Our results suggest that approximately 20% of 16-17-year-old boys and girls fulfilled the current WHO recommendations for PA. Boys were more physically active than girls, as they accumulated more minutes in MVPA and higher CPM. However, steps per day were similar between boys and girls. Moreover, both boys and girls were more active during weekdays than weekends.
Our results are at large in accordance with other studies assessing PA by accelerometry in adolescents (10, 18-20). A challenge when comparing different studies of accelerometer measured PA is the lack of standardization of cut-points for intensity categories (18). For example, the lower cut-point for MVPA ranges from 1000 CPM to 3000 CPM (18), affecting comparison between studies. A cross-sectional study by Ruiz et. al. (2011) including nine European countries (the HELENA study) using compatible cut-points for MVPA showed that 41% of adolescents (mean age 14.9 years) met the recommended activity levels (27.5% of the girls and 56.8% of the boys) (31). These proportions are substantially larger than in our study, but the HELENA study included a wider age-span and the sample was somewhat younger than ours. A recent review suggested that the compliance with meeting PA recommendations ranged from 0-60%, depending on intensity threshold used (18), emphasizing the need for data harmonization for cross-study comparisons.
The higher activity levels in boys in our study is consistent with previous studies (18, 20, 32). Even though boys and girls accumulate about the same amount of steps, there is a general agreement that MVPA is essential for health benefits (33), and step counts do not assess the intensity of PA. The difference between girls and boys in this study seems to be more similar to national studies performed on children and adults (9, 10), than to international studies performed on adolescents (18, 31, 34).
We expected the PA levels in our sample of adolescents aged 16-18 years to be lower than in younger children but higher than in adults. However, we found that the mean CPM in our sample was similar as that previously observed in Norwegian adults (9, 35). Here, a decline in PA of 30 % in females and 35 % in males between adolescents aged 15 years and adults between 20 and 64 years of age, was found (9). Although speculative, comparing these results with those from this study, suggests that this decline occurs at the age of 16 to 18 years, when adolescents move from lower secondary school to upper secondary school.
We found lower PA during weekends compared to weekdays, which is in line with previous studies (18, 32). Also worth mentioning is that the variation is larger during weekends, as some of the adolescents increase their activity.
The positive association between self-perceived health and PA is consistent with the findings in several other studies (36-40). This is a young and physically healthy population, but despite this we found a significant correlation between the level of PA and self-perceived health status. This study did not investigate causality, and it is therefore not possible to ascertain the direction of this association. Nevertheless, a low level of PA might contribute to a lower health status over time, which again may lead to even less PA.
We have not been able to find other studies comparing levels of PA in different school programs. It might be considered obvious that students in a sports class are more physically active than peers in general studies and vocational studies. This raises the question of whether these students are more active because they are attending a sports study program, or if they attend the sports study program because they lead a more active lifestyle. The two are not mutually exclusive. This study did not differentiate between school time and after school activity. However, several studies imply that increased PA during the school day increases total PA (41-43). We consider our result to be in accordance with these studies.
Strengths and weaknesses
We consider the high participation rate and the objective PA measurements as the main strengths of our study. The ActiGraph wGT3X has high validity compared with self-reported PA (44) and compared with other accelerometer devices (45, 46) and is used in several other cohort studies (10, 18, 19, 47). However, accelerometer measurements have limitations, such as being unable to accurately assess the intensity while graded walking, carrying loads such as groceries or a rucksack, and cycling (48). Recommendations for PA for both children, adolescents and adults include strength conditioning exercises, and many adolescents tend to shift from team-sports to gym based strength exercising (49), which is not measured accurately by accelerometry (50). The accelerometer was mounted on the hip with a belt and was removed when sleeping and during water activities. This may increase non-wear time if participants forgot to attach the monitor after these activities. Therefore, continuous 24-hour measurements with waterproof equipment are preferable. We chose to use the uniaxial data to be able to compare our results to previous studies.
PA levels tend to fluctuate during the day, week, and between seasons. A limitation of this study is that the measurements were done during one single week, and do not capture seasonal variability. Previous studies have documented lower PA levels during the winter and during periods with poor weather conditions (51-53). In Norway, and particularly in the northern part with substantial difference in temperature and daylight between winter and summer, it is likely that the seasonal variability affects PA levels. The measurements in our study were conducted between September and May, covering 3 seasons. However, for practical reasons students from the same school and study program were measured during the same period. Although the difference between study programs were as expected, it precludes robust analyses of the influence of season.