught with errors, especially when the goal is to detect changes in fat-free mass, and no information is available from these methods regarding changes in the major tissue components of fat-free mass—muscle and skeletal tissue.

Muscle

Skeletal muscle is the largest tissue mass in the body. It is the main energy-consuming tissue and provides the propulsive force for movement. Muscle represents about 23-25 percent of body weight at birth and about 40 percent in adults, although there is a wide range of “normal” (Malina, 1986, 1996). Postnatal muscle growth is explained largely by increases in cell size (hypertrophy) driving an increase in overall muscle mass. The increase in muscle mass with age is fairly linear from young childhood until puberty, with boys having a small but consistent advantage (Malina, 1969, 1986). The sex difference becomes magnified during and after puberty, driven primarily by gender-related differences in sex steroids. Muscle, as a percentage of body mass, increases from about 42 percent to 54 percent in boys between ages 5 and 11, whereas in girls it increases from about 40 percent to 45 percent between ages 5 and 13 and thereafter declines (Malina et al., 2004). It should be noted that absolute mass does not decline; rather, the relative decline reflects the increase in the percentage of weight that is fat in girls. At least part of the sex difference is due to differences in muscle development for different body regions (Tanner et al., 1981). The growth rate of arm muscle tissue during adolescence in males is approximately twice that in females, whereas the sex difference in the growth of muscle tissue in the leg is much smaller. The sex difference that develops during puberty persists into adulthood and is more apparent for the musculature of the upper extremities.

Sex-related differences in muscular development contribute to differences in physical performance. Muscle strength develops in proportion to the cross-sectional area of muscle, and growth curves for strength are essentially the same as those for muscle (Malina and Roche, 1983). Thus the sex difference in muscle strength is explained largely by differences in skeletal muscle mass rather than muscle quality or composition. Aerobic (endurance) exercise has little effect on enhancing muscle mass but does result in significant improvement in oxygen extraction and aerobic metabolism (Fournier et al., 1982). In contrast, numerous studies have shown that high-intensity resistance exercise induces muscle hypertrophy, with associated increases in muscle strength. In children and adolescents, strength training can increase muscle strength, power, and endurance. Multiple types of resistance training modalities have proven effective and safe (Bernhardt et al., 2001), and resistance exercise is now recommended for enhancing physical health and function (Behringer et al., 2010). These adaptations are due to muscle fiber hypertrophy and neural adaptations, with muscle hypertrophy playing a more important role in adolescents, especially in males. Prior to puberty, before the increase in anabolic sex steroid concentrations, neural adaptations explain much of the improvement in muscle function with exercise in both boys and girls.

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, reflecting 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 intermediate markers of bone health, such as bone mineral content and density. 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 observational 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 density 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 osteogenic 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 exercise interacts with frequency, intensity, and duration. Taken together, however, 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 mineral accrual represents approximately the same amount of bone mineral that most people will lose in their entire adult lives (Arlot et al., 1997). The increase in mineral contributes to increased bone strength. Mineral is accrued on the periosteal surface of bone, such that the bone grows wider. Increased bone width, independent of the increased mineral mass, also contributes to greater bone strength. Indeed, an increase of as little as 1 mm in the outer surface of bone increases strength substantially. Adding bone to the endosteal surface also increases strength (Parfitt, 1994; Wang et al., 2009). Increases in testosterone may be a greater stimulus of periosteal expansion than estrogen since testosterone contributes to wider and stronger bones in males compared with females. Retrospective studies in tennis players and gymnasts suggest structural adaptations may persist many years later in adulthood and are greatest when “impact” activity is initiated in childhood (Kannus et al., 1995; Bass et al., 1998). RCTs on this issue are few, although the available data are promising (McKay et al., 2000; Fuchs et al., 2001; MacKelvie et al., 2001, 2003; Lindén et al., 2006). Thus, impact exercise begun in childhood may result in lasting structural changes that may contribute to increased bone strength and decreased fracture risk later in life (Turner and Robling, 2003; Ferrari et al., 2006).

