All students in grade 10 (aged 15–16 years, later referred to as 15 years) in Oslo during the school years 1999–2000 and 2000–2001 were invited to enter the youth part of the Oslo Health Study. This was a questionnaire study conducted in schools. All parents received written information, and the students completed a consent form before participation. The students completed questionnaires during two school classes. For those not present on the day of the survey, questionnaires were left for them at school. A copy was mailed to the home address of those who did not respond, together with a stamped return envelope. A more detailed description has been published elsewhere 2930. From the total population of 10^th graders (both 1999–2000 and 2000–2001), 7343 (88%) participated. The 3811 subjects participating in the year 2000–2001 comprised the baseline of our longitudinal study. During the school year 2003–2004, a second study was conducted with the same adolescents.

The follow-up study was carried out partly as a school-based survey and partly through mail, as described elsewhere 3031. The procedure for the school-based part of the study was similar to the baseline procedure. All 32 secondary high schools in Oslo took part, and the senior year students (aged 18–19 years, later referred to as 18 years) completed the questionnaire during one school class. The participants in the baseline study (year 2000–2001) who were not enrolled in the senior year of secondary high school in Oslo and who had consented to participate in the follow-up were invited to participate by mail.

Of the 3811 participants in the baseline study, 2489 (65.3%) participated in the follow-up and gave their consent to link their data between the two surveys. Only the adolescents who participated in both surveys were included in our analyses (1,112 boys and 1,377 girls) (Figure 1). Of the participants in the follow-up, 20% had an ethnic minority background. Ethnic minorities were defined as those having both parents born in a country other than Norway 32. The parents' countries of birth were provided by Statistics Norway and were linked to the data file. Most of the ethnic minority youth (96%) came from non-Western countries. The largest ethnic minority groups were from the Indian subcontinent (42%), the Middle East (16%), and Eastern Europe (11%). Fifty-four percent of the minority group were born in a country other than Norway (first-generation immigrants).

Both protocols were evaluated by the Regional Committee for Medical Research Ethics and were approved by the Norwegian Data Inspectorate. The baseline study and the part of the follow-up study carried out in the schools received approval from the school authorities in Oslo.

Seventy percent of the ethnic Norwegian students and 54% of the ethnic minority students participated in the follow-up study. In ethnic Norwegian participants, baseline physical activity did not differ between those who participated in the follow-up and those who were lost to follow-up. In the ethnic minority participants, physical activity level at baseline was higher in those who completed the follow-up than in those lost to follow-up. To test whether the physical activity level among the immigrants could be biased by the length of stay in Norway, we compared the physical activity level of those lost to follow-up and in those participating at both times in first- and second-generation immigrants. Weekly hours of physical activity by both boys and girls did not differ between first- and second-generation immigrants. To predict how attrition might bias the results, others have shown that subjects who participate after reminders are fairly similar to the non-responders 36. In our analysis, the change in physical activity did not differ significantly between adolescents from ethnic minorities who participated after the first invitation and those who participated after reminders [mean difference in change in weekly hours of physical activity (SE), -0.28 (0.59), p = 0.59].

The data were stratified by sex and partly by ethnicity (ethnic Norwegians and ethnic minorities). To compare physical activity levels at baseline (T1) and follow-up (T2), the change in physical activity from baseline to follow-up (T2-T1), and the associations between physical activity and socio-demographic factors at the two times, we classified hours per week as a continuous variable and analysed the data using analysis of variance (ANOVA). The patterns of association were compared between ethnic Norwegians and ethnic minorities by testing the interaction terms.

Cohen's kappa and percentage of agreement were used to study the stability in physical activity (0–2 hours versus 3 hours or more) from baseline to follow-up in the four subgroups.

Finally, we studied the association between socio-demographic factors and change in physical activity level (T2-T1) using a linear regression model. The estimated regression coefficients (β) and 95% confidence interval (95% CI) are presented in the text. The socio-demographic factors that showed associations with physical activity at T1 or T2 were entered collectively in the adjusted model. Only participants who provided information on all variables in the adjusted analysis were included in the crude comparisons. Weekly physical activity level did not differ significantly between participants excluded and those included in this analysis. The level of significance was set at p < 0.05. The data were analysed using SPSS version 14.

