Physical activity has important health benefits in youth. Physical activity is associated with more favorable biological cardiovascular disease risk-factor profiles in children and adolescents, such as lower blood pressure, more favorable serum lipid and lipoprotein levels, and decreased adiposity (16). Moreover, physical activity has been associated consistently with better psychological health, such as higher levels of self-esteem and lower levels of anxiety and stress. Physical activity in childhood and adolescence is also important to attain and maintain an appropriate bone strength, and it contributes to normal skeletal development (16). Physical activity habits developed in early life may persist into adulthood; however, this association has been questioned (59,64).
Despite these health benefits, a rapid decline in physical activity during adolescence can be seen (30). Better understanding of the correlates of physical activity and sedentary behavior in children and adolescents will support the development of effective interventions that promote an active lifestyle and prevent a sedentary lifestyle.
Many studies have been conducted focusing on correlates of children's and adolescents' physical activity and sedentary behavior. Sallis et al. (53) summarize the studies on correlates of physical activity in children and adolescence published before 1999 (53), and Gorely et al. (25) have conducted a review on correlates of television viewing among youth (25). Since the publication of the review on correlates of physical activity, many more studies have been published on this topic. Therefore, the present review elaborates on the Sallis et al. (53) review and describes biological, demographic, psychological, behavioral, social, and physical correlates of physical activity and sedentary behavior reported in studies published from 1999 to January 2005. The purpose of this review is to present a systematic update of correlates of children's and adolescent's physical activity, insufficient physical activity, and sedentariness.
METHODS
Identification of studies.
Computer searches (PubMed and PsycINFO) were conducted in the English-language. The following key words were used (MeSH, title words, and text words): activities and habits; child behavior; adolescent behavior; health behavior; social behavior; mental processes; health knowledge, attitudes, practice; attitude to health; exercise/psychology; social environment; self-efficacy; parenting; and lifestyle. The titles and abstracts of the papers were screened for potential correlates of physical activity and sedentary behavior by two researchers (K.v.d.H., M.C.A.P.) independently, to come to a first selection of relevant papers. Subsequently, all papers were read to come to the final selection. To be included in the review, a study had to meet all the following criteria: subjects were healthy young people in the age range of 4-18 yr old (or the mean age was in this range); the dependent variable should be a measure of overall physical activity, insufficient physical activity, or sedentary behavior; and the variables had to have been tested for their association with the dependent variable. Studies reporting on data from a control group or baseline data in an intervention study were included, and studies reporting on the effects of an intervention were excluded. Dissertations also were excluded.
Categorization of variables.
The correlates were categorized into five groups: demographic and biological variables (e.g., age and parental overweight); psychological, cognitive, and emotional variables (e.g., self-esteem, attitude, and perceived benefits); behavioral attributes and skills (e.g., previous physical activity and sedentary time); social and cultural variables (e.g., parental encouragement); and physical environment variables (e.g., access to facilities). This categorization also was used by Sallis et al. (53) and is based on the ecological approach of health behavior. Some conceptually similar variables were combined (e.g., self-efficacy and perceived behavior control; sport ability beliefs and sport competence; physical self-perception and self-worth; sedentary lifestyle and watching television).
In the literature, physical inactivity can refer to insufficient physical activity or sedentary behavior. In this review, a distinction was made between sedentary behaviors (e.g., watching television and reading) and insufficient physical activity (i.e., not spending at least 1 h·d−1 in a physical activity of at least moderate intensity, or at least three sessions per week of moderate to vigorous activities that last 20 min or more). For physical activity, only variables that were studied in at least three studies were added in the tables and described in the text. For sedentariness and insufficient physical activity, all studies were taken into consideration because of the limited amount of studies included in this review examining correlates of these behaviors.
Level of evidence.
