Having valid and reliable measures of physical activity among young people is critical for identifying dose–response associations with a variety of health, developmental, and cognitive outcomes, and for identifying which aspects of physical activity are important for such outcomes. Moreover, valid and reliable measures are necessary for estimating physical activity participation and monitoring compliance with recommendations, for quantifying the biological and environmental correlates of physical activity, and for assessing the efficacy and effectiveness of interventions to promote physical activity behaviour.1
In the development of national physical activity recommendations for young people,2, 3, 4, 5, 6, 7 most scientific background reports identify a lack of dose–response evidence,3, 6, 7 which is often attributed – in part – to an inability to accurately measure both the behaviour of physical activity and its predictors. In this special issue of the Journal of Science and Medicine in Sport, several articles on assessment and methodological issues in young people's physical activity that arose from the Sixth National Physical Activity Conference held in Adelaide, Australia in 2007 (which was jointly sponsored by the National Heart Foundation of Australia) were submitted and peer-reviewed to provide an international update on research in this important area.
Knowing what physical activity assessment instruments to use for which age group and socio-demographic characteristic, and under which circumstance, can be challenging as the article by Dollman and colleagues illustrates. Dwyer, Baur and Hardy argue that very young children's physical activity is unique as they engage in short sporadic bursts of high-intensity activity followed by brief periods of rest. Further, unlike adults, much of their physical activity is acquired through informal play and games and may therefore difficult to capture using self- or proxy-report retrospective or recall measures. Adolescents’ physical activity is more similar to that of adults, however, their activities are still unique (e.g., school-based physical activity and sport).6 As argued by Stathi, Gillison and Riddoch it is therefore important that future research into young people's physical activity overcomes previous limitations, such as the lack of specificity of measures of different types of activity. An original study by Veitch, Salmon and Ball addresses the issue of specificity by reporting the reliability and validity of a new parental proxy-report measure to assess the frequency with which young children play in various outdoor locations. Also using a survey methodology, Ridley and colleagues examine the intra-individual variations in children's physical activity across days of the week based on a computerised time-use instrument which asks children to report everything they did from the time they woke up to the time they went to bed on the previous day.
The challenges in assessing children's physical activity using survey instruments has led to an increase in popularity of objective measures, such as pedometers and accelerometers. With a burgeoning product market, selecting the right instrument to use can be difficult. Mcclain and Tudor-Locke provide an overview of accelerometers and pedometers currently available and most commonly in use; and Cliff, Reilly and Okely discuss issues to consider when using accelerometry in young children (e.g., how many days of monitoring are needed, epoch length, etc.). The review by Duncan, Badland and Mummery describes the use of global positioning system (GPS) devices that provide an extra dimension to the assessment of physical activity data collection. Duncan, Badland and Schofield report on the findings of an original study to assess the feasibility of combining GPS and heart rate monitoring for assessing children's physical activity.
In addition to having accurate and reliable measures for assessing young people's physical activity, it is also important to be able to assess the factors that influence these behaviours. Such influences may take the form of correlates or determinants (factors that explain variance in or predict behaviour) or may moderate behaviour (e.g., sex, age, environmental factors), or may provide a pathway or avenue for mediating changes in behaviour (e.g., enjoyment, self-efficacy). Brown, Hume and Chin A Paw describe the psychometric properties of measures of potential mediators of children's physical activity. The validity of Stage of Change theory constructs across varying intensities of physical activity among young people is reported by Haas and Nigg. A further original study by Rose and colleagues assesses the validity of a scale to assess young children's self-reported attraction to physical activity. Cutumisu and Spence describe a unique technique for quantifying urban form influences on spatial behaviour called ‘space syntax’; a methodology that was recently introduced to public health from the field of architecture.
It is hoped that this Special Issue, which features commentaries, reviews and original studies from experts in the field of young people's physical activity from around the world (Australia, Canada, Germany, the Netherlands, New Zealand, the United Kingdom, and the United States of America) provides useful and up-to-date information on the methodological challenges and some potential solutions for assessing the influences on and participation in physical activity among young people.
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