Adipose tissue

The adipose “organ” is composed of fat cells known as adipocytes (Ailhaud and Hauner, 1998). Adipocytes are distributed throughout the body in various organs and tissues, although they are largely clustered anatomically in structures called fat depots, which include a large number of adipocytes held together by a scaffold-like structure of collagen and other structural molecules. In the traditional view of the adipocyte, the cell provides a storage structure for fatty acids in the form of triacylglycerol molecules, with fatty acids being released when metabolic fuel is needed (Arner and Eckel, 1998). While adipocytes play this critical role, they are also involved in a number of endocrine, autocrine, and paracrine actions and play a key role in regulating other tissues and biological functions, for example, immunity and blood pressure, energy balance, glucose and lipid metabolism, and energy demands of exercise (Ailhaud and Hauner, 1998; Frühbeck et al., 2001). The role of adipocytes in regulation of energy balance and in carbohydrate and lipid metabolism and the potential effects of physical activity on adipocyte function are of particular interest here, given growing concerns related to pediatric and adult obesity (Ogden et al., 2012) and the associated risk of cardiometabolic disease (Weiss et al., 2004; Eisenmann, 2007a,b; Steele et al., 2008). Metabolic differences among various fat depots are now well known (Frühbeck et al., 2001), and there is significant interest in the distribution of adipose tissue, the changes that occur during childhood and adolescence, and their clinical significance.

Adipocytes increase in size (hypertrophy) and number (hyperplasia) from birth through childhood and adolescence and into young adulthood to accommodate energy storage needs. The number of adipocytes has been estimated to increase from about 5 billion at birth to 30 billion to 50 billion in the nonobese adult, with an increase in average diameter from about 30-40 μm at birth to about 80-100 μm in the young adult (Knittle et al., 1979; Bonnet and Rocour-Brumioul, 1981; Chumlea et al., 1982). In total the adipose organ contains about 0.5 kg of adipocytes at birth in both males and females, increasing to approximately 10 kg in average-weight-for-height males and 14 kg in females (Malina et al., 2004). There is wide interindividual variation, however, and the difficulty of investigating changes in the number and size of adipocytes is obvious given the invasiveness of the required biopsy procedures; understandably, then, data on these topics are scarce in children and adolescents. Also, since only subcutaneous depots are accessible, results must be extrapolated from a few sites.

Based on such information, the average size of adipocytes has been reported to increase two- to threefold in the first year of life, with little increase in nonobese boys and girls until puberty (Malina et al., 2004). A small increase in average adipocyte size at puberty is more obvious in girls than in boys. There is considerable variation in size across various subcutaneous sites and between subcutaneous and internal depots. The number of adipocytes is difficult to estimate. Available data suggest that the cellularity of adipose tissue does not increase significantly in early postnatal life (Malina et al., 2004). Thus, gain in fat mass is the result of an increase in the size of existing adipocytes. From about 1-2 years of age and continuing through early and middle childhood, the number of adipocytes increases gradually two- to threefold. With puberty the number practically doubles, followed by a plateau in late adolescence and early adulthood. The number of adipocytes is similar in boys and girls until puberty, when girls experience a greater increase than boys.

The increases in the number of adipocytes during infancy and puberty are considered critical for enlargement of the adipose tissue organ and for the risk of obesity. Since size and number are linked, the number of adipocytes can potentially increase at any age if fat storage mechanisms are stimulated by chronic energy surfeit (Hager, 1981; Chumlea et al., 1982). Energy expenditure through regular physical activity is a critical element in preventing energy surfeit and excess adiposity. While cellularity undoubtedly is strongly genetically determined, regular physical activity, through its contribution to energy expenditure, can contribute to less adipocyte hyperplasia by limiting hypertrophy.