Varieties of statistical techniques (e.g., correlations, logistic regression, ANOVA) were used to evaluate the associations. Most studies not only reported univariate but also multivariate analyses, with adjustments for other potential correlates investigated. Whenever possible findings reported were those from the fully adjusted model. Studies reporting findings for boys (M) and girls (F) separately provided two independent samples for the analysis of the results, consistent with the approach outlined by Sallis et al. (53). In the studies reviewed, either a positive (+), inverse (−), or no significant (0) association between the variable of interest and physical activity, insufficient physical activity, and sedentariness was found; these results are shown in Tables 1-3. A final summary association code for each correlate was derived as follows: when 50-75% of the associations were in a similar direction, it was considered evidence for either a positive (+), an inverse (−), or no association (0). When more than 75% of the associations were in a similar direction, it was considered strong evidence for a positive (+), an inverse (−), or no association (00). When exactly 50% of the associations were in a positive or inverse direction, or if there was considerable lack of consistency in the findings (showing both positive and inverse associations), the evidence was considered inconclusive (?).
;)
TABLE 1: Summary of studies examining correlates of physical activity in children and adolescents.
TABLE 2: Summary of studies examining correlates of sedentary behavior and insufficient physical activity in children and adolescents.
TABLE 3: Overview of the results of the present review compared with the results of Sallis et al. (
53).
RESULTS
Identification of relevant studies.
Of the 1466 studies identified from PubMed and PsycINFO and manual searches, 84 met the criteria for inclusion in the review on the basis of title and abstract. After reading these 84 papers, the two reviewers (K.v.d.H., M.C.A.P.) agreed to include 57 papers. The vast majority of the studies described in these papers were cross-sectional (N = 51). Six prospective studies were included as well (4,11,22,39,42,51). The most important reasons for exclusion of studies were the study design, the age of the population, and studies being aimed at testing a theoretical model instead of studying the association of variables with physical activity, insufficient physical activity, or sedentariness. Thirteen studies had children as study population (aged 4-12 or mean age < 12) (1,4,5,12,13,17,20,32,33,44,51,61,62), and 40 studies focused on adolescents (aged 13-18 or mean age > 12) (2,3,6-11,15,18,19,21-24,26-28,34,37-40,42,45-47,50,52,55-57,60,63,65-70). Four publications included both children and adolescents (14,31,54,58), of which two studies reported their results for children and adolescents separately (14,54). One study was categorized among "children" because the mean age of the study population was not reported (age range 6-15) (31), and another paper was categorized among "adolescents" because the mean age of the study population was 12.8 for boys and 12.9 for girls (58). Finally, 16 papers were included among children and 43 papers among adolescents.
Correlates of children's physical activity and sedentariness.
Tables 1-3 present a summary of the associations between physical activity, insufficient physical activity, and sedentary behaviors, and the correlates that were reviewed.
The associations between demographic and biological variables and children's physical activity were examined in nine studies. These variables were gender (4,31,32,44), age (1,4,31,32,44,61), ethnicity (32,44,51,54,61), parental education (31,44,51,54), BMI or skinfolds (1,4,13,31,51), and having a single parent (32,51,54). There was evidence for a positive association between gender (male) and physical activity. No associations were found for age, ethnicity, BMI/skinfolds, and single-parent status with physical activity. For parental education, the evidence was inconclusive.
Seven studies examined psychological variables as correlates of physical activity-namely, self-efficacy (44,61,62), self-perception (51,54), enjoyment or activity preference (51,54), and barriers to physical activity (14,20,54). There was strong evidence for a positive association between self-efficacy and physical activity. No association was found between physical activity and self-perception, enjoyment, and barriers towards physical activity.
One behavioral variable (watching television/playing video games) was examined in three studies (31,51,61). According to these three studies, there was evidence for no association between watching television and physical activity.
Five studies examined parental physical activity/modeling (44,51,54,62) and parental support (1,44,51,54) in the category social variables. There was evidence for a positive association between parental physical activity and boys' physical activity, but not for girls' physical activity. There was also evidence for a positive association between parental support and children's physical activity.
