This study aimed to examine trends in adolescents’ ACS over time in 28 countries from 5 WHO regions in Africa, the Americas, and Asia. Included countries were predominantly LMICs, and the need to increase physical activity research in LMICs is an established research gap in the existing literature [5, 25].
Our study results showed that ACS is not a homogeneous phenomenon across countries. Indeed, we found stable patterns over time for the majority of countries (16 out of 28), decreasing trends for some countries (7/28), and increasing trends for a few (5/28). Time trend studies are scarce in LMICs, but our results are not completely in line with the few that are available since they consistently show decreasing trends in ACS over time (Brazil, between 2005 to 2012 ; Vietnam, 2004–2017 ; China, 1997–2006 . Notwithstanding, caution is urged when interpreting such comparisons since distinct operationalization of ACS variables and research methodologies are present (e.g., the number of time points available in the previous studies ranged from 2 to 5).
Several factors may potentially explain the decreasing trends in ACS found for some countries. First, it is well documented that LMICs are experiencing rapid socioeconomic development and growth in motorized vehicles . In addition, demographic transitions are occurring rapidly in LMICs with people moving from rural to urban areas , and it has been suggested that people living in urban areas are more likely to use passive transportation [16, 18, 26, 29]. In addition, it may be hypothesized that the rapid urban sprawl, and sub-urbanization may have increased distance to school in some settings, or specialized or preferred schools are at increasing distance from where people live. Research conducted in high-income countries identified distance to school as a main barrier to ACS , and showed that living in sub-urban areas has limited people’s ability to actively commute and reinforced car-dependency [30, 31].
The present study also found stable patterns in ACS for most of the included countries across different regions. It is possible that the aforementioned social and economic megatrends that affect society have occurred at a much more rapid speed in the past but have been relatively stable over the past fifteen years, which may partially explain such steady prevalence in ACS over time observed in the majority of the countries in our study. For instance, previous studies based on industrialized countries (Canada, Australia, US, Switzerland) showed that declines in ACS occurred over more than three decades ago (1986–2006) [32,33,34,35,36], but more recent studies (2006–2018) based in Europe suggest that these patterns remained stable [12, 13], (with the exception of Germany and Czech Republic that reported decreasing trends from 2003 to 2017, and between 2006 and 2010, respectively) [12, 17]. Alternatively, it is possible that the societal megatrends do not directly shape ACS over time, but that this behavior might be triggered automatically and strongly underpinned by habits (behaviors repeated regularly with little or no conscious thought) that are difficult to modify . This, together with the fact that the promotion of active transportation is usually neglected from a public health perspective and receives scarce attention from local governments and authorities may contribute to the stable patterns found in the majority of the countries.
Importantly, several countries reported very low levels of ACS with less than a quarter of their adolescents engaging in ACS on at least 3 days/week. In addition, it should be highlighted that the low ACS levels remained stable (United Arab Emirates, Kuwait, Lebanon) or even worsened over time (Oman). Weather may be an important factor that influences travel behaviors, since such countries were all located in the Eastern Mediterranean Region. This region is characterized by persistent hot and dry weather conditions (especially during summer), and the aforementioned countries also have high levels of vehicle ownership , which may explain in part the low levels of ACS found in the present study. Efforts towards designing and testing tailored and contextually adapted interventions to increase ACS in such contexts is needed. Unfortunately, little is known about moderators and mediators that influence travel behavior change, in this population. Relatedly, research in this area is in its infancy and previous intervention studies to increase ACS from high-income settings (US, Europe, Australia) reported small effect sizes and the quality of evidence was weak [39, 40].
Our study also revealed increasing trends in ACS for a few countries (i.e., Namibia, Trinidad & Tobago, Morocco, Cook Islands, and Philippines). While the reason for this trend is only speculative, it is possible that with rapid urbanization occurring in some settings, particularly LMICs, vulnerable populations (i.e., socioeconomically disadvantaged) have settled in the periphery of large cities, with perhaps no schools in the local community, which force adolescents to actively commute to/from school. Alternatively, economic development may increase the number of schools for some countries or urban contexts, and the higher availability of schools located nearby may have shortened distance to schools and fostered active commuting choices. In addition, low affordability of public transportation and potential increases in fares may be obstacles for low-income urban residents to use this mode of transportation, and they may have opted for alternative and cheaper modes to travel to school such as active commuting. It is also possible that there has been a more widespread access to education for rural communities in LMICs, which could also have contributed to increases in ACS over time.
