Some young people are quite active but many others are not, prompting the question, "Why are some young people more physically active than others?" Researchers have attempted to answer this question by looking at psychological, social, and environmental factors (i.e., correlates) that may affect a young person's level of physical activity. This section describes three theoretical models used in this research and identifies distinct factors that have been associated with youth physical activity.

Conceptual Models.
Several theoretical models of human behavior have guided the research on the determinants of physical activity in youth. The predominant model has been Bandura's Social Cognitive Theory (SCT; previously Social Learning Theory) ⁵, which asserts that personal (knowledge, self-efficacy, beliefs, intentions), environmental (physical and social), and behavioral (self observation, judgement, and reaction) factors interact (reciprocal determinism) to determine the final action of the individual. A central concept of SCT is self-efficacy: people's sense of personal effectiveness at exercising some control over events that affect their lives ⁵ . The Theory of Planned Behavior (TPB)¹ also has been used to explain physical activity behavior in adults and children. The TPB incorporates the concepts of attitudes towards the behavior, the perceived societal norms, motivation to comply with those norms, and perceived behavioral control, which takes into account that not all behaviors are under volitional control. The presence or absence of volitional control, in turn, influences one's intentions and behaviors. This concept is particularly important for children, who have limited control over their activities due to parental and other constraints. Both SCT and TPB were developed to explain adult behavior, then applied to younger populations. While certain concepts from these models may also apply to children and youth, it should not be assumed that children and adults are influenced in the same way.

Recently, Welk ⁹³proposed a model to specifically predict physical activity behavior in youth. The Youth Physical Activity Promotion (YPAP) model "synthesize[s] existing research into a more usable, heuristic model..." (Figure 4) ⁹³. In this model, two domains related to psychological issues (self-efficacy, beliefs, enjoyment, and attitudes) form "predisposing factors." The environment and the young person's physical abilities are considered "enabling factors." Social influences are grouped within "reinforcing factors." Personal demographics affect all three types of factors. The YPAP model is a promising tool for explaining and predicting physical activity behavior in youth, but its ability to do so has not yet been tested.

Demographic Factors.
Age, gender, race, and other demographic factors influence physical activity in young people. Data from the YRBS¹⁹ and the National Health and Nutrition Examination Surveys ³, as well as a comprehensive review of physical activity correlates by Sallis et al. ⁶⁸, indicate that younger children are more active than adolescents ^{15 19 31}, boys are more active than girls (especially in the older age groups), and white children are more active than African-American and Hispanic children and youth^{19 54 89}. Socioeconomic status does not appear to be correlated with physical activity in young people (although it is in adults)⁶⁸, but it may be associated with physical inactivity ³⁴.

Individual Factors.
Physical traits such as the individual's fitness level, body composition, and motor skill development influence physical activity levels. Approximately 60% of an individual's aerobic fitness level is genetically determined¹⁴; consequently, those who are naturally more fit may be predisposed to be physically active. Obesity and other body composition measures may be partially genetically determined^{84 95}, and could also affect an individual's inclination to be active. In fact, several studies have identified an inverse relationship between physical activity and obesity^{3 23 76 86 92 96}, although others have not ^{4 16 65 72 96}. Because these studies have only identified associations among these variables, causality between physical activity and obesity cannot be determined. Motor skill development also may play a role in a child's physical activity level, with the earlier and more developed children having a greater self-confidence to participate 51. Little research has been done in this area, however, and the associations are not well understood.

Individual factors also include a person's beliefs, attitudes, and expectations about a behavior. One of the most frequently studied constructs is self-efficacy: a person's confidence in his or her ability to be physically active⁵. A number of studies have found self-efficacy to be positively associated with physical activity in young people^{9 21 89 90 96}. Other individual factors include the young person's beliefs about the behavior 1690, perceived benefits of the behavior^{21 29 31 96}, and enjoyment of physical activity and physical education classes^{29 83 90 96}. Research on how these factors influence physical activity is not conclusive, however, and additional work is needed to fully explain the relationships among these correlates and physical activity.

Environmental Factors.
Environmental factors that affect physical activity include the physical environment in which a young person lives (e.g., home, neighborhood, school) and social and cultural influences on his or her life (e.g., parental support, peer influences, cultural norms regarding physical activity). The physical environment can strongly promote children and youth to be physically active, or virtually ensure that they will not be. Young people with access to community physical activity programs^{16 29 90}, for example, are more likely to be active, as are those with access to activity equipment and facilities (e.g., balls, skates, bike trails, gyms)^{3172 83}. Simply spending time outdoors promotes physical activity; studies have shown that children who spend the most time outside are the most active^{8 45 72}.

Conversely (and not surprisingly), the more time a young person spends in sedentary activities (such as watching television), the less likely he or she is to be active. Simply decreasing the amount of time spent doing sedentary activities can increase a young person's level of physical activity^{26 27 66}. One interesting study found that the level of physical activity in obese children could be increased simply by moving the television set. When the television was moved to another building and exercise equipment was placed in the room with the children, the obese children used the equipment rather than walk several minutes to watch television²⁶.

The strongest social influence in the lives of most children is their parents. Parents can play a significant role in encouraging their children to be physically active. Although verbal support and role modeling provide encouragement^{10 24 30 31 62 69 70 74 90 96}, studies have shown that parents who financially support (e.g., purchase physical activity equipment), transport (e.g., drive their children to sports practice or to a physical activity facility) ^{75 96}, and are active with their children^{69 83} typically have the most physically active children. Teachers, coaches, health professionals, and active peers can also influence a young person to be physically active^{9 83 96}.

The identification of correlates of physical activity in young people is a relatively new field of study and much work remains to be done. The more health professionals, parents, and teachers know about the factors that influence physical activity and inactivity in children and youth, the more likely they will be to provide the

environment and promote the behavioral skills they need to be active during childhood, adolescence, and adulthood. Already, researchers and health professional are using what is known about self-efficacy, beliefs and attitudes, and physical and social environments to develop and test intervention programs to increase physical activity in young people. The following section describes several of these programs.