Fat distribution

Fat distribution refers to the location of fat depots on the body. The metabolic activities of fat depots differ, and small variation can have a long-term impact on fat distribution. Differences in metabolic properties across depots also have clinical implications. Visceral adipose tissue in the abdominal cavity is more metabolically active (reflected by free fatty acid flux) than adipose tissue in other areas (Arner and Eckel, 1998), and higher amounts of visceral adipose tissue are associated with greater risk of metabolic complications, such as type 2 diabetes and cardiovascular disease (Daniels et al., 1999; He et al., 2007; Dencker et al., 2012). In contrast, subcutaneous fat, particularly in the gluteofemoral region, is generally associated with a lower risk of cardiometabolic disease. Age- and sex-associated variations in fat distribution contribute to age- and sex-associated differences in cardiometabolic disease prevalence. Girls have more subcutaneous fat than boys at all ages, although relative fat distribution is similar. After a rapid rise in subcutaneous fat in the first few months of life, both sexes experience a reduction through age 6 or 7 (Malina and Roche, 1983; Malina and Bouchard, 1988; Malina, 1996). Girls then show a linear increase in subcutaneous fat, whereas boys show a small increase between ages 7 and 12 or 13 and then an overall reduction during puberty. The thickness of subcutaneous fat on the trunk is approximately one-half that of subcutaneous fat on the extremities in both boys and girls during childhood. The ratio increases with age in males during adolescence but changes only slightly in girls. In males the increasing ratio of trunk to extremity subcutaneous fat is a consequence of slowly increasing trunk subcutaneous fat and a decrease in subcutaneous fat on the extremities. In girls, trunk and extremity subcutaneous fat increase at a similar rate; thus the ratio is stable (Malina and Bouchard, 1988). As a consequence, the sex difference in the distribution of body fat develops during adolescence. It is important to note that changes in subcutaneous fat pattern do not necessarily represent changes in abdominal visceral adipose tissue.

Tracking of subcutaneous fat has been investigated based on skinfold thicknesses and radiographs of fat widths in males and females across a broad age range (Katzmarzyk et al., 1999; Campbell et al., 2012). Results indicate that subcutaneous fat is labile during early childhood. After age 7 to 8, correlations between subcutaneous fat in later childhood and adolescence and adult subcutaneous fat are significant and moderate. Longitudinal data on tracking of visceral adipose tissue are not available, but percent body fat does appear to track. Thus children and especially adolescents with higher levels of body fat have a higher risk of being overfat at subsequent examinations and in adulthood, although variation is considerable, with some individuals moving away from high fatness categories, while some lean children move into higher fatness categories.

In cross-sectional studies, active children and adolescents tend to have lower skinfold thicknesses and less overall body fat than their less active peers (Loftin et al., 1998; Rowlands et al., 2000; Stevens et al., 2004; Lohman et al., 2006), although the correlations are modest, reflecting variation in body composition at different levels of physical activity, as well as the difficulty of measuring physical activity. Longitudinal studies indicate small differences in fatness between active and inactive boys and girls. Although some school-based studies of the effects of physical activity on body composition have reported changes in BMI or skinfolds in the desired direction (Gortmaker et al., 1999; McMurray et al., 2002), most have not shown significant effects. High levels of physical activity are most likely needed to modify skinfold thicknesses and percent body fat. In adults, visceral adipose tissue declines with weight loss with exercise. In contrast, in a study of obese children aged 7-11, a 4-month physical activity program resulted in minimal change in abdominal visceral adipose tissue but a significant loss in abdominal subcutaneous adipose tissue (Gutin and Owens, 1999). In adults, decreases in fatness with exercise are due to a reduction in fat cell size, not number (You et al., 2006); whether this is true in children is not certain but appears likely. Given that adipocyte hypertrophy may trigger adipocyte hyperplasia (Ballor et al., 1998), energy expenditure through regular physical activity may be important in preventing excess adipose tissue cellularity. Regular physical activity also affects adipose tissue metabolism so that trained individuals have an increased ability to mobilize and oxidize fat, which is associated with increased levels of lipolysis, an increased respiratory quotient, and a lower risk of obesity (Depres and Lamarche, 2000).

