Our results indicate that the PA questionnaire in HUNT 1 is reproducible and provides a useful measure of leisure-time PA for men. The questionnaire is very short, and compared favourably with much longer instruments for assessment of more vigorous PA. It should be an appropriate tool for use in further epidemiological studies, particularly when the interest is in aspects of PA reflected in fitness or METs greater than 6.

Background: There is no standardized method for the assessment of physical activity (PA). Therefore it is important to investigate the validity and comparability of different measures. The International Physical Activity Questionnaire (IPAQ) has been developed as an instrument for cross-national assessment of PA and has been validated in 12 countries. These instruments have acceptable measurement properties for monitoring population levels of PA among 18-65 yearold adults in diverse settings. However, there are some concerns that IPAQ may over-report PA.

Background: To create and find accurate and reliable instruments for the measurement of physical activity has been a challenge in epidemiological studies. We investigated the reliability and validity of two different physical activity questionnaires in 71 adolescents aged 13-18 years; the WHO, Health Behaviour in Schoolchildren (HBSC) questionnaire, and the International Physical Activity Questionnaire (IPAQ, short version).

The HUNT 2 question for "hard" LTPA has acceptable repeatability and appears to be a reasonably valid measure of vigorous activity, as reflected in moderate correlations with several other measures including VO(2max), and with corresponding results from IPAQ and ActiReg. The HUNT 2 question on occupational activity had good repeatability and appears to best reflect time spent in moderate activity, with moderate associations with measured time at intermediate intensity levels. The "light" activity question from HUNT 2 had poor reproducibility and did not correlate well with most of the comparison measures. Thus, the "hard" PA and the occupational activity question should be useful measures of vigorous PA, if time and space allow only very brief assessment. The utility of the "light" PA questions remains to be established.