Frequency, duration and intensity are quantitative dimensions which describe the amount of physical activity taking place during a given time-frame (e.g. one week, one month, one year).
The time over which the physical activity takes place. This can vary from very short bouts of only a few seconds, to many hours of continuous activity (e.g. marathon running).
- Intensity is the rate of energy expenditure of the activity. The more intense the activity, the greater the metabolic cost per unit of time.
- This is a continuous dimension which ranges from very low intensity (or even sedentary) behaviours, to those which are vigorous and demand high energy expenditure.
- The intensity of activity can vary from moment to moment, as illustrated by Figure P.1.5.
- The intensity of an activity can be described in terms the Metabolic Equivalent Task or MET, which is the multiple of the individual’s resting energy expenditure.
As a categorical variable
- Activities can be grouped according to their intensity, with MET values providing reference points; categories such as light (< 3 METs), moderate (3 to 6 METs), and vigorous (> 6 METs) physical activity are commonly used (see Figure P.1.4).
- The use of these categories, and the MET values associated with them, is much debated.
- Moderate-to-vigorous physical activity, or MVPA, is an important category of activity due to its relationships with health.
Figure P.1.4 Classification of activity by intensity using metabolic equivalents (METs). Categories of activity include, light, moderate and vigorous. Sleep and sedentary behaviours (SB) can also be classified by METs.
The number of occasions of activity in the given time frame (e.g. one day, one week, one year).
Together the product of frequency, intensity and duration describe the overall volume of physical activity in a given time frame (e.g. one day, one week, one year).
Figure P.1.5 Variation in physical activity intensity with time throughout a 24 hour period. The graph shows a number of activity bouts of different duration and with variable intensity both within and between bouts. The area under each bout is representative of its energy expenditure, while the area under the entire curve is the total daily physical activity energy expenditure.