Access to facilities was the only physical environmental variable examined. No significant associations were found between perceived access to facilities (1), perceived access to play space (54), or for perceived access to sporting and/or fitness equipment at home (61,62) and children's physical activity.
Only three studies were included in the review examining correlates of sedentary behaviors (television/video watching and computer games) (12,32,33), and one study examined correlates of insufficient physical activity (44). Evidence about the examined associations in these studies was insufficient.
Correlates of adolescents' physical activity and sedentariness.
Twenty-four studies examined demographic and biological variables as correlates of adolescents' physical activity, including gender (3,10,24,27,38,45-47,50,63,68,70), age (26,27,38,42,46,47,56,57,63), ethnicity (7,24,37,38,42,54,56,60), socioeconomic status (7,9,19,24,27,56), parental education (24,38,54,56), and BMI or skinfolds (11,28,38,42,54) (Table 1). There was evidence for a positive association for gender (male) and parental education with physical activity. No association between socioeconomic status or BMI and physical activity was found. For age and ethnicity, the evidence was inconclusive.
Twenty-eight studies examined 10 psychological correlates of adolescents' physical activity: attitude (9,46,55), self-efficacy (2,3,9,39,40,42,50,55,57,60,63,68-70), intention (9,40,55,60), barriers to physical activity (2,3,9,14,42,54,69,70), perceived benefits of physical activity (9,14,42,69,70), sport competence (6,11,42,67), goal orientation (6,10,66,67), self-perception (6,11,42,54,58), fun/enjoyment (9,15,42,54,66), and depression (21,27,42,45,56). Attitude, self-efficacy, and goal orientation/motivation were found to be positively associated with physical activity. For perceived benefits, self-perception, fun/enjoyment, and depression, no association with physical activity was found. The evidence for the associations between physical activity and intention, perceived barriers, and sport competence was inconclusive.
Smoking (9,27,46), watching television/sedentariness (3,9,42,57), and physical education/school sports activity (9,24) were examined in the category behavioral variables. There was evidence for a positive association between physical education/school sports and physical activity. No association was found between watching television/sedentariness and physical activity. The evidence for and association between smoking and physical activity was inconclusive.
Thirteen studies examined social variables as correlates of adolescents' physical activity; variables included parental activity/modeling (9,15,37,54,63,70), family influences (9,18,27,37,42,54,55,57,63,69), and peer influences/friend support (9,18,47,69,70). There was evidence for a positive association between family influences and friend support and physical activity. No association between parental activity and adolescent physical activity was found.
Five studies (9,15,19,24,52) examined the association between physical activity and availability or proximity to sports facilities (environmental variables). Only one study used objectively measured environmental variables (52), whereas the other studies reported on perceived environmental variables. No association between availability of facilities and adolescents' physical activity was found.
Twelve studies were included in the review examining correlates of insufficient physical activity (10,22,34,45) or sedentary behaviors (television/video watching and computer games) (21,23,24,28,34,37,42,56,65) (Table 2). For gender (male), a positive association with watching television and video was found, and an inverse association was found with insufficient physical activity. For ethnicity (Caucasian), socioeconomic status, and parental education, an inverse association with watching television and video was found, whereas BMI and depression were positively associated with watching television and video. For the other variables, there was insufficient evidence to draw conclusions about an association with insufficient physical activity or sedentary behavior.
DISCUSSION
Physical activity.
To identify correlates of physical activity and sedentary behavior in children and adolescents, we reviewed the literature from 1999 to January 2005. Sallis et al. (53) have reviewed the literature between 1970 an 1998, and our study elaborates on this review (53). The results from our review suggest that correlates of physical activity for children are gender, self-efficacy, parental physical activity for boys, and parental support. Correlates for adolescents' physical activity were gender, parental education, attitude, self-efficacy, goal orientation/motivation, physical education/school sports participation, family influences, and friend support. Gender, self-efficacy, and family/parental support were associated with physical activity, both in children and adolescents.