Cardiorespiratory System

The ability to perform sustained activity under predominantly aerobic conditions depends on the capacity of the cardiovascular and pulmonary systems to deliver oxygenated blood to tissues and on the ability of tissues (primarily skeletal muscle) to extract oxygen and oxidize substrate. By age 2 the systems are fully functional, although young children lack the cardiorespiratory capacity of older children and adults because of their small size (Malina et al., 2004). Children's aerobic capacity and consequently their ability to exercise for longer periods of time increase as they grow. Maximal aerobic power (liters per minute) increases fairly linearly in boys until about age 16, whereas it increases in girls until about age 13 and then plateaus during adolescence (Malina et al., 2004; Eisenmann et al., 2011). Differences between boys and girls are small (~10 percent) during childhood and greater after the adolescent growth spurt, when girls have only about 70 percent of the mean value of boys. Changes with age and sex differences are explained largely by differences in the size of the relevant tissues. Dimensions of the heart and lungs enlarge with age in a manner consistent with the increase in body mass and stature (Malina et al., 2004). The increase in the size of the heart is associated with increases in stroke volume (blood pumped per beat) and cardiac output (product of stroke volume and heart rate, liters per minute), despite a decline in heart rate during growth. Similarly, increase in lung size (proportional to growth in height) results in greater lung volume and ventilation despite an age-associated decline in breathing frequency. From about age 6 to adulthood, maximal voluntary ventilation approximately doubles (50–100 L/min) (Malina et al., 2004). The general pattern of increase as a function of height is similar in boys and girls. In both, lung function tends to lag behind the increase in height during the adolescent growth spurt. As a result, peak gains in lung function occur about 2 years earlier in girls than in boys.

Blood volume is highly related to body mass and heart size in children and adolescents, and it is also well correlated with maximal oxygen uptake during childhood and adolescence (Malina et al., 2004). Blood volume increases from birth through adolescence, following the general pattern for changes in body mass. Both red blood cells and hemoglobin have a central role in transport of oxygen to tissues. Hematocrit, the percentage of blood volume explained by blood cells, increases progressively throughout childhood and adolescence in boys, but only through childhood in girls. Hemoglobin content, which is related to maximal oxygen uptake, heart volume, and body mass, increases progressively with age into late adolescence. Males have greater hemoglobin concentrations than females, especially relative to blood volume, which has functional implications for oxygen transport during intense exercise.

Growth in maximal aerobic power is influenced by growth in body size, so controlling for changes in body size during growth is essential. Although absolute (liters per minute) aerobic power increases into adolescence relative to body weight, there is a slight decline in both boys and girls, suggesting that body weight increases at a faster rate than maximal oxygen consumption, particularly during and after the adolescent growth spurt (Malina et al., 2004). Changes in maximal oxygen consumption during growth tend to be related more closely to fat-free mass than to body mass. Nevertheless, sex differences in maximal oxygen consumption per unit fat-free mass persist, and maximal oxygen consumption per unit fat-free mass declines with age.

Improvements in cardiorespiratory function—involving structural and functional adaptations in the lungs, heart, blood, and vascular system, as well as the oxidative capacity of skeletal muscle—occur with regular vigorous- and moderate-intensity physical activity (Malina et al., 2004). Concern about the application of invasive techniques limits the available data on adaptations in the oxygen transport system in children. Nevertheless, it is clear that aerobic capacity in youth increases with activity of sufficient intensity and that maximal stroke volume, blood volume, and oxidative enzymes improve after exercise training (Rowland, 1996). Training-induced changes in other components of the oxygen transport system remain to be determined.

PSYCHOSOCIAL HEALTH

Research supports the positive impact of physical activity on the overall psychological health and social engagement of every student. A well-designed physical education curriculum provides students with social and emotional benefits (NASPE, 2001). Simultaneously, exposure to failure experiences, emphasis on competitive sports, and elitism for naturally inclined athletes, along with bullying and teasing of unfit, uncoordinated, and overweight youth, may be important factors discouraging participation in current and future physical activity (Kohl and Hobbs, 1998; Sallis et al., 2000; Allender et al., 2006). School-based physical activity, including physical education and sports, is designed to increase physical activity while also improving motor skills and development, self-efficacy, and general feelings of competency and engaging children socially (Bailey, 2006). The hoped-for psychosocial outcomes of physical education and other physical activity programs in the school setting have been found to be critical for continued physical activity across the life span and are themselves powerful long-term determinants of physical activity (Bauman et al., 2012). Unfortunately, significant gaps exist between the intent and reality of school-based physical education and other activity programs (HHS, 2013).

