What Is Physical Activity

Physical activity has been defined by Caspersen et al. as 'any bodily movement produced by skeletal muscles that results in energy expenditure' (21). Components of total energy expenditure in a relatively sedentary individual include basal metabolic rate, which typically encompasses 50-70% of total energy, and the thermic effect of food, which accounts for another 7-10% (22). The remaining 20-43% is composed of energy expended through some type of 'physical activity', which can then be subdivided into energy expended in general activities of daily living (such as bathing, feeding, and grooming) and occupation, transportation, sporting and other leisure activities (see Figure 5.1).

Since the relative contribution of each of these components can vary considerably both within and among individuals and populations, valid and appropriate measurement of physical activity becomes a challenging task This measurement is further complicated by the fact that there are several health-related dimensions of physical activity (23). For example, in addition to quantifying physical activity based on the amount of energy expended, it can be quantified based on the manner in which energy is expended. In other words, physical activity can be measured according to its effects on different systems of the body by assessing attributes such as aerobic intensity, muscular resistance, degree of weight-bearing, and range of motion involved (see Figure 5.2). These

The Epidemiology of Diabetes Mellitus. An International Perspective. Edited by Jean-Marie Ekoe, Paul Zimmet and Rhys Williams. © 2001 John Wiley & Sons Ltd.

Basal metabolism ggsis;»

Activities of daily living

(a agnificant proportion of total energy expenditure in older populations; usually not measured in younger populations)

Occupational activity

(high proportion in developing countries; low proportion is industrialized cultures)

Thermic effect of food

Sports and leisure activities

(high contribution to total energy expenditure in more active individuals such as in younger and male populations)

Figure 5.1

Schematic representation of components of total energy expenditure

Physical activity

Physical activity

Figure 5.2 Health-related dimensions of physical activity

qualitative differences in physical activity may have implications for the prevention of specific diseases. For example, 100 calories burned swimming may be particularly beneficial to cardiovascular health and the prevention of related diseases but 100 calories expended weight-training may have a more favorable effect on bone mass or osteoporosis risk. Similarly, 100 calories expended in range of motion or flexibility training may be important for maintenance of physical function or avoidance of disability in older populations.

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