The future of physical activity: from sick individuals to healthy populations

Although the notion that movement can be beneficial to one’s health has been around for centuries,1 it was only in the 1950s that the first epidemiologically oriented studies on the benefits of physical activity for health were published. Through a series of publications, Morris and coworkers showed that occupational2–4 and leisure-time physical activity5 reduced the risk of coronary heart disease. In the following two decades, prospective evidence from studies such as the Harvard Alumni Health Study6 confirmed the findings by Morris and colleagues and expanded the range of health outcomes benefiting from engagement in physical activity, including hypertension,7 all-cause mortality8 and some cancer types.9,10 The growing evidence on the benefits of physical activity for health motivated the American Heart Association11 and the American College of Sports Medicine12 to produce statements on exercise testing, training and prescription in the 1970s. Although the early statements were primarily on fitness, since the late 1970s and the early 1980s, growing recognition of the distinctions between health-related physical activity and fitness has been observed. The shift of paradigm from individually targeted exercise science to public health-focused physical activity required the field’s terminology to be defined. In 1984, the organizers of a Centers for Disease Control and Prevention (CDC) Workshop suggested a definition for physical activity as: ‘any bodily movement produced by skeletal muscles that results in energy expenditure’.13 Definitions were also suggested for exercise: ‘a subcategory of exercise that is planned, structured, repetitive and has as a final or an intermediate objective the improvement or maintenance of one or more components of physical fitness’, and for physical fitness: ‘a set of attributes that are either health- or skill-related’. Previously, the terms had been used almost interchangeably, and the purpose for the proposed definition was to facilitate the communication of research findings. The goal of the meeting itself was to establish physical activity as an important public health issue. The following year, Geoffrey Rose’s seminal article, Sick Individuals and Sick Populations, appeared in the International Journal of Epidemiology.14 In the article, Rose discusses the differences between the determinants of individual cases of disease (sick individuals) and the determinants of the incidence of disease in populations (sick populations). He notes that social norms and environmental factors may foster behaviours that are so common within a population that they may not be recognizable as risk promoters. Rose points out that treating only those with the highest values of a risk factor (cholesterol in East Finland, in his example) would have relatively little impact on the overall incidence of the outcome (coronary heart disease, in his example). Rose14 also described two fundamentally different approaches to preventing disease, which he called the high-risk strategy and the population-based strategy. The high-risk approach identifies those individuals at highest risk and provides treatment to them, usually a medicinal treatment. The population-based strategy strives to shift the risk-factor distribution curve to safer levels, occasionally with a medicinal approach (e.g. vaccines), but the risk factor of concern more often requires changes in social norms, environmental factors and public policies. In the paper, Rose discussed the pros and cons of each approach14. Our understanding of his discussion is that the high-risk strategy for sick individuals is most appropriate for genetically determined risk factors such as haemoglobin S or BRCA1 or BRCA2 mutations, and the population-based strategy for sick populations is most appropriate for culturally/socially determined risk factors such as physical activity, diet, tobacco and substance abuse, seat-belt utilization or occupational hazards. A major benefit (Rose called it radical) of the population-based strategy is that it actually reduces the prevalence of the risk factor and, thereby, the incidence of disease. In addition, as the policies, environments and social norms change, the strategy becomes easier to implement and maintain. The thinking and proposals described in Rose’s article14 have been insufficiently applied to our efforts to encourage health-enhancing physical activity. During the past 40 years, the preponderance of prevention efforts in the field of physical activity have largely focused on behavioural change at the individual level, highlighting the individual risks of inactivity and using behavioural change theories to stimulate individuals to be more active. Physical activity has been perceived as an attribute of individuals, and not as a property of populations. Most efforts still push individuals to achieve a specific goal. However in recent years, evidence of the importance of environmental, social and policy determinants of physical activity has been mounting.15 The time has come to acknowledge the powerful influence on physical activity of cultural/social norms (such as those relating to gender and race) and environmental and regulatory policies (such as transportation and recreation), as recognized in socio-ecological models.16 Since the publication of Rose’s article14 and the papers from the CDC Workshop,13 the field of physical activity experienced a massive growth of worldwide interest, and a wide range of physical and mental health benefits for individuals and a plethora of environmental and social benefits to communities have been identified. For example, dementia has been described as ‘the greatest global challenge for health and social care in the 21st century’.17 It is now widely recognized that physical activity benefits cognition, with a recent systematic review estimating a population attributable fraction around 7% for physical activity as it relates to dementia.18 Recently, a comprehensive publication identified possible benefits of physical activity promotion for 15 of the 17 United Nations Sustainable Development Goals (SDGs). For example, 3—good health and wellbeing—and 11—sustainable cities and communities.19 Unfortunately, many of these benefits are unachieved, but could be achieved if more individuals across the globe became more physically active.20,21 This wide range of benefits of physical activity has caused some to claim that physical activity should be discussed within the human rights sphere. For example, the 2015 UNESCO’s International Charter of Physical Education, Physical Activity and Sport stated that: ‘every human