The present study shows a consistent decrease in PA level with increasing BMI. Overweight and obese participants had a lower overall PA level, took fewer steps each day, and performed less daily moderate and vigorous PA and MVPA performed in bouts of ≥10 minutes than did normal weight participants. Obese participants also accumulated more sedentary time, compared with normal weight participants.
The results of the present study are consistent with those of studies that used accelerometers to measure PA in large populations of adults and older people. Tudor-Locke et al. (2010) showed that, among Americans, overall PA decreased consistently with increasing BMI and that men had a higher overall PA than women, within each BMI category. The gradient between BMI categories was similar in the present study, indicating that the decrease in overall PA with increasing BMI is a consistent finding. However, only negligible sex differences within each BMI category were observed in our study. Norwegian women are consistently more active than American women, whereas Norwegian men are consistently less active than American males across all BMI categories, independent of age 
. This finding also agrees with Swedish data showing a similar decrease in overall PA with increasing BMI but no apparent sex difference within each BMI category 
The relative differences in PA between BMI categories in the present study were larger for intensity-specific PA than for the indicators of overall PA. Normal weight women performed twice as much MVPA in bouts as obese women. Similar relative differences between intensity-specific PA stratified by BMI have been reported by others 
. The larger relative difference in intensity-specific PA between BMI categories than in overall PA may be explained partly by thermodynamics. Because of the greater body mass, resting energy expenditure is higher in obese compared to normal weight individuals; the greater body mass is associated with a higher metabolic cost of PA for heavier individuals. An accelerometer calibration study showed that the true MVPA intensity threshold is substantially lower for obese compared with normal weight individuals 
. Although the metabolic cost of exercise increases with body mass, we are confident that the differences in PA between BMI categories are real and are important to public health, although care must be taken when interpreting the results for intensity-specific PA. It should also be recognized that BMI category related differences in PA might be underestimated in the present study. A study of PA using pedometers showed that a larger percentage of obese individuals increased their PA compared to those who decreased their behaviour, when monitored over 1 year 
. If a collective behaviour of increased PA among overweight and obese in order to affect weight is picked up in the present study, this might moderate the gradient in the relationship between PA and weight status.
According to the recommendation for PA and public health set forward by the Nordic Councils of Ministers, those who are physically inactive may achieve the greatest health gains of increasing their regular PA, independent of age 
. Although cross-sectional, the linear reduction in relative odds for being overweight or obese observed with higher levels of physical activity indicates the importance of PA to weight management. The odds of being overweight or obese differed by 53–71% between the least and most active quintile of PA and the relationship between PA and risk reductions associated with higher quintiles of PA appears to be about linear.
To our knowledge, BMI related differences in hourly activity patterns of overall PA (counts per minute) across weekdays and weekends have not been examined in large and randomly selected samples of adults and older people. A study of 108 participants by Cooper et al. (2000) showed that although the obese participants were consistently less active than non-obese participants, no significant differences were observed while participants were at work 
. Although time at work could not be identified in the present study, the patterns of overall PA suggest that differences were least pronounced between the hours of 09
00 and 16
00 on weekdays and largest around midday and early afternoon on weekends. Further, compared to normal weight participants, obese participants displayed 19% lower overall PA on weekdays and a 25% lower overall PA on weekends. As the majority of the analysed sample reports working either full time (59%) or part time (11%), the observed larger relative difference in overall PA between obese and normal weight participants on weekends compared to weekdays implies that overweight and obese participants are more likely to pursuit sedentary behaviours when not constrained by work.
The findings of this study must be interpreted in light of the following limitations. We acknowledge the limitations of a cross-sectional design in establishing a causal relationship between level of activity and weight status. However, it clearly shows quantitative differences in amount of PA performed as well as differences in patterns of activity. Further, although BMI is the most commonly used measure to identify and grade overweight and obesity in populations, the method’s reliability had been questioned in individuals at the extremes of age, muscle mass, and height 
. BMI accurately predicts obesity-related morbidity and mortality in epidemiological studies 
, and it provides a reliable and robust estimate of height-independent body fatness. Another limitation is that height and weight were self-reported, which might introduce bias because of the suspected underestimating that occurs when participants self-report body weight 
. In order to control this source of error, trained test personnel measured the weight and height of a randomly selected sub sample of the initial participants (n
904), in a laboratory. The largest discrepancy between the self-reported and objectively measured anthropometrical data was observed for overweight women who on average underestimated their weight by 1.4 kg, indicating that a bias as a result of self-reported weight is not a threat to the validity of the present study. Among men, a small, but significant, underestimation of weight was only observed in the normal weight category (0.44 kg).
We acknowledge that accelerometers are unable to register water activities such as swimming and to accurately assess movement associated with non-ambulatory activity such as cycling 
. To try to account for this potential source of error, participants reported the frequency and duration of cycling and swimming performed during the week of assessment. No significant differences in the total time spent performing such activities were observed between the participants in the different weight categories (data not shown) indicating that the omission of these activities from the accelerometer counts did not affect the results.
Another limitation of the present study is the relatively low participation rate. Given the declining response rates in Norway, and in other countries 
, and the risk for selection bias, it is important to describe the non-responders in studies that attempt to examine samples that are representative of the general population 
; however, such analysis is rarely available 
. Analysis of the non-responders in our study by the use of registry linkage showed that they were more likely to be either at the younger or older end of the age spectrum, unmarried and not of Norwegian origin and had lower educational and income levels, compared to the responders 
. This has also been observed in most population-based surveys 
. Further, the sample included participants from throughout Norway, and the prevalence of overweight or obesity and other non-communicable diseases such as type 2 diabetes was similar to other national estimates. This indicates that the results from the present study have a general validity corresponding to similar studies and that the study sample was fairly representative of the general population in Norway. The study is the first epidemiological study to objectively show differences in activity patterns across weight categories and to demonstrate the contribution of PA to the prevalence of overweight and obesity in Norway.
The worldwide obesity epidemic shows no signs of abating, and, given the health risks and costs of the condition, it is crucial to understand as much as possible about the relationship between PA and weight status. Although we acknowledge that multiple factors other than PA, such as the energy intake, consummation of specific foods and beverages, alcohol use, and television watching. 
, play vital roles in the development of overweight or obesity, we believe that the findings of the present study provides additional information on the relationship between PA and BMI and suggests that there might be a particular scope for targeting the weekend as a source of increased PA among overweight and obese individuals.
Both indicators of overall PA and intensity-specific PA differ between BMI categories and the risk of being overweight or obese increased with decreasing PA level. The BMI category related difference in overall PA is largest on weekends, with obese participants displaying an overall PA level 25% lower than the normal-weight participants. These findings indicate the need for planned interventions to increase the overall level of PA in the population to counteract the environmental forces that are producing a gradual weight gain in the population. The continuing use of accelerometers to monitor longitudinally the level of activity in the general population is vital for identifying the dose response relationship between PA and the prevention and treatment of overweight and obesity.