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Ancient philosophers and medical scientists, English Lords, and United States Presidents have noted the health benefits of living an active life for thousands of years. Attempts to quantify the role of physical activity and health using scientific methods have been much more recent, beginning in earnest during the mid-twentieth century with a series of foundational epidemiological studies.1,2 In the decades since, cohort epidemiological studies have demonstrated clear and consistent benefits of physical activity on a broad range of health outcomes and mortality using a variety of different techniques to quantify activity. National and global surveillance systems provide population-level physical activity prevalence data, which is used to guide public health recommendations for optimal health. To understand how epidemiology informs our understanding the role of lifestyle and physiology in preventive medicine and disease treatment, it is important to define the different aspects physical activity and fitness.

Defining Physical Activity and Physical Fitness

Physical activity is defined as any bodily movement produced by skeletal muscles that results an increase in metabolic rate over a period of time.3 This concept is intentionally broad in scope, and there are numerous domains of physical activity including leisure-time physical activity (which include exercise and sport), work-based physical activity, household physical activity, and transportation physical activity (Fig. 183-1 for average energy expenditure by each domain for a representative U.S. sample of adults). Structured exercise is one component of leisure-time physical activity and is defined as an activity that is performed repeatedly over an extended period of time (bout) with the specific external intention of improvement in fitness, physical performance, or health.3 Sport is another form of leisure-time physical activity, and is generally defined as a competitive activity under the context of rules (though some parts of the world consider all exercise as sport).3 Work, household, and transport physical activity refer to bodily movement that occurs while performing task related to the relevant domain (lifting boxes, caring for a child, biking to work, etc.). Fitness refers the ability to perform work, and can be either health- or performance-related. Performance aspects of fitness can include muscular power, speed, endurance, agility, etc. Health-related fitness can refer to muscular, motor, metabolic, and cardiorespiratory components.

FIGURE 183-1

Energy expenditure by activity domain estimated from the National Human Activity Patterns Survey. (Source: Adapted from Dong L, Block G, Mandel S. Activities contributing to total energy expenditure in the United States: Results from the NHAPS Study. Int J Behav Nutr Phys Act. 2004;1(1):4.)


Epidemiologists have been exploring the association between physical activity and health for over six decades using a variety of techniques. Before the establishment of a national surveillance system related to physical activity, Morris et al. creatively compared London transportation workers ...

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