Physical activity measurement: Definitions and scientific importance

General definitions and overview
Importance in epidemiology and other disciplines

General definitions and overview

Physical activity refers to any “bodily movement produced by skeletal muscles that results in energy expenditure” (p. 126) [9]. While physical activity can be broadly conceptualised in terms of energy expenditure, the goal of research is often to understand and measure various qualitative and quantitative aspects of physical activity [10]. For example, a study may wish to compare the health benefits of physical activity derived from work versus during leisure time. Such contextual information can be useful for identifying the most suitable domains to focus on to promote physical activity. Another study might be interested in determining the optimal type, frequency, intensity, and duration of physical activity to develop an intervention. Understanding these aspects of physical activity are of particular importance for estimating physical activity energy expenditure and understanding physical activity within the context of cardiometabolic health [11].

The study of physical activity has given rise to several subfields of research, the most recent of which is sedentary behaviour. Sedentary behaviour refers to any behaviour where the body expends less than 1.5 metabolic equivalents (METs) of energy, such as sitting or lying down [12]. Another example includes the study of exercise science. Exercise is a subset of physical activity that involves structured forms of activity that are routinely repeated to improve or maintain physical fitness [9]. The range of methods for conceptualising physical activity make it a diverse and complex field of study.

Importance in epidemiology and other disciplines

In epidemiological research, the relationship between physical activity and disease has been well-established for many years. In 1953, Morris et al. demonstrated that those people in physically inactive professions such as bus drivers and telephonists, had a higher incidence of coronary heart disease than their peers with physically active jobs, such as bus conductors and postmen [13]. Since these landmark findings, a series of prospective longitudinal studies have found a consistent relationship between physical activity and the incidence of cardiovascular disease and various cardiovascular risk factors, such as blood pressure and obesity [14-17]. Findings from prospective longitudinal studies have also demonstrated that physical activity levels are related to the incidence of several other major non-communicable diseases, such as diabetes and multiple forms of cancer [17-19], regular exercise is associated with muscular skeletal benefits and promotes healthy ageing of muscle [20, 21]. Furthermore, the importance of physical activity appears to extend beyond physical health, with prospective evidence suggesting an association with the risk of various neurological and psychiatric conditions, including dementia, depression and anxiety disorders [22-24].

Physical activity is measurable at a population-level and its relationship with such a broad range of outcomes makes the quantification of physical activity an important goal. Importantly, physical activity is a modifiable behaviour. Through a range of informational, social and behavioural approaches, it is possible to increase physical activity levels in people of different ages, social groups, countries and communities [25]. The consequences of promoting physical activity at a population level are significant. One report estimates that decreasing physical inactivity by 25% worldwide would prevent 1.3 million deaths each year [19]. We may also investigate causal associations of physical activity on health outcomes using emerging analysis approaches, such as bi-directional causal modelling (e.g. Mendelian randomisation) [26]. The collection of population-level physical activity data will continue to play an important role in understanding and reducing the global burden of disease so we can chart changes by age (within individuals), period, and longitudinal study.

Go to the next section: ‘Physical activity domains’

Learn more about the individual studies covered by this guide and their measurement of physical activity:

Explore the measures by physical activity domain and their cross-study comparability:

Further information:

This page is part of the CLOSER resource: ‘Physical activity across age and study: a guide to data in six CLOSER studies’.