For both genders, daily MVPA and VPA exhibited a significant inverse relationship with grade level. Among male subjects, the average grade group difference (relative to the previous grade level) for MVPA was 35.1%, with the largest difference (40%) occurring between grades 1–3 and 4–6. For VPA, the average grade group difference was 31%, with the largest difference (45%) occurring between grades 4–6 and 7–9. Among female subjects, the average grade group difference (relative to the previous grade level) for MVPA was 35.3%, with the largest difference (48.8%) occurring between grades 1–3 and 4–6. For VPA, the average grade group difference was 37.9%, with the largest difference (55.6%) occurring between grades 1–3 and 4–6.
Means and standard deviations for the weekly number of 5-, 10-, and 20-min bouts of MVPA are shown in Table 2. Across all grade groups, boys exhibited more sustained bouts of MVPA than girls; however, only the differences observed among students from grades 1–3 and 4–6 reached statistical significance. For both genders, weekly 5-, 10-, and 20-min bouts of MVPA exhibited a significant inverse relationship with grade level, with the largest differences occurring between grades 1–3 and 4–6. For boys, significant differences (relative to the previous grade group) were noted in grades 4–6 and 7–9. For girls, significant differences in weekly 10- and 20-min bouts of MVPA were noted in grades 4–6, whereas the number of 5-min bouts differed significantly in girls from grades 4–6 and 7–9.
Means and standard deviations for weekly 5-, 10-, and 20-min bouts of VPA are shown in Table 3. On average, boys and girls from all grade levels exhibited few bouts of VPA over the 7-d monitoring period. Across all grade groups, boys tended to perform more bouts of VPA than girls; however, only the differences observed among students from grades 1–3 and 4–6 for vigorous 5- and 10-min bouts reached statistical significance. Weekly bouts of VPA tended to be higher in elementary school students than middle school and high school students, but only the difference observed in boys from grades 4–6 and 7–9 and girls from grades 4–6 for vigorous 5-min bouts reached statistical significance. When bouts were defined in a more lenient manner (≥90% of the time in activity), the average number of 10- and 20-min bouts of MVPA increased by approximately 75%; however, the trends with respect to gender and grade group were almost identical to those observed for continuous 10- and 20-min bouts. Inclusion of the interruption interval had little to no effect on the number of vigorous bouts.
No previous study has employed a state-of-the-art accelerometer to evaluate age and gender physical activity differences in a population-based sample of children and adolescents. Our results strongly support the concept that physical activity declines rapidly during childhood and adolescence. However, contrary to previous reports (3,11,18,19,22,24,27), we observed the greatest age-related differences to occur during elementary school, rather than during the teen years. This discrepancy is perhaps due to the small number of studies and/or the small samples that have been studied at younger ages. Across all grade groups, boys were more active than girls; however, for overall physical activity, the magnitude of the gender differences was noticeably smaller than that reported by studies using self-report methods to assess physical activity.
The results of this study highlight the utility of objective monitoring devices in field-based studies involving children and adolescents. The CSA accelerometer, which can detect and store movement on a real-time basis, is a particularly effective assessment tool because it provides reliable information about physical activity patterns within a given day or over several days. Moreover, the availability of age-specific count cutoffs that take into consideration age-related differences in economy and resting metabolism (16,17) allows for the estimation of daily participation in moderate and vigorous physical activity.
Traditionally, objective measures of physical activity have been used in smaller descriptive studies (9,10,21,24). However, our success in collecting 7-d of complete monitoring data from approximately 400 youth ranging in age from 6 to 18 yr indicates that accelerometers are feasible alternatives to self-report methods in moderately sized population-level surveillance studies.
Across the four grade groups, boys were consistently more active than girls. However, it is informative to examine the gender differences in physical activity in the context of exercise intensity. For daily VPA, the average gender gap was substantial at around 45%. In contrast, the average gender difference for MVPA was quite modest, at around 11%. This indicates that the boys and girls within our sample did not differ markedly with respect to daily participation in moderate physical activity and that the majority of the gender gap in overall physical activity was accounted for by the girl’s low participation in vigorous-intensity activities. Other studies have reported similar findings. van Mechelen et al. (27) evaluated physical activity behavior in a cohort of Dutch youth over a 15-yr period, beginning at age 13 and ending at age 27. During adolescence (age 13–16), girls reported significantly greater participation in moderate-intensity physical activity than boys; yet, by virtue of their low participation in vigorous physical activity, girls were found to have significantly lower levels of total physical activity. Fuchs et al. (8) longitudinally examined physical activity behavior in a population-based sample of German children in grades 6 and 7. Boys reported significantly greater participation in total physical activity, despite the fact that participation in moderate physical activity was similar for boys and girls. The consistent observation that boys participate in substantially greater amounts of vigorous physical activity than girls underscores the need of physical activity intervention programs for girls of all ages.
