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What is physical activity epidemiology?

This is an excerpt from Research Methods in Physical Activity-8th Edition by Jerry R. Thomas,Philip E. Martin,Jennifer L. Etnier & Stephen J. Silverman.

Epidemiology has been defined as “the study of the distribution and determinants of health-related states or events in specified populations, and the application of this study to the control of health problems” (Last, 1988, p. 141). Caspersen defined physical activity epidemiology as a two-part process (Caspersen, 1989, p. 425):

  1. First, as a science, “it studies the association of physical activity, as a health-related behavior, with disease and other health outcomes; the distribution and the determinants of physical activity behavior(s); and the interrelationship of physical activity with other behaviors.”
  2. Second, as a practice, “it applies that knowledge to the prevention and control of disease and the promotion of health.”

Based on these definitions and concepts, physical activity epidemiology can be concisely defined as “the study of the distribution and determinants of physical activity, its associations with health-related outcomes, and the application of this study to disease prevention and health promotion.” Physical activity can be either an exposure or outcome depending on the study design and purpose. When studying the associations of physical activity with disease and other health outcomes, it is an exposure. However, when studying the determinants of physical activity, it is an outcome. From both clinical and public health perspectives, physical activity epidemiology is an important growing area of science and practice for the prevention of disease and health promotion.

Components of Epidemiological Research

Distribution
Frequency—prevalence, incidence, mortality rate
Patterns—person, place, time

Determinants
Defined characteristics—associated with change in health

Application
Translation—knowledge to practice

Distribution

The distribution of disease relates to the frequency and patterns of disease occurrence in a population. Frequency, or how often the disease occurs, is typically measured as the prevalence, incidence, or mortality rate of a disease. Disease prevalence refers to the number of people in a given population that have a disease at a particular point in time. The frequency of disease occurrence may also be calculated as the rate of new disease or health events, as incidence or mortality rates (i.e., new cases or new deaths from a specific disease within a specified period). For example, the mortality rates for cardiovascular disease were 219.4 per 100,000 U.S. adults in 2017 (Virani et al., 2020).

A presentation of the frequency of disease occurrence relative to the number in the population of interest enables a comparison of prevalence, incidence, or mortality rate across populations. For example, the prevalence of cardiovascular disease is about 50% higher for men than it is for women in the U.S. (Virani et al., 2020).

Evaluating the basic patterns of disease occurrence within a specified population is often useful for developing hypotheses about risk factors for the disease. Patterns of disease occurrence refers to characteristics related to person, place, and time. Personal characteristics include demographic factors such as age, sex, and socioeconomic status. Characteristics of place include geographic differences, urban–rural variation, and, particularly important in the history of physical activity epidemiology, differences in occupation. Historically, differences in occupational classifications enabled researchers to make crude comparisons of occupational physical activity levels and were used in some of the first epidemiological studies of the relationship between physical activity and heart disease.

Time of disease occurrence refers to annual, seasonal, or daily patterns of occurrence. Quantification of temporal changes in disease rates often leads to hypotheses that generate a more detailed examination of the factors that caused this change. A good example of this was the observation that the incidence of upper-respiratory infections (i.e., common colds) in runners increased in the 14-day period after running an ultramarathon (56K) (Peters & Bateman, 1983). Of the 141 marathoners studied, 47 (33%) came down with a cold after the race, whereas only 19 (15%) of the 124 people who did not run the marathon had a cold during the same period. Thus, the incidence of colds was more than twice as high among the runners, apparently because of the exposure of the marathon event. This observation led to intensive research in the area of exercise immunology and ultimately resulted in a greater understanding of the relationship between exercise and the immune system (Nieman, 1994).

In physical activity epidemiology, distribution can also refer to the prevalence of meeting the physical activity guidelines (as opposed to the occurrence of disease) in a certain population, area, or time. For example, in 1,922 community-recruited adults (mean age 65) with or at high risk for knee osteoarthritis, more than 50% of men and nearly 80% of women failed to meet the U.S. Physical Activity Guidelines, defined as at least 150 weekly minutes of moderate-to-vigorous-intensity physical activity measured by accelerometers, which suggests a sex difference in physical activity in this population (Chang, Song, Lee, Chang, Semanik, & Dunlop, 2020).

Determinants

A determinant is “any factor, whether event, characteristic, or other definable entity, that brings about change in a health condition, or other defined characteristic” (Last, 1988, p. 500). In physical activity research, the goals are usually to test the hypothesis that activity is or is not a determinant for a particular disease outcome, or to identify the determinant of physical activity behaviors such as sex, age, race, income, occupation, and environment. Determinants of disease are often called risk factors because they increase a person’s risk for disease. Epidemiological studies have been instrumental in identifying risk factors for heart disease, including obesity, high blood pressure, high LDL cholesterol, low HDL cholesterol, and physical inactivity. Epidemiological studies have also identified that women are less likely than men and older individuals are less likely than younger individuals to meet the physical activity guidelines. The identification and surveillance of specific determinants of a particular disease or physical activity behaviors allow for targeted health promotion campaigns that present this new health knowledge to the public.

Associations

The associations of physical activity with various health-related outcomes (e.g., cardiometabolic diseases, cancer, mental health, cognition, quality of life, mortality) are the central part of physical activity epidemiology. Findings from these studies provide important data (e.g., how much of the risk of experiencing a heart attack can be reduced in physically active compared to inactive individuals) to develop effective public health strategies and policies such as the U.S. Physical Activity Guidelines. The associations of physical activity with health outcomes are commonly expressed as odds ratios or hazard ratios from observational studies and the differences in the primary outcomes (e.g., body weight, blood pressure) before and after exercise intervention from randomized controlled trials. These epidemiological measures and study designs are described in detail with examples in the following sections.

Application

The application of the established understanding of the causal factors related to disease is a major goal of public health. Thus, after epidemiologists have identified the cause of disease, health educators interact with communities to make them healthier places to live.

Other names for the application of research in a community setting are translation and dissemination. Successful public health dissemination strategies are those that engage the community using a variety of methods, from motivating people to change their behaviors to affecting public policy. This is referred to as the ecological model for health promotion (McLeroy, Bibeau, Steckler, & Glanz, 1988). Public health disseminators translate knowledge from epidemiological studies to help increase physical activity among individuals and within social groups and community organizations. Strategies are also used to affect the community environment (e.g., by building walking trails) and encourage policy makers to enact legislation or appropriate funds to enable people to lead physically active lives.