International comparisons of key non-communicable disease (NCD) risk factors, such as obesity and tobacco use, are commonplace [19, 20], but comparisons of physical activity prevalence among countries have not been possible due to the lack of standardised and validated instruments. Previous reviews have demonstrated very different prevalence estimates across countries, and these differences were as likely to be due to variations in questions and survey methods as they were to true differences in prevalence [21, 22]. The present study reports population-level prevalence estimates and patterns of physical activity in a diverse set of countries, using a comparable, reliable and validated survey instrument. In this study, the IPAQ short form was administered to over 52,000 adults aged 18–65 years, using a standard protocol in 20 countries.
The results show substantial variation in the population estimates of meeting the IPAQ "high active" category, a threshold developed to reflect an amount greater than than standard recommended levels, but more suited for use with a multiple domain measure such as IPAQ. Eight of the 20 countries had "high activity" rates over 50%, and these countries came from several continents. We also observed different patterns by gender and age, with most countries showing younger men more active than younger women, but this gender difference was less marked among older adults. Further, in these countries, age-related declines in physical activity were much more frequently observed among men than among women.
As shown in Figure 1, countries achieved high physical activity in different ways, with four of the most active countries (Australia, Canada, New Zealand, USA) showing a greater volume of vigorous-intensity physical activity relative to moderate-intensity activity and walking. These same countries seem to have relatively well developed facilities for recreational activity and a history of long-term promotion of exercise. Four of the countries with substantial rates of high physical activity had more than 30% of overall physical activity derived from walking (Canada, China, Colombia, Czech Republic) suggesting that countries with an infrastructure or culture that supports walking can achieve high levels of physical activity with lesser contribution from vigorous activity. However, substantial proportions from walking (Hong Kong SAR, Japan, Spain, Taiwan) and vigorous activity (Belgium, Brazil, Taiwan) were also found in countries with low overall physical activity prevalence rates (< 30% in the 'high' category), so there is no indication that an emphasis on promoting one domain of activity will lead to high levels of overall physical activity at the population level. One conclusion from these results is that different patterns of physical activity are associated with high prevalence estimates, so countries could tailor physical activity promotion strategies to local infrastructure, available programs, and culture.
There are a few European multi-country physical activity surveys that provide a context for interpreting present results. The 2002 Eurobarometer study used the IPAQ instrument  and identified low prevalence estimates for Belgium and Sweden, similarly to IPS and reported the highest levels of physical activity in the Netherlands and Germany (these two countries did not participate in IPS). However, earlier European-only studies had observed slightly different rankings [14, 23]. A Pan-European Union Survey on Consumer Attitudes to Physical Activity had found a similar low prevalence of activity in Belgium and Portugal, but reported a high prevalence rate in Sweden . Another study used information from the 2002–3 WHO World Health Survey to collect IPAQ short-form physical activity data from 51 countries, mostly from population samples in developing countries . Physical inactivity prevalence data from the four comparable countries were remarkably similar to the findings in this IPS study; for example, inactivity rates were low for China and the Czech Republic [around 10% in both the IPS and the World Health Survey], and high levels of inactivity reported in both surveys for Brazil [close to 30% inactive in both studies], and close to 25% for Spanish adults in both studies. The similarity of these estimates was noteworthy, despite different survey methods, suggesting that IPAQ data may be consistent, at least within country.
One interesting further comparison can be made from representative cross sectional data of adolescents in Europe and North America through the contemporaneous 2001–2002 Health Behaviour in School-aged children survey (HBSC) . Looking at the group closest to the IPS age group (the 15 year old samples) the countries with the most active boys and girls included USA, Canada, Czech Republic and Lithuania in the upper quartile, and Belgium, Norway and Portugal in the lowest quartile. This distribution for physical activity among adolescents was similar to that observed in the IPS among adults from the same countries.
The findings from this study indicate that the majority of the population in most participating countries or regions appeared to undertake at least a moderate amount of physical activity when assessed using the multi-domain IPAQ. This suggests that most adults in these countries are obtaining some activity, yet the global problem of rising prevalence of obesity remains. Thus, it appears total physical activity in most countries remains insufficient to ensure energy balance and prevent obesity  or that the ratio of energy expenditure to dietary intake is unbalanced to maintain weight stability.
Strengths of IPAQ include its measurement of multiple domains, and the separate assessment of walking behaviour, compared to many current PA surveillance systems . There are some limitations with IPAQ, including difficulties with respondents in distinguishing moderate and vigorous activities. It is also well recognised that self-reported measures can over-estimate physical activity , and the IPAQ may do this more than other surveys [29–31]. Although there is a benefit in IPAQ assessing multiple domains of physical activity as part of global surveillance, it could contribute to higher overall PA estimates than previous surveys that captured leisure-time activity alone . There is also the possibility of differential measurement error using IPAQ, with some countries or population subgroups potentially giving relatively accurate estimates of their behaviour, while other populations may over-estimate or under-estimate their physical activity; this between-country variability appeared even greater in the World Health Survey, which was comprised of mostly developing countries . The samples here were large-scale population samples, but eight of them were regional, not national samples, so these prevalence estimates cannot be generalised to the country level (for example, to all of China or Argentina). Based on education level (Table 1), these samples showed similar or slightly higher educational attainment to national or regional levels for those countries, suggesting comparability on this attribute.
Other methodological issues in the IPS included the variations in response rates across countries, and despite many efforts to standardize protocols, there were undoubtedly important differences in implementation. While all surveys were reviewed for consistency in the translation and cultural adaptation for participating countries, variations in how respondents understood the survey may have contributed to difficulties in interpreting findings of this international prevalence study. Though it is likely impossible to eliminate all challenges to the comparability of survey results across countries, subsequent surveillance studies could benefit from additional training of investigators, development of more rigorous translation and adaptation procedures, on-site inspections for quality control of data collection, centralized data entry system, and other methods for enhancing data quality. Furthermore, in low literacy populations, standardised showcards or illustrations should be used to depict types or intensity of different physical activities, but these cannot be used with telephone-based survey administration.
In addition to the abovementioned methodological issues, concerns remain with self report measures. Whilst they remain the most feasible and affordable instruments for global surveillance, objective population measures of physical activity, such as pedometers  or accelerometers [33, 34], may be beneficial to determine if differences across countries and between groups revealed in the present study represent true differences in physical activity behaviour.
The International Prevalence Study on Physical Activity provided data that allow 20 countries to be compared on physical activity behaviours for the first time. For some countries, these data represent their first large scale measurement of physical activity. Several countries have adopted the IPAQ and IPS methods as their national or regional surveillance system, and these data contribute to current WHO and European surveillance systems. A major contribution of IPS was to produce internationally comparable physical activity prevalence estimates and to demonstrate the feasibility of standardised data collection on physical activity using a common protocol. Though questions remain about the precision of the derived prevalence estimates, an important next step is the continued use of established instruments for the collection and monitoring of physical activity as part of ongoing non-communicable disease risk factor surveillance systems. Ongoing research will compare the IPAQ results with the Global Physical Activity Questionnaire (GPAQ), the World Health Organization's instrument also being used for international surveillance [21, 35], and compare both of these against objective measures.
The results from this international surveillance study on physical activity can provide useful baseline data, at least within countries, and studies could be repeated to ascertain population trends in physical activity. One important feature of a useful health monitoring system is the use of consistent measures over time . Country-specific data and trends can be used to monitor efforts to promote physical activity and improve public health. Although surveillance data alone are not sufficient to motivate or guide the implementation of national policy, consistent trend data are an essential underpinning of the case for public health action.