Modern industrialized societies are witnessing an increase in serious chronic diseases and morbid conditions, such as diabetes (17) and obesity (9), which are associated with the economic and health burdens of cardiovascular disease for European countries (16). An increase in these global epidemics seems to be directly related to adverse behavioral patterns, such as pursuing a sedentary lifestyle. The World Health Organization (30) has estimated that in developed and developing countries around the world, 60-85% of people have sedentary lifestyles.
Several scientific and medical institutions have determined a sedentary lifestyle to be one of the most serious public health problems in modern societies, especially because it affects a growing number of children and adolescents (24,30). To achieve health benefits, efforts to ensure that adolescents participate in regular physical activity (PA) of moderate to vigorous intensity are worth pursuing (22).
Recent years have seen a wealth of epidemiological studies involving children and adolescents that have examined the prevalence of PA by sex, age, race/ethnicity, and socioeconomic status (1,5,7,14,23,25), as well as some studies that have examined an array of characteristics thought to explain the variation seen in PA levels and patterns among adolescents (22). Several studies have shown that males have greater levels of participation in PA than do females (5,7,11,13,23,25,26) and that in the United States and northern Europe, PA declines with increasing age (7,23). No such data exist for southern Europe or Mediterranean countries, particularly Portugal, where important differences may exist in lifestyle behaviors and in environmental, climatic, and geopolitical circumstances that may also influence school-based, sport, and leisure-time PA. For example, throughout the country, physical education is mandatory across all school levels, and a sport school system exists that engages many students during their free time. Another factor relates to the infrastructure of Portuguese cities, with safe and large sidewalks and parks providing ample opportunities for the establishment of active and healthy lifestyles.
The aims of this cross-sectional study are to examine (1) the age- and sex-specific PA patterns of Portuguese adolescents aged 10-18 yr, and (2) whether changes in PA occur during potentially key developmental periods.
For the 2003-2005 school years, we sampled native Portuguese male and female students aged 10-18 yr. We selected students who were enrolled in public and private schools from regions having 66 teachers enrolled in the masters program of the Faculty of Sports of the University of Porto. This sampling scheme was necessary because these well-trained teachers were recruited to oversee aspects of the study, and, most importantly, they were responsible for the systematic administration of the study questionnaires. As a result, these schools were located in rural and urban areas in three regions nearest the city of Porto (Porto, Vila Real, and Viseu) and one island-based region (Região Autónoma Açores).
In Portugal, the majority of basic and secondary schools comprise grades 5-12, which resulted in a total enrollment of 225,149 students from 364 schools in the four regions. We sampled one class per grade from these schools to achieve a good representation of students. Because the number of students in any class in Portugal must not exceed 24 students on average, that is the same number of students achieved per classroom. Our school-based samples represented 18, 35, 18, and 14%, respectively, of relevant schools found in the four regions noted above. The final sample comprised 12,672 students, representing 4, 10, 6, and 10%, respectively, of all available students by region. During the 2 yr of data collection, questionnaires were administered between March and June, to limit any effects of seasonal variation. The rate of completing the physical activity questionnaire ranged from 95 to 99.5% relative to class samples of students (N = 12,577). We should point out that whereas our sample comprises Portuguese adolescents, it may not be fully representative of all adolescents in Portugal. The project was approved by the research committee of the Faculty of Sport Sciences and Physical Education of the University of Porto, and by school authorities. In addition, parents and participating students provided informed consent. The characteristics of the sampled students in terms of age and sex are displayed in Table 1.
PA was estimated with the Baecke etal. (4) questionnaire, which is considered a reliable and valid instrument (19). This questionnaire has been shown to be reliable in different subsamples of the Portuguese population aged 10-18 yr, where intraclass correlation estimates for PA have ranged from 0.80 to 0.87 (26).