Boys were more physically active than girls at both 15 and 18 years, and this was independent of ethnic background (Table 1). We found ethnic differences in physical activity levels in girls but not boys. At both ages 15 and 18 years, ethnic Norwegian girls were more physically active than ethnic minority girls (Table 1).

Ethnic minority adolescents had parents with lower income and lower education level than ethnic Norwegians (p < 0.001). Compared with their Norwegian counterparts, more ethnic minority boys and girls lived in eastern regions of Oslo and had parents that were married/cohabiting (both sexes p < 0.001). Ethnic minority girls perceived poorer family economy than did ethnic Norwegian girls (p = 0.01). No difference was observed in boys (p = 0.38).

Physical activity level measured as weekly hours declined in all groups (Δ, change) from age 15 to 18 years (Table 1). The change in physical activity level from age 15 to 18 years did not differ between ethnic Norwegian and ethnic minority boys or girls (Table 1).

More ethnic Norwegian girls and boys were physically active for three hours or more at both times (stability) than were ethnic minority girls and boys (Figure 2). Eighty-two percent of the ethnic minority girls had a persistently low physical activity level (0–2 hours per week) at both times. Fifty-four percent of ethnic minority girls who were physically active for more than three hours per week at age 15 years reduced their activity level at age 18 years (Figure 2). The kappa (κ) values for stability of physical activity level are presented in Figure 2 according to sex and ethnicity. The κ values varied between 0.15 and 0.31 for both sexes and ethnic groups. The lowest κ value was seen in ethnic minority boys.

We found similar associations in the crude and adjusted analyses for the association between socio-demographic variables and change in physical activity analysed by sex. The only significant associations found in the adjusted analyses were that physical activity level declined more in girls with mothers with compulsory education than in girls with mothers with tertiary education [β, -0.75; 95%CI, -1.38, -0.12)], and declined more in boys with fathers with low income than in boys with fathers with high income [β, -0.90; 95% CI, -1.66, -0.14).

The association between socio-demographic factors and change in physical activity level did not differ between ethnic Norwegians and ethnic minorities (i.e. no interaction).

In our study, boys were more physically active than girls, as has been reported earlier 6737. We found ethnic differences in activity level, an observation that is consistent with previous findings from the UK 20 and USA 1318, although other studies in the USA reported no differences 21 or higher physical activity level in some ethnic minority groups 16. The difference between the USA and Europe may reflect differences in the immigrants' ethnicity, history, and time since immigration. Discrimination and racism are other factors that might cause differences in sport participation between ethnic minority and host adolescents 3839. Fear of being exposed to racism could be keeping ethnic minorities away from organized sport 40. However, our results do not allow us to conclude whether racism influences the choice of physical activity type.

As observed in other studies 1318, we found that ethnic differences in physical activity were more apparent in girls than in boys. Differences in physical activity may reflect the influence of factors such as the religion and culture in the country of origin. In our study, 96% of the ethnic minorities came from non-Western countries, the largest group being from the Indian subcontinent. The difference between boys and girls could relate to gender segregation in some religions (Islam), girls having more household responsibility, or stricter rules set by parents 414243. The ethnic minority girls' low activity level might also relate to the structure of organized sports in Norway, which includes fewer differences according to gender than in many of the ethnic minorities' countries of origin 44. Another contributing factor could be the low physical activity level among ethnic minority women in Oslo 45. Parents' physical activity patterns probably influence their children's physical activity through modelling, social influence, and social support 1.

Consistent with other studies, 242526 we found few significant associations between socio-demographic factors and physical activity level. Such associations seem to be dependent on the SES measure used and the characteristic of the subgroup being studied.

One relatively consistent finding in our study was the lower physical activity level among ethnic Norwegians who perceived family economy as poorer than other Norwegian families. Those who perceived themselves as having low family income might be restricted in their physical activity choices and opportunities because of the cost involved.

