Invited commentary: comparing physical activity across countries--current strengths and weaknesses.

Physical activity is thought to be important for various health outcomes, but population levels are suspected to be low. There is a lack of large-scale comparable data with which to assess temporal trends and make between-population comparisons. Continued increases in the use of objective monitoring, especially in longitudinal studies, would be very valuable in public health research, and both self-reported and objective data may help to start developing explanations regarding any observed population differences. There is much scope for more international surveillance of physical activity levels using historically comparable measurement tools, as well as making current data available for reanalysis. The continued use of objective measurement tools with transparent research protocols and data reduction strategies would also be beneficial for future research. Prospective objective physical activity data across different countries would allow us to learn from areas successful in maintaining or even increasing population physical activity levels. Physical activity surveillance using objective measures is needed worldwide, not only in Western countries but also in developing countries, as obesity and related metabolic disorders are a global problem, and it is therefore appropriate that the solution is similarly large scale in nature.