Most of the Baecke questionnaire is made up of 16 items that call for a Likert-type response (possibilities from 1 to 5) and that are designed to assess different categories of the broad concept of PA (work/school, sport, and leisure). The work index in the standard version of the Baecke questionnaire is based on eight items intended to assess different aspects of work activities of 20- to 32-yr-old Dutch men and women (4). Because our study was conducted in adolescent students, we reworded these questions to reflect different aspects of their PA at school that were independent of mandatory physical education programs or school-based sport activities. We then calculated the work/school index (WSI) from eight equally weighted items (questions 1-8). Items included asking students how often at school they sat, stood, walked, lifted heavy loads, sweated, and left school physically tired. They were also asked to compare their school activity with that of their peers. The sport index (SI) (questions 9-12) was scored, in part, from the two most frequently played sports, for which the number of hours per week, months per year, and estimated intensity were reported. The SI also included the frequency of overall participation in sport, the frequency of sweating, and a subjective comparison of participation in exercise relative to others one's own age. The leisure-time index (LI) (questions 13-16) summarized the frequency of television viewing (which was negatively weighted), cycling, and walking (which also included time spent walking daily). The score of the three indexes varied from 1 to 5, and, when summed, the result was a total PA index (PAI) whose score ranged from 3 to 15.
Training of teachers in study and questionnaire procedures.
Sixty-six teachers of physical education from the selected regions participated in our study and were responsible for administering the study questionnaires. Participating teachers were 24 yr of age or older, and each received 8 h of training, provided by the principal investigators. The training included an explanation of the study objectives and its design, a reading and explanation of each questionnaire and their associated instructions, and suggestions of ways to address students' questions or difficulties that teachers might encounter. Because the principal investigators emphasized the importance of having complete questionnaires in the classroom setting, there were, fortunately, no missing data.
Descriptive statistics summarized the four PA indexes. Differences by sex across all age groups for each index were tested with a factorial analysis of variance. The Scheffé test for multiple comparisons was used to check for specific differences by age and sex. For each of the four indexes, the average annual rate of change in mean values was calculated by first finding the difference in means between successive ages, then dividing each result by its initial value to get its percent change, and then averaging the summed percent changes by the number of years corresponding to the span of the three age groups (i.e., by 3 yr for the 10- to 13-yr and 13- to 16-yr groups, and by 2 yr for the 16- to 18-yr group). The three age groups correspond to the structure of the Portugal school system: 10-13, second level; 13-16, third level; and 16-18, the secondary level. Statistical significance criterion was 0.05. SPSS 14.0 was used in all analyses.
For all four PA indexes, males had higher mean values than females at every age (Table 2). Whereas males (except for LI, which peaked at age 16) had their highest mean values at age 18 and their lowest at age 10, among females, the highest mean values occurred at age 16 and the lowest at age 10 (for WSI and for LI) or age 11 (for SI and PAI).
All four indexes had significant differences in their mean values by age and sex, as well as a significant interaction between age and sex (P < 0.05). This significant interaction suggests that the age effects in the PA indexes differed between males and females; it can be visualized by referring to Figure 1. Among males, with the exception of LI, mean values of the indexes tended to rise with age; among females, mean values generally started to decline after 16 yr. For LI, females of the youngest age groups had lower mean values than males, with general convergence of this index at older ages.
For the PAI and SI, the mean difference between male and female scores at the various ages ranged from 0.53 to 0.80 and from 0.39 to 0.61, respectively (each difference was significant (P < 0.05)). For WSI, male values were always at least slightly higher than those for females, and significant differences by sex were seen at ages 11, 12, 14, and 18 (differences ranged from 0.06 to 0.13). After age 17, there was a drop for females and an increase (albeit nonsignificant) for males. For LI, among both sexes there was an increase from age 10 to 16, then small decreases. All differences by sex were, again, significant (except at age 18), and these differences ranged from 0.06 to 0.19.
The average annual rate of change in the mean values of each PA index is shown in Figure 2 by sex for the three age groupings. For females, in the 10-13 and 13-16 age groups, the average annual rate of change was positive in seven of eight cases, with increases ranging from 0.7% per year (for PAI) to 1.6% per year (for SI and LI); the lone decrease was −0.2% per year for SI in the 10-13 group. Values for the 16-18 group were all negative and ranged from −1.0% per year (for LI) to −2.1% per year (for SI).