A large number of psychological and social outcomes have been examined. Specific aspects of psychosocial health showing a beneficial relationship to physical activity include, among others, self-efficacy, self-concept, self-worth (Haugen et al., 2011), social behaviors (Cradock et al., 2009), pro-school attitudes, motivation and goal orientation (Digelidis et al., 2003), relatedness, friendships (de la Haye et al., 2011; Macdonald-Wallis et al., 2011), task orientation, team building, bullying, and racial prejudice (Byrd and Ross, 1991). Most studies are descriptive, finding bidirectional associations between psychosocial outcomes and physical activity. Reviews and meta-analyses confirm a positive association between physical activity and self-esteem, especially for aerobic activities (McAuley, 1994).

Among psychosocial factors, self-efficacy (confidence in one's ability to be physically active in specific situations) has emerged as an important correlate of physical activity from a large body of work based on the durable and practically useful social learning theory (Bandura and McClelland, 1977; Bandura, 1995). Bandura's theory compels consideration of the psychosocial and physical environments, the individual, and in this case the behavior of physical activity. Using this framework, physical activity itself has been shown to be a consistent positive correlate as well as a determinant of physical activity in children and adolescents. A large amount of reviewed research has found that physical education and physical activity experiences can increase children's confidence in being active and lead to continued participation in physical activity (Bauman et al., 2012). RCTs have shown that both self-efficacy and social interactions leading to perceived social support influence changes in physical activity (Dishman et al., 2009). Skill mastery, confidence building, and group support are well-known strategies for advancing student learning and well-being in many educational domains in the school setting and apply equally to school physical education and other physical activity. Early observational studies of physical, social, and environmental determinants of physical activity at home, school, and recess indicated that prompts to be active (or not) from peers and adults accounted for a significant amount of the variance in directly observed physical activity (Elder et al., 1998). One longitudinal study following the variability and tracking of physical activity in young children showed that most of the variability in both home and recess activity was accounted for by short-term social and physical environmental factors, such as prompts from others and being outdoors (Sallis et al., 1995). Another study, examining activity among preschool children, found that, contrary to common belief, most of the time spent in preschool was sedentary, and correlates of activity were different for preschool boys and girls (Byun et al., 2011). In addition, significant variation in activity by preschool site was noted, indicating that local environmental conditions, including physical environment and equipment, policies, and teacher and administrative quality characteristics, play an important role in promoting physical activity (Brown et al., 2009).

Studies in middle and high school populations have strengthened the evidence base on relationships among self-efficacy, physical activity, and social support (from adults and peers). This research has highlighted the central contribution of self-efficacy and social support in protecting against a decline in activity levels among adolescent girls (Dishman et al., 2009, 2010). Evidence indicates further that these impacts spread to activities outside the school setting (Lytle et al., 2009). Findings of a related study suggest that leisure-time physical activity among middle school students was linked to motivation-related experiences in physical education (Cox et al., 2008).

A recent review of reviews (Bauman et al., 2012) found that population levels of physical activity are low and that consistent individual-level correlates of physical activity are age, sex, health status, self-efficacy, and previous physical activity. Physical activity declines dramatically as children progress from elementary through high school (Nader et al., 2008). Boys are consistently found to be more active than girls from ages 4 to 9. For other age groups of children and adolescents, sex is correlated with but not a determinant of activity (Bauman et al., 2012). These findings suggest the need to tailor physical education and physical activity programs for youth specifically to increase self-efficacy and enjoyment of physical activity among girls (Dishman et al., 2005; Barr-Anderson et al., 2008; Butt et al., 2011).

In summary, a broad range of beneficial psychosocial health outcomes have been associated with physical activity. The promotion of more physical activity and quality physical education in the school setting is likely to result in psychosocially healthier children who are more likely to engage in physical activity as adults. Schools can play an important role in ensuring opportunities for physical activity for a segment of the youth population that otherwise may not have the resources to engage in such activity. It makes sense to assume that, if physical activity experiences and environments were once again structured into the daily school environment of children and adolescents, individuals' feelings of self-efficacy regarding physical activity would increase in the U.S. population.