In agreement with previous studies, we observed elementary school students to be significantly more active than their middle and high school counterparts (19,20,24). However, the magnitudes of the grade level differences were not consistent, with the largest differences occurring between grades 1–3 and 4–6. For students in North America, this time interval corresponds roughly to late elementary school. Previous descriptive studies have reported the greatest declines to occur during middle adolescence, or more specifically, during the transition form middle school to high school (3,11,22,27). However, it is important to note that none of these studies included children under the age of 9 and none used objective measures of physical activity. One longitudinal study that employed an objective measure of physical activity was the 10-yr NHBLI Growth and Health Study (11). In that study, participants wore a Caltrac accelerometer for 3 consecutive days during years 3, 4, and 5 of the study. During this period, which corresponded to the ages of 11–14, physical activity, as measured by the Caltrac, activity declined by 21%. Interestingly, this figure compares favorably with the observed 27% difference in MVPA between girls in grades 7–9 and 10–12 in the present study.
Over the course of a week, the children and adolescents in this study performed very few sustained bouts of physical activity. Indeed, for vigorous physical activity, the mean number of weekly 20-min bouts approached zero for all grade levels. Participation in sustained bouts of MVPA was also relatively low, with girls in grades 10–12 averaging less than a single 20-min bout of MVPA a week. There was, however, a clear trend for greater participation in the shorter 5- and 10-min bouts of MVPA. Previous descriptive studies have reported similar findings. Armstrong et al. (1) utilized continuous heart rate monitoring to assess the physical activity patterns of British school children aged 11–16. On weekdays, 77% of boys and 88% of girls demonstrated no 20-min bouts of MVPA, with these percentages increasing to 88% and 96.7%, respectively, on Saturday. In contrast, 72.9% of boys and 87% of girls exhibited three or more 5-min bouts of MVPA during weekdays. In similar fashion, Baranowski and colleagues (2) observed no 20-min bouts of MVPA in 24 third- to sixth-grade children during 48 d of direct observation. The consistency of this finding across studies and modes of activity assessment highlights the need to consider the unique activity patterns of children and adolescents when devising physical activity recommendations or guidelines for this population (15).
The present study had several limitations that warrant mention. First, our recruitment rate was relatively low (36.6–40.3%). Consequently, we are unable to completely rule out the possibility of selection bias. However, there is little reason to believe that children with markedly different physical activity profiles would have chosen to (or not to) participate in the study. Second, our use of grade levels rather than specific age groups may have lead to some loss of sensitivity, particularly for the younger groups. Third, it is important to note that accelerometers do not accurately assess exercise intensity during nonweight-bearing activities such as cycling and may not be sensitive to many of the complex movement patterns exhibited by youth. Therefore, our estimates of MVPA and VPA probably underestimate the true level of physical activity.
In summary, objective monitoring of physical activity in population-based samples of children and adolescents appears to be a feasible alternative to traditional self-report methods. Future studies should employ objective physical activity measures in larger, more diverse representative samples of youth, taking care to include subjects from minority groups and children from younger age groups. Furthermore, studies examining the mediators of age- and gender-related differences in objectively measured physical activity (e.g., differences in the quality and quantity of physical education) are warranted. Clearly, the cross-sectional design of this study precludes our ability to make definitive conclusions regarding age-related trends in objectively measured physical activity behavior. Consequently, longitudinal objective monitoring studies with long-term follow-up are warranted.
This study was supported from a grant from the Cowles Media Foundation. The authors wish to thank Edward Debold and Devra Hendelman for their assistance with data collection.
Address for correspondence: Stewart G. Trost, Ph.D., School of Human Movement Studies, The University of Queensland, Brisbane, QLD 4072 Australia; E-mail: firstname.lastname@example.org.
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Keywords:© 2002 Lippincott Williams & Wilkins, Inc.
CHILDREN; EXERCISE; ACCELEROMETER; ASSESSMENT