Young people aged 10–24 years constitute 24% of the world's population.1 This population includes, as per the Lancet Commission on adolescent health and wellbeing,2 younger adolescents (10–14 years), older adolescents (15–19 years), and young adults (20–24 years), and will be referred to as such throughout this Series paper.3 Both the Lancet Commission2 and the WHO Global Accelerated Action for the Health of Adolescents4 concluded that investing in adolescent health and wellbeing will yield a triple benefit—ie, today, into adulthood, and for the next generation.2 Although adolescence is generally considered a healthy period in a person's life, many non-communicable diseases (NCDs) that manifest later in life are, partly, the result of modifiable risk behaviours established during this time, such as smoking, unhealthy dietary patterns, and low levels of physical activity.5, 6 During the past three decades, there have been major global trends in adolescent health.7 Although a decrease in adolescent disease burden has been observed in many countries during the past 25 years, almost one in five (324 million; 18%) adolescents globally are now overweight or have obesity,7 and there is an increasing burden of adolescent mental health disorders (including depression and anxiety).8, 9 It is estimated that 962·8 million adolescents (53% globally) now live in multi-burden countries, where they face a so-called triple burden of health problems, including infectious diseases, injury and violence, and NCDs.7 Global agendas, therefore, encourage increased efforts to develop a better understanding of, and potential solutions for, health and wellbeing during adolescence.2, 4, 10, 11 To support these efforts, The Lancet launched the 2020 Campaign on child and adolescent health.12
Physical inactivity is associated with many NCDs and substantial economic costs on a global scale.13, 14 Physical inactivity has been estimated to account for 5·3 million deaths per year,13 and is estimated to cost at least US$54 billion in direct health-care costs, of which $31 billion is paid by the public sector.14 Although physical inactivity is recognised as a global pandemic,15 much of the evidence has come from studies of adults,13, 16, 17 in whom its effects on NCDs become apparent.18 However, evidence suggests that the prevalences of NCDs (eg, type 2 diabetes19) and NCD risk factors (eg, hypertension20 and obesity21) in adolescence are increasing. Acknowledging the health risks of long-term neglect of adolescent health and wellbeing, the Lancet Commission on adolescent health and wellbeing2 proposed 12 headline indicators to track progress in adolescent health. However, despite the known health risks of physical inactivity across the life course,13, 17, 22 and the alarmingly low levels of physical activity in the global population,23 no indicator related to physical activity was included. Therefore, physical activity appears to have low priority in adolescent health. It is crucial and timely to refocus the global prevention agenda in adolescence to include physical activity.
It is important that a better understanding of adolescent physical activity is developed, such that effective strategies can be implemented. The implementation of policies and interventions to promote physical activity has the potential to contribute to achieving many of the UN Sustainable Development Goals (SDGs) for 2030.24 WHO's Global Action Plan on Physical Activity 2018–203025 shows how the promotion of physical activity can help reach multiple SDGs. Beyond its direct contribution to SDG 3 (good health and wellbeing),25 co-benefits of promoting physical activity in adolescents include contributions to SDG 5 (gender equality) and, based on the increasing evidence linking physical activity to academic achievement26, 27 and the crucial role of physical education in high-quality education, SDG 4 (quality education). This Series paper provides an overview of up-to-date evidence on adolescent physical activity behaviours, including prevalence, determinants, and consequences, and provides recommendations for action in research and practice. The term physical activity behaviours is used to capture both physical activity and sedentary behaviour; we will indicate when evidence is specific to a type of behaviour.
Key messages
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The amount of physical activity is low and screen use is ubiquitous among adolescents across the globe; within-country socioeconomic differences vary by country context—adolescents from high socioeconomic backgrounds have better activity profiles than those from low socioeconomic backgrounds in high-income countries (HICs), with the reverse true for low-income and middle-income countries (LMICs)
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Adolescent physical inactivity probably contributes to key global health problems, including cardiometabolic and mental health disorders, but the evidence is weak; obesity and mental health problems might become auxiliary drivers of physical inactivity, increasing the risk of morbidity and mortality
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Supportive social and built environments are key drivers of adolescent activity behaviour, and successful policy action should aim for directing change in these areas; adolescents benefit from built environments that promote a range of activity behaviours (including active travel, play, and sport), and a supportive social environment in and out of school; access to supportive built environments is unequally distributed, particularly in LMICs
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Schools offer an effective avenue to increase physical activity among adolescents, but school-based initiatives have had little success overall and research involving older adolescents (15–19 years) is scarce; there is a need for sustained implementation of multicomponent programmes, co-designed with adolescents, and such interventions require context-specific support for schools to ensure effective implementation and sustainability
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Many young people across the globe, particularly those aged 15–24 years, are not in educational settings and alternative strategies to reach this population are required; with widespread access to the internet and some evidence of effectiveness, the potential contribution of eHealth and mHealth approaches contextualised to adolescents' needs and life circumstances should be explored
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The reasons to instigate change are different for decision makers in health, policy, education, and among adolescents themselves; understanding the benefits associated with physical activity, and tailoring messaging around the outcomes most salient to the specific audience will help drive change at multiple levels of a complex system
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Observational and interventional evidence on adolescent physical activity behaviours comes largely from HICs and younger adolescents (10–14 years); increased knowledge from LMICs, individuals not in school, and older adolescents going through major life transitions (eg, starting employment and parenthood) is urgently required to curb rapid rises in the health consequences of physical inactivity