MENTAL HEALTH

Mental illness is a serious public health issue. It has been estimated that by 2010 mental illness will account for 15 percent of the global burden of disease (Biddle and Mutrie, 2008; Biddle and Asare, 2011). Young people are disproportionately affected by depression, anxiety, and other mental health disorders (Viner and Booy, 2005; Biddle and Asare, 2011). Approximately 20 percent of school-age children have a diagnosable mental health disorder (U.S. Public Health Service, 2000), and overweight children are at particular risk (Ahn and Fedewa, 2011). Mental health naturally affects academic performance on many levels (Charvat, 2012). Students suffering from depression, anxiety, mood disorders, and emotional disturbances perform more poorly in school, exhibit more behavioral and disciplinary problems, and have poorer attendance relative to mentally healthy children. Thus it is in schools' interest to take measures to support mental health among the student population. In addition to other benefits, providing adequate amounts of physical activity in a way that is inviting and safe for children of all ability levels is one simple way in which schools can contribute to students' mental health.

SUMMARY

Good health is the foundation of learning and academic performance (see Chapter 4). In children and youth, health is akin to growth. An extensive literature demonstrates that regular physical activity promotes growth and development and has multiple benefits for physical, mental, cognitive, and psychosocial health that undoubtedly contribute to learning. Although much of the evidence comes from cross-sectional studies showing associations between physical activity and various aspects of health, available prospective data support this cross-sectional evidence. Experimental evidence, although more limited for younger children, is sufficient among older children and adolescents to support the notion that children and young adults derive much the same health benefits from physical activity.

Moreover, many adult diseases have their origins in childhood. This finding, together with the finding that health-related behaviors and disease risk factors may track from childhood into adulthood, underscores the need for early and ongoing opportunities for physical activity.

Children's exercise capacity and the activities in which they can successfully engage change in a predictable way across developmental periods. For example, young children are active in short bursts, and their capacity for continuous activity increases as they grow and mature (see Figure 3-2). In adults and likely also adolescents, intermittent exercise has much the same benefit as continuous exercise when mode and energy expenditure are held constant. The health benefits of sporadic physical activity at younger ages are not well established. However, the well-documented short-term benefits of physical activity for some aspects of mental and cognitive health suggest that maximum benefit may be attained through frequent bouts of exercise throughout the day.

FIGURE 3-2

Changes in physical activity needs with increasing age of children and adolescents. SOURCE: Adapted from Malina, 1991. Reprinted with permission from Human Kinetics Publishers.

Children require frequent opportunities for practice to develop the skills and confidence that promote ongoing engagement in physical activity. Physical education curricula are structured to provide developmentally appropriate experiences that build the motor skills and self-efficacy that underlie lifelong participation in health-enhancing physical activity, and trained physical education specialists are uniquely qualified to deliver them (see Chapter 5). However, physical education usually is offered during a single session. Therefore, other opportunities for physical activity can supplement physical education by addressing the need for more frequent exercise during the day (see Chapter 6). In addition to the immediate benefits of short bouts of physical activity for learning and for mental health, developmentally appropriate physical activity during those times, along with the recommended time in physical education, can contribute to daily energy expenditure and help lessen the risk of excess weight gain and its comorbidities. Specific types of activities address specific health concerns. For example, vertical jumping activities contribute to energy expenditure for obesity prevention and also promote bone development (via the resulting ground reaction forces), potentially contributing to lower fracture risk. Other activities contribute to prevention of chronic disease. Since different types of physical activity contribute to distinct aspects of physical, mental, and psychosocial health, a varied regimen is likely to be most beneficial overall.

The quality of physical activity programming also is critical; psychosocial outcomes and improvements in specific motor skills, for example, are likely the result of programming designed specifically to target these outcomes rather than just a result of increases in physical activity per se. These psychosocial outcomes also are likely to lead to increased levels of physical activity in both the short and long terms, thereby conferring greater health benefits. Unstructured physical activity or free play also confers unique benefits and is an important supplement to more structured opportunities. Quality physical activity programming that makes these activities attractive, accessible, and safe for children and youth of all skill and fitness levels is critical to ensure that all youth participate in these activities and can therefore derive the health benefits.

Sedentary activities, such as screen viewing and excessive time spent sitting, may contribute to health risks both because of and independent of their impact on physical activity. Thus specific efforts in school to reduce sedentary behaviors, such as through classroom and playground design and reduction of television viewing, are warranted.

In sum, a comprehensive physical activity plan with physical education at the core, supplemented by other varied opportunities for and an environment supportive of physical activity throughout the day, would make an important contribution to children's health and development, thereby enhancing their readiness to learn.

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