The findings for children differ to some extent from the findings of Sallis et al. (53) (Table 3). In the review of Sallis et al., associations were found for access to facilities, barriers to physical activity, and preference for physical activity. These factors were not associated with physical activity in the present review. The positive association for gender (male) and the lack of an association between single-parent status and self-esteem was shown both in the review of Sallis et al. (53) and in our update. Looking at the results for adolescents, our findings for gender, socioeconomic status, BMI/skinfolds, goal orientation, enjoyment, smoking, parental activity, family support, support from friends or significant others, and availability of facilities are consistent with the findings from Sallis et al. (53). The observed differences between the reviews may result from the fact that some variables or theories were studied more often in a certain time period, that there were fewer studies included in our review compared with the review of Sallis et al. (53), and by the shorter time-period covered by our review.
The evidence from the review by Must and Tybor (41), which includes prospective observational studies, suggests the expected inverse relationship between physical activity and obesity, except for the youngest children (41). Our review found no evidence for an association between body mass index and physical activity in children and adolescents.
Regarding ethnicity, no association was found in children, whereas for adolescents the evidence was inconclusive. However, a limited number of studies have included children and adolescents from ethnic minorities. The review of Sallis et al. (53) found inconclusive evidence for an association between ethnicity and physical activity in children and a positive association in adolescents. These findings suggest that in childhood there are no ethnic differences in physical activity. These differences might develop later when children grow into adolescence. For inactivity, the same pattern can be seen. No association between ethnicity and inactivity was found for children (partly because of the limited number of studies), whereas for adolescents an inverse association was found, indicating that minority populations are more often inactive. Therefore, public health efforts aimed at promoting physical activity and decreasing sedentary behaviors should particularly target adolescents from ethnic minorities.
Looking at the different categories, it is noteworthy that in the behavioral and environmental categories, only one of the variables shows an association with physical activity-namely, participation in school sports and physical education. Despite the focus in the literature on physical environmental correlates of physical activity, few studies have examined environmental correlates of physical activity in children and adolescents. Most studies on environmental determinants of physical activity rely on self-reports of environmental factors. This represents the perceived, rather than the actual, features of the physical environment. Little is known about such perceived measures or whether objectively measured factors show stronger associations with physical activity in youth.
There was considerable inconsistency between the studies included in the review. Only few correlates were consistently associated among all studies examining the correlate. Some possible explanations for this inconsistency are, for example, the different measures of physical activity, different sample characteristics, and different statistical analyses. For instance, physical activity was measured with validated and unvalidated self-report measures, and with objective measures such as the accelerometer. The samples were also very different. Some studies included children at risk of overweight, whereas others included populations of normal-weight children or adolescents who were not at risk of overweight. In addition, many different analysis strategies were used in the studies. Multivariate techniques resulted in fewer significant associations than did other strategies such as bivariate techniques.
Sedentary behavior and insufficient physical activity.
Only few studies have focused on correlates of sedentary behavior or insufficient physical activity. Consequently, for most results there was insufficient evidence available to draw conclusions. Associations with television- and video viewing were found for gender, body mass index, depression, ethnicity, socioeconomic status, and parental education. Gender (male) was also found to be associated with insufficient physical activity. In a recent review by Gorely et al. (25), in which the correlates of television viewing were studied, ethnicity, parental income, parental education, body weight, number of parents in the house, parents' television-viewing habits, weekend, and having a television in the bedroom were found to be associated with television viewing. In the review by Must and Tybor (41), body weight was found to be associated with television viewing in children younger than age 10 but not in older children around age 12 (41). Both the reviews of Sallis et al. (53) and our review found no evidence for an association between body mass index and physical activity in children and adolescents. Considering the available information, it seems fair to conclude that variables, which are consistently associated with physical activity such as self-efficacy, do not always have the opposite association with sedentary behavior. Physical activity and sedentariness have their own, unique determinants (24,32,56). Neither our review nor the review of Sallis et al. (53) found an association between sedentary behavior and physical activity. This is not consistent with the hypothesis that sedentary behaviors replace physical activity. One should bear in mind that in most studies, moderate to vigorous physical activity was measured. Surprisingly, some authors suggest that sedentary behaviors particularly replace low-intensity physical activity behaviors, such as walking and playing, instead of more vigorous activities such as running and sports (34,57). Studies on time spent on watching television confirm this association (48,49).
