Physical activity plays a vital role in the health of children and adolescents . Along with a high caloric diet, low levels of physical activity and increased participation in sedentary leisure-time activity are two important lifestyle behaviors that have contributed to the increased prevalence of overweight and obesity among youth and adults [2, 3]. In children and adolescents, overweight and obesity are associated with an increased risk of high blood pressure, dyslipidemia, impaired glucose tolerance, cardiovascular disease, and type II diabetes [4, 5]. Furthermore, overweight children are highly likely to become overweight adults, which may reflect the tracking of obesity-risk behaviors (i.e., physical activity and diet) from childhood into adulthood [4, 6].
The social environment comprises the physical surroundings, social relationships and cultural milieu within which people function and interact . It has been shown to influence obesity-risk behaviors in adults [8, 9]; those reporting low social support from family and friends are more likely to be insufficiently active for health benefits compared to those with high levels of social support . The social environment also plays an important role in relation to children’s physical activity and sedentary behavior. The social environment of children includes the influence of parents, siblings, friends, neighbors, teachers, and coaches [10, 11]. While parents are the most important source of influence in early-life, parental influence on their child’s day-to-day behavior becomes less evident as the child matures [12, 13]. Children and adolescents spend a significant portion of their time at school with friends and peers. Evidence suggests that the dietary behavior of a friend or group of friends influences the dietary behavior of the individual , with similar results observed for sports participation  and sedentary behavior .
The pathways by which behaviors may be similar among groups of friends during childhood, however, are complex. Similar behaviors among friends likely reflect the processes of homophily or selection (i.e., an individual with certain behaviors seeking out others who also share similar behaviors) and peer influence or peer contagion (i.e., the influence of friends’ behaviors causing changes in an individual’s behavior) . Several mechanisms may explain the processes of peer influence and contagion on physical activity and sedentary behavior including: behavioral modeling (i.e., observing a peer perform a behavior leading to increased motivation to perform a behavior); peer pressure (i.e., direct attempts to impose a certain behavior on a peer); group norms (i.e., the underlying attitudes and behaviors shared among a group of peers), and; co-participation (i.e., undertaking a behavior with a peer potentially contributing to behavioral reinforcement) [17, 18].
Social network analysis or sociometry  provides a means of studying the inter-relationships among friends themselves and does not rely on an individual recalling or reporting the behavior of his/her friends or peers. Social network analysis is a quantitative method for assessing the structure and patterns of the ties or relationships among a set of entities (e.g., people or organizations) . It can provide information about an individual’s local relations (e.g., who he or she is friends with) and network position (e.g., whether he or she is centralized within a given network) as well as measures of the entire network itself (e.g., number of connections between people, and degrees of separation ). In child and adolescent health, social network analysis has been used extensively to investigate behaviors such as smoking, substance use, and delinquency in relation to individual-level network measures [21–24]. For example, popularity, or being nominated as a friend by many others, is associated with higher odds of drinking alcohol among thirteen and fifteen year olds , while substance use is associated with receiving fewer friendship nominations . Smoking , delinquency , substance abuse , and depression  studies that have used social network analyses suggest that the attitudes and behaviors of adolescents influence the attitudes and behaviors of others in their friendship networks (i.e., peer contagion). Moreover, the influence of peer contagion might also be gender-specific. Mercken et al.  found that teenage girls, but not boys, were influenced by their peer group to initiate smoking, while delinquent behavior in friends may be more influential in boys than girls .
Regarding physical activity, some evidence derived from social network analysis suggests that higher physical activity levels within friendship groups could be associated with higher levels of participation among individual group members . Much of this evidence is based on individual-level or ego-network measures (i.e., a direct link between individuals) rather than an individual’s position in the network of a class or school or the characteristics of the networks themselves. Furthermore, similar to other behaviors, there is preliminary support for gender-specific relationships between individual measures of friendship networks and physical activity. Jago et al.  found that moderate-to-vigorous physical activity of boys’ best friends, but not girls’ best friends, was positively associated with an individual’s moderate-to-vigorous physical activity.
Little is known about how specific network ties (i.e., local relations) and specific network roles (i.e., positions within the network) might influence physical activity and sedentary behaviors among children and adolescents. For example, a non-reciprocated friendship nomination (i.e., person ‘A’ says ‘B’ is my friend, but person ‘B’ does not say ‘A’ is my friend) may have a different influence on behavior compared to a reciprocated nomination. The concept of reciprocation in a friendship network can indicate the presence of strong ties (reciprocated nomination) and weak ties (non-reciprocated nomination) between individuals. Strength of ties may also be related to degree of friendship separation (i.e., friend of a friend) , or intimacy of friendship (i.e., first nominated friend, second nominated friend) . Specific roles within a network may also influence behavior, such as being an isolate (i.e., no ties to other individuals) or liaison (i.e., providing ties between groups within a network) . While studies have identified relationships between specific network roles (e.g., isolates) and smoking , as well as network characteristics (e.g., density) and delinquency , these relationships in the physical activity and sedentary behavior literature are still poorly understood. Knowledge of the dynamics of friendship networks in relation to physical activity and sedentary behavior could be useful for informing health promotion interventions within social settings (i.e., schools).
A recent systematic review found strong similarities between a child or adolescent’s level of physical activity and that of his/her close friends and wider peer group, but limited evidence on the role of social networks in influencing sedentary behavior . These authors, along with others , suggest that better interventions may come from better understanding of friendship networks and behavior. To do so, however, requires a deeper understanding of the psychology and sociology of networks, such as who should be recruited to interventions and how experiences and messages can be amplified (or diluted) across the group . School-based, peer-group interventions in drug use lacked this sophistication, with consequent modest or negligible effects .
The purpose of this review was to expand and reassess the conclusions of a previous synthesis  by undertaking a systematized literature review of studies examining the association between friendship networks and both physical activity and sedentary behavior. A systematized review encompasses several, but not all aspects of a full systematic review . The objectives of this review were to: 1) examine the association between a friend’s level of physical activity and sedentary behavior and an individual’s levels of physical activity and sedentary behavior; 2) determine if the number of friends a child or adolescent has influences his/her own physical activity or sedentary behavior, and; 3) identify and differentiate the effects of different types of social network measures, for example, network ties and positions, that are potentially associated with physical activity and sedentary behavior, especially as they operate at gender-specific levels.