For males, the PAI and WSI were similar in that they showed increases in all three age groupings, which ranged from 0.2% per year for PAI among those aged 16-18, to 1.5% per year for WSI in the 10-13 group; for both SI and LI, the pattern was two increases and one decrease, with the largest decrease being 1.3% per year for 16- to 18-yr-olds in LI. The largest increase was 1.9% per year for 10- to 13-yr-olds in SI.
In this cross-sectional study of adolescents ages 10-18 yr from four regions of Portugal, we found that males had greater levels of PA than did females. We also found in these analyses that mean PA levels increase among both sexes from age 10 to 16 yr, but after age 16, female students decrease their PA, whereas their male counterparts increase it, except for PA during leisure time. Should these cross-sectional results hold true within a longitudinal context, we would consider an age greater than 16 a "very sensitive period" for adolescents, a time during which their PA may often decline.
Our finding that male adolescents in Portugal are more active than their female counterparts accords with the work of several other authors (5,7,11,13,23,25,26). Weinberg and Gould (27) contend that such sex differences may be grounded in social and cultural factors: historically, males assimilate roles based on more active work-related activities, and females assimilate roles directed more toward the family and housekeeping (12,15). Additionally, those authors (27) contend that males are given more permission than females to explore their physical environment, which might explain greater amounts of walking and cycling by males.
Our findings run counter to those from some studies of northern European and American youth (5,7,13,23,25) that show progressive declines in PA levels with increasing age. Two other studies, however, have results that are in many ways similar to ours (7,23). In a longitudinal study of Finnish youth, Telama and Wang (23) report that in a comparison of youth ages 12-15 yr with those ages 15-18 yr, percent declines in the mean PA index became greater with increasing age (rising from 2 to 2.3% in males and from 1.2 to 1.5% in females). Thus, at about 16 yr, the declines in PA became greater. We did not find similar progressive declines in our Portuguese sample from ages 10 to 16 yr. Instead, mean PA values increased by 0.4-1.9% per year for males and 0.7-1.6% per year for females (except for SI). Even so, after age 16, females decreased their PA values by 1-2.1% per year, whereas males increased their PA levels for all indexes except leisure time (−1.3% per year). In the other study, Caspersen et al. (7) report that U.S. youth ages 15-18 yr were at greater risk of declining PA than were those ages 12-15 yr when five different patterns of PA were examined. Between 15 and 18 yr, the authors found annual decreases in prevalence to be greater than 5% for regular sustained and regular vigorous patterns of PA. We found the greatest declines after age 16, which would fit in the 15-18 yr age group. We did not, however, find such dramatic changes for similar ages in our Portuguese sample, but our assessment procedure could not examine such distinct PA patterns as were examined in the study by Caspersen and colleagues (7). Our leisure and sport indexes, which most closely approximate the two activity patterns assessed by Caspersen et al. (7), are also mostly congruent with their overall findings, especially the sport index for females.
Compared with the United States, the absence in our study of an earlier age-dependent decline in PA during adolescence may be attributable to sociocultural factors, different environments, and neighborhood conditions unique to Portugal, as well as a highly pronounced decline in sport-specific participation among U.S. adolescents with age (6,18). In addition, we should note that in Portugal, limited available studies suggest that age-related declines across either childhood or adolescence do not occur (11,26), which may be attributable to the daily routines of Portuguese students that require mandatory physical education classes, involve plentiful school sports programs, and allow free recess time, during which PA may ensue. In fact, during the last 10 yr, a large increase (16.8%) in extracurricular sports programs (which include two or three training practices per week, plus a weekend competition) has been noted (20). During roughly the same period (from 1998 to 2004) in Portugal, Adelino et al. (2) report that the number of sports participants outside of school had increased by 24.3% (from 116,759 to 145,148). Finally, the majority of Portuguese students walk to and from school, and city planners have sought to design safe walkways to support active transportation (8).