Limitations of the review.
A potential limitation of this systematic review is the completeness of the literature search. It is possible that we missed some published studies with other key words or with unclear titles or abstracts. Especially for sedentary behavior, it is possible that we missed some studies because we chose to use key words for inactivity and sedentary behavior only, instead of adding key words for television watching and computer use as well. Better search terms need to be defined to retrieve all studies concerning various sedentary behaviors and insufficient physical activity. The review is limited to published studies, so it may be biased toward significant findings because of publication bias. Another limitation of this review is that the main outcome was overall physical activity, without making a distinction between, for instance, moderate and vigorous physical activity, or physical activity at home versus at school. Therefore, possible differences in correlates associated with the various specific behaviors could not be established. A problem concerning sedentary behavior is the measurement of this behavior. In some studies, sedentary behavior was defined as a low level of physical activity, or insufficient physical activity according a guideline, and in other studies, the amount of television viewing was used as an indicator of sedentary behavior. Only a few studies used a composite measure for sedentary behavior, including behaviors such as television viewing, playing computer/video games, reading, listening to music, and chatting with friends. From a study by Marshall et al. (35), it has become clear that measuring the amount of television viewing alone does not represent sedentariness (35). Other ways in which children and adolescents can be inactive should be taken into account, especially given the increase in the use of computers and video games.
Most of the research conducted on correlates of physical activity behavior has involved cross-sectional designs. Therefore, only the direction of the associations could be established-not the cause and effect. For instance, a high body mass index can be either a cause or a result of a low level of physical activity. Because of the diversity of variables, samples, measures, and analyses, it was not possible to assess the overall strength of the associations.
CONCLUSION
In conclusion, this systematic review shows some evidence for associations between physical activity and the following correlates: gender, self-efficacy, parental physical activity for boys, parental support, parental education, attitude, goal orientation/motivation, physical education/school sports participation, family influences, and friend support. Gender, body mass index, depression, ethnicity, socioeconomic status, and parental education were associated with adolescents' sedentary behavior, and gender was also found to be associated with insufficient physical activity.
Most of the studies included in this review were cross-sectional, and to gain more insight into the determinants of change in physical activity levels or the decline in physical activity during adolescence, more prospective studies are needed. Moreover, further research is needed on the determinants of insufficient physical activity and sedentary behaviors. Sedentariness is not the opposite of physical activity; it comprises several types of behavior (such as watching television and playing video games) with their own determinants. More information on these determinants is needed to develop effective interventions that stimulate children and adolescents to diminish the time they spend on inactive behaviors. An active childhood may be helpful in maintaining an active lifestyle during adolescence and adulthood. Therefore, further research is needed on the determinants of physical activity in children, because most studies are conducted on adolescents' physical activity. From intervention studies, it is known that parental influences, peer support, school-based physical education, and length of physical activity time at school (29,36,43) are important, changeable determinants of physical activity in youth. Several correlates cannot be modified, such as gender, age, ethnicity, parental education, and socioeconomic status; these should be used to target high-risk subgroups that need special attention. To achieve substantial behavior change, preventive interventions should target changes in important determinants from different categories (social, psychological, and environmental) simultaneously.
This research was funded by grants from the European Commission; [email protected], Research Center on Physical Activity, Work and Health, TNO-VU University Medical Center; and the EMGO Institute, VU University Medical Center, Amsterdam, the Netherlands.
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