Finally, we found that most countries showed no differences in ACS trends between boys and girls, which is in line with previous findings in Brazilian and German adolescents [16, 17]. However, in the current study, differences in ACS over time between boys and girls were apparent in some countries. Specifically, girls in Argentina showed decreases in ACS while the trend for boys remained stable. In Namibia and Lebanon, trends did not change in girls and only significant increases were observed for boys. Several reported barriers by parents may possibly explain in part such decreasing or stable patterns for girls, including concerns related to perceived traffic, personal safety concerns (e.g., violence, rape, harassment, or crime safety) [41,42,43].
In United Arab Emirates, Seychelles and Kuwait, abrupt decreases occurred in boys, while significant increases in ACS were found for girls in Kuwait (in United Arab Emirates and Seychelles, this remained stable). Cultural/religious barriers to overall physical activity have been previously documented in females from Arab countries (e.g., lack of encouragement, conservative clothing not suitable for physical activity, time constraints from academic/family responsibilities, being in public spaces accompanied, paucity of gender segregated facilities, etc..) [44, 45]. Indeed, in our study, we observed notable differences in the prevalence of ACS among girls and boys in some countries (e.g., Kuwait in 2011, Oman in 2015, United Arab Emirates in 2010), where girls were much less likely to engage in ACS. Given that these were all Arab or Mainly Muslim countries, it is possible that there are religious and cultural barriers for girls to engage in ACS. However, it seems that these factors are unlikely to explain the observed trends, since we found significant increasing or stable trends for girls in United Arab Emirates, Seychelles and Kuwait. Other factors such as the political stand, changes in societal/gender norms and potential changes in women’s rights have contributed to increasing or stable ACS for girls . Importantly, given the limited research on this topic, the aforementioned hypotheses are speculative in nature, and clearly, more research is warranted on the correlates and determinants of travel behaviors according to sex/gender in different settings.
The promotion of physical activity was previously based on theoretical approaches that primarily target personal level factors. However, psychosocial and environmental variables best explain physical activity behavior . Indeed, it is well known that physical activity − and ACS in particular − , are behaviors that are influenced by a complex interplay of personal, behavioral, social/psychological and environmental factors . Thus, the prioritization of environmental rather than individual approaches for physical activity promotion has already been advocated . Recently, the Lancet has published the second Series on urban design, transport, and health which is pioneer work on how to facilitate the creation of worldwide sustainable cities that encourage urban and population health . Importantly, research on city planning and capacity building in LMICs is a current research gap that should guide future research endeavors .
Importantly, ACS is considered a sustainable behavior that has positive consequences for the health of individuals and societies, but it may also entail important safety risks. LMICs experience 90% of the worlds’ traffic fatalities and injuries , and pedestrians and cyclists are the group that is most vulnerable to such events . In addition, several studies conducted in developing countries suggest that those with lower socioeconomic backgrounds (e.g., living in rural or suburban areas, least wealthy families, lower socio-economic schools) are more prone to use active transportation [16, 18, 26, 29]. The overall lack of public transportation infrastructure in rural areas, schools located further, and the low car ownership rates for the poorest segments of the population may force the adolescents to actively commute to/from school. Indeed, socioeconomic determinants not only play an important role and directly influence overall travel behaviors, but they foster increased vulnerability to road traffic injuries. Additionally, the efforts to improve ACS in adolescence should also take into consideration safe environments, especially among girls, in which they need to feel secure. Altogether, this highlights the need for protecting such users by providing safe and high-quality infrastructure to promote ACS and overall active commuting behaviors.
This study contributed to building a stronger evidence base and to expand physical activity-related surveillance in under-represented countries. The large sample size and the use of standardized comparable measures of ACS allowed for direct comparisons across countries. Nonetheless, the present findings should be interpreted in light of several limitations. First, there is no gold standard or established consensus related to ACS measurement. In addition, the present study is based on active commuting to school, but active commuting purposes for other day-to-day activities may potentially be important. Second, similar to previous studies conducted on the topic, the climate linked to the timing of data collection may have influenced the ACS estimates since weather conditions may affect transportation choices (e.g., rainy season). Finally, not all surveys were conducted in the same years for all the countries which makes the estimates not entirely comparable across countries. Relatedly, there were some cases where the time frames do not overlap. For example, the timeframe of Jordan was between 2004 and 2007, while in Kuwait, Lebanon, Cook Islands, Samoa, and Vanuatu, the timeframe was from 2011. Thus, data should always be interpreted in conjunction with the year in which the surveys were conducted. Further, temporal trends are more accurate in the countries that provided more than two datapoints.