Influence of Physical Activity on Energy Balance

In the simple locomotor activities that involve walking, running, or cycling, the energy cost of activity is readily determined and can be shown to be a function of speed: where body mass is supported, as in running, or where it must be moved against gravity, as in cycling uphill, then body mass is also an important factor in determining the energy cost. For walking, running, and cycling at low speeds, there is a linear relationship between velocity and energy cost, if the energy cost is expressed relative to body mass. Across a range of speeds, the cost of locomotion is approximately 1 kcal kg-1km-1. Therefore, energy expenditure depends on the distance covered and the body mass and is not influenced by walking speed. In purposeful walking, where the aim is to get from one place to another, the distance is set, but where walking is part of a physical activity program, activity is more often measured by time rather than distance, so walking speed becomes an important factor in determining the energy cost. At higher speeds, the relationship between energy expenditure and speed becomes curvilinear and the energy cost increases disproportionately.

It is often recommended that 20-30 min of moderate intensity exercise three times per week is sufficient exercise to confer some protection against cardiovascular disease: if this exercise is in the form of jogging, aerobics, or similar activities, the energy expenditure will be about 4MJ (1000 kcal) per week for the average 70-kg individual, or an average of only about 150 kcal day-1 (Table 1). However, even a small daily contribution from exercise to total daily energy expenditure will have a cumulative effect on a long-term basis. For obese individuals, whose exercise capacity is low, the role of physical activity in raising energy expenditure is necessarily limited, but this effect is offset to some degree by the increased energy cost of weight-bearing activity.

Very high levels of daily energy expenditure are now rarely encountered in occupational tasks. The average daily metabolic rate of lumberjacks has

Table 1 Estimated average energy cost of physical activity, expressed as METS (multiples of BMR) and in kJperkg body mass per h

Activity

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