Although we do not have any overall findings of declines in PA with increasing age, other authors (28,29) who have noted declines in PA during adolescence have found them to be greater among females than among males. The declines in PA indexes we observed in females after age 16, for which increases were seen in their male counterparts, are somewhat congruent with those findings. Wold and Andersen (28) and Wolf et al. (29) have suggested that greater observed age-related declines among females are grounded in social, psychological, and biological changes, primarily in identity, perceived sexuality, peer pressure, and social environment. This cluster of factors may produce changing attitudes toward participation in PA. Female adolescents frequently abandon or stop doing less important social activities, and they prefer those that are socially relevant, reinforcing, and consistent with established sex rules. The decrease we have noted may have been directly related to an increase in academic work needed to prepare for university entrance examinations. On the other hand, another possible explanation is that the decline might have occurred as a means to compensate for time spent in sport and physical education participation. In contrast to childhood, during which females readily participate in PA of any energy demand, during adolescence females usually prefer activities of low energy demand (13), which may also be viewed as congruent with posited attitudes, roles, and circumstances unique to adolescent females (28,29).
The decrease in the leisure PA index after 16 yr was the only decline we noted for males during this sensitive period. To understand this further, we examined more closely our LI, which was based on four questions regarding usual walking habits, usual cycling, TV viewing, and time spent in walking, and we found that males had a declining prevalence of never watching television and of moderate to intense walking, cycling, and time spent walking between the ages of 14 and 16 yr (data not shown). This tends to correspond with reports from some U.S. authors (3,10) who have noted that during this age period, males reduce the number of physical and sports activities practiced inside or outside of school, instead preferring to spend their free time in sedentary pursuits such as being with friends, watching television, or going to the cinema. Among adolescents from four regions of Portugal, whether for males or for females, as noted above, this decline in leisure activity may occur because of rapidly approaching higher education, as the transition to a university may force changes in a student's interest toward spending his or her free time in less active leisure pursuits.
There are several limitations associated with our analyses. First, our data were based on self-reports, and, thus, they may have resulted in estimates different from what would have been produced by objective assessment, albeit that has its own limitations. Second, we used the Baecke questionnaire, which may yield different estimates of PA than those obtained in studies of similarly aged youth that used different instruments. In addition, some common conditioning activities, such as running and strength training, may not have been fully captured by the sport index of the Baecke questionnaire if the student had not perceived them as sports. Unfortunately, most studies rarely use the same instrument, making the ability to compare their results somewhat tenuous (21). Because children and adolescents are often unable to accurately supply the details regarding the frequency, intensity, and duration of multiple, different types of physical activities (21), it is difficult to declare which instrument is truly the best to use. Despite such nuances between questionnaires, we were at least able to see mean annual percentage declines that were somewhat similar to those reported for Finnish youth (23). There is at least one additional advantage of the Baecke questionnaire in that it offers estimates for sport, leisure, and work/school that are forged from items representing both positive and negative contributions to each index. Third, we used trained teachers to administer the questionnaire and to address students' concerns, which might have induced a potential bias different from that associated with using only study investigators or staff. Fourth, there are potential clustering effects resulting from influences of school and class, for which we could not adjust, given the data available. Finally, we used a cross-sectional design, which may not accurately reflect individual change in PA over time, as would be found with a longitudinal cohort of, say, 12-yr-old children measured repeatedly until age 18. Our cross-sectional design, nonetheless, allowed the gathering of data for young people representing each of 9 yr of age at one time, thereby saving time and money. It also afforded a more consistent assessment methodology free of temporal effects when, for example, climate or other environmental circumstances change over a prolonged period of monitoring. The cross-sectional design has an additional advantage because it averts any intervention effect that might occur from repeated questioning about PA, which would occur with a longitudinal design.
In summary, our findings of greater activity levels among males than among females in these four Portuguese regions, and the decline for females during the sensitive period afterage 16, suggest that efforts be made to establish andmaintain sufficient PA levels among girls and young women as an essential part of a healthy lifestyle. These efforts should involve family, friends, and school staff, particularly the physical education teacher, as important agents of behavioral change.
This study was supported by the Portuguese Foundation of Science and Technology: SFRH/BD/20166/2004.The findings and conclusions in this report are those of the author(s) and do not necessarily represent the views of the Centers for Disease Control and Prevention.
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