being has a fundamental right of access to physical education, physical activity and sport’.22 Others have attempted a holistic definition of physical activity, that includes ‘culturally specific … contexts, and an array of interests, emotions, interactions, and relationships’.23 In addition to the widening range of health and social benefits is the recognition that substantial health improvements among adults can be achieved by increases in physical activity to a level below the widely recommended cut-point of 150 min per week of at least moderate-intensity physical activity.24 The curvilinear relationship between risk of disease and moderate- to vigorous-intensity physical activity clearly shows a steep rate of decline in risk, starting at any minute per week of leisure-time activity above zero. It has also been pointed out that there is no obvious ‘best’ amount to be achieved.25 In addition, recent evidence has unequivocally established the benefits of physical activity of light-intensity (not covered by existing guidelines).26,27 New statistical methods have also been applied, such as compositional data analyses,28 that take into consideration the fixed 24-h day: an increase in one kind of activity (say, of light intensity) must be compensated by a similar decrease in another kind of activity (e.g. sedentary behaviour) or a combination of other kinds of activities. These results advocate in favour of the field of physical activity and health adopting a population approach, focused on moving the entire physical activity distribution to more active levels, akin to that highlighted in Rose’s famous example of blood pressure.14 Although most countries still rely on the traditional cut-off-based recommendations on physical activity (high-risk approach), Japan is innovating by creating the +10 strategy,29 aimed at adding 10 minut to whatever one’s level of physical activity is. This is one of the few examples of the population approach being used in physical activity policy. To accompany the shift from high-risk to population-based strategy, public health officials should develop surveillance systems that reflect the shift. One possibility would be to report not the proportion of individuals who fail to achieve a cut-point of achievement, but on the overlap between the actual distribution of physical activity behaviour and a preferred distribution. The goal will be to increase the overlap. A good example is the difference in the distribution of serum cholesterol levels in South Japan and Eastern Finland, alluded to by Rose14 and depicted by Blackburn.30 Instead of focusing on moderate- to vigorous-intensity physical activity, the distribution curve should represent total physical activity, utilizing total step counts, total accelerometer counts or similar measures. Individual efforts are still valid and needed. However, governments at all levels should be held accountable for making physical activity a societal priority. Global governmental policies and practices deserve greater emphasis and monitoring. Efforts to determine what to surveil and to do it have already begun.31–33 The Caspersen and coworkers paper13 further pointed out that various categorization schemes, such as intensity or volume categories, would facilitate study and communication. The inclusion of physical activity in all aspects of daily life, as the authors suggested, offers a wide variety of categorical schemes. The segmentation into ‘identifiable portions of daily life’, such as leisure time, occupation, transportation and household, has been helpful. However, in the past three decades we have also seen more research from low- and middle-income countries (LMICs) that has underscored additional issues that respond to context. One acknowledged but little-used dimension of physical activity is that of choice versus necessity.34 For example, exercise is usually a choice, whereas occupational and—in many cases in LMICs—transport physical activity is a necessity. This distinction is relevant considering the so-called ‘physical activity paradox’, where some studies have suggested occupational physical activity to be associated with adverse health outcomes, in contrast to the beneficial associations observed for leisure-time activity.35 The industrial and technological revolutions have caused a marked reduction in the volume of required physical activity. Efforts to encourage inactive individuals—the ‘sick individuals’ approach—have been insufficient to counter the decline in required physical activity, leaving a large portion of health benefits unclaimed. Some of the failure is because some individuals have no access to places for physical activity whereas others find the available options to be unsafe or unattractive.16 If we are to improve this situation, greater use of the sick populations approach is required and that will require governmental leadership and action. One action government could undertake is to make access to places for physical activity, e.g. public parks and greenways, more equitable. Assuring that these places are safe and attractive is an important component of this.36 A second example is to create regular enjoyable events or opportunities such as the popular open street events or Ciclovias.37,38 A third would be to make physically active transportation—walking, biking, public transport—easier, more pleasant, safer and less expensive than the use of motorized vehicles.39 All these strategies target the entire population, and not only high-risk individuals. Thus, based on developments in the field over the past few decades and the recognition that a healthy populations approach inspired by Rose’s framework14 is needed in physical activity, we propose the use of the following as a complement to the definition. Physical activity is any bodily movement produced by skeletal muscles that results in energy expenditure. Governments are responsible for making it easy for individuals to choose physical activity as part of routine living. Access to pleasant, safe, healthy, equitable and purposeful physical activity must be a societal priority. Moving forward, we hope the field will be mindful of these additional considerations that go beyond the physiological definition of physical activity. By doing so, the field of physical activity and health can transition to a public health focus by replacing a ‘sick individuals’ with a ‘healthy populations’ approach.14 P.H. drafted the first version of the article. All authors critically reviewed the article and helped re-write sections of it. All authors substantially contributed to the final version and approved its submission. No AI tools were used in the preparation of this article. None declared.