The lack of association between physical activity and SES in ethnic minorities may indicate a different relationship between SES and health and disease, and a different influence on health behaviour in ethnic minorities than in ethnic Norwegians. A Norwegian study focusing on determinates of diabetes in different ethnic adult groups reported a negative association between the prevalence of diabetes and SES among ethnic Norwegians and Westerners but almost no association in ethnic minorities 45.

We found a decline in mean hours per week of physical activity from age 15 to 18 years, which was similar in ethnic Norwegians and ethnic minorities. In the UK, Asian adolescents and black girls are less active than white girls at age 11–12 years, and this difference does not change over the next five years 20. McMurray et al. 16 observed a similar decline in physical activity from age 8 to 16 years in African-American and Caucasian girls but a greater decline in Caucasian boys than in African-American boys. Another longitudinal study from the USA 13 found a substantial decline in physical activity level that was higher in black girls than in white girls.

The relatively low κ scores in our study suggest low stability of physical activity levels within groups. Anderssen et al. 12 used a similar physical activity questionnaire with youths in the western part of Norway and reported κ values, based on tertiles, of 0.26 for boys and 0.21 for girls over a three-year period (age 16–19 years). The only study that assessed physical activity stability in different ethnic groups (Caucasians and African-Americans) reported low κ values (0.03–0.22) 16. The discrepancy in results might be because κ appears to be higher for shorter time periods, whereas our study and the study by Anderssen et al. 12 studied stability over a three-year period, and McMurray et al. 16 studied stability over seven years. We also dichotomized physical activity level at two times, whereas the other studies divided the physical activity variable into three groups.

McMurray et al. 16 also reported that more Caucasian than African-American youth remained in the low physical activity group during the follow-up. In our study, the highest percentage of persistently low physical activity was found among the ethnic minority girls. As suggested previously, the different findings might be explained by the different immigrant histories and cultures of origin of the ethnic minorities in the USA and in Norway. The discrepancy in results may also be caused by different sample sizes, age, and the definition of "low physical activity".

The only socio-demographic factors that were associated with change in physical activity level were mother's education in girls, and father's income in boys. The few associations observed might reflect the influence of more important circumstances experienced by this group of youth. These adolescents had experienced changes in schools, increasing homework level, and the biological, social, and psychological changes that accompany puberty. These factors, alone or in combination, may affect physical activity significantly and might "overrule" the importance of socio-demographic factors.

Measuring physical activity level by questionnaire is associated with difficulties 46. The measure used to capture physical activity level in this study is general: "activities outside of school that make you feel sweaty and out of breath". Such a crude overall measure does not capture all the physical activities that promote health. However, simple, self-reported questions on overall physical activity have been used in several studies and correlated significantly with other activity measures 4748, and with other indicators of physical activity such as maximal oxygen uptake 49 and physical fitness 50. It seems reasonable to assume that the question captures the level of physical activity, although we do not know how accurate the adolescents were in reporting hours per week. However, we compared associations, change in, and stability of physical activity between groups and believe that the measure is reasonably reliable and valid for this purpose. Generally, the more unreliable the measure, the greater the chance of not finding differences or associations that exist. Therefore, we probably underestimated rather than overestimated any associations and difference between the groups.

Performing a large number of tests, as we did when studying the relationship between physical activity level and socio-demographic factors (aims 2 and 4), increases the risk of type 1 error. Hence, we emphasize the pattern of our findings and interpret the single significant associations with caution.

We also note the variety of ethnic minorities represented by our sample. Even though about 96% of the ethnic minorities in our study population were from non-Western countries, they were not a homogenous group. Studying all ethnic minorities together might conceal differences in physical activity levels between different ethnic groups.

The attrition is also a concern. Of those participating in the baseline study, 70% of the ethnic Norwegians and 54% of the ethnic minorities participated in the follow-up. Ethnic minorities participating at follow-up reported more physical activity at baseline than did the ethnic minorities lost to follow-up; there was no corresponding difference at baseline in the ethnic Norwegians. However, in the ethnic minorities, the physical activity level did not differ between those who completed the questionnaire after reminders compared with those who participated after the primary invitation, indicating that the selection probably did not influence the results substantially 36.