Measuring physical activity

How did we measure physical activity?

Physical activity is any bodily movement produced by skeletal muscles that requires energy expenditure (WHO 2018). Physical activity includes sport and leisure activities; everyday activities, such as doing household chores; and muscle-strengthening activities, such as lifting weights.

Being physically active improves mental and musculoskeletal health and reduces other risk factors such as overweight (including obesity), high blood pressure and high blood cholesterol (AIHW 2022).

Australia’s Physical Activity and Sedentary Behaviour Guidelines (the guidelines) recommend minimum levels of activity for different age groups to promote good physical and mental health and wellbeing (the Department 2021). For example, it is recommended that people aged 18 to 64 do a minimum of 150 minutes of moderate- to vigorous-intensity activity (such as playing tennis or high-intensity interval training) a week.

The guidelines reflect acceptable activity levels after assessing the risks and benefits between population exposure and subsequent health impacts. However, the methods applied in this analysis are based on a theoretical exposure level that results in minimal risk of disease. This theoretical level differs from recommended guidelines because they are used for different purposes. See the Technical notes section for further details on the estimation of risk factor attributable burden.

Physical activity can be measured using the metabolic equivalent of tasks (METs), with 1 MET equivalent to 1 kcal/kg/hr, multiplied by the time spent active and the intensity of the activity to give MET-minutes.

Understanding METs: Julianne and Yolanda’s activity levels

To calculate the amount of weekly physical activity using metabolic equivalent of tasks (METs), it is necessary to look at the amount of time spent being active and the intensity of each activity. This information is required for activity related to leisure, a person’s activity at work, transport, and household chores (including gardening) to give the total amount of weekly activity in units of weekly MET-minutes (MET-mins).

Yolanda, aged 33, reports riding her bicycle to work each day, a 1-hour return commute (MET intensity score for moderate-intensity activity of 5). Her total MET score from travelling to work is 1,500 weekly MET-mins (60 minutes x 5 days a week x 5). At her job, she is on her feet doing physically intensive work for 2-hour shifts each day of the working week (MET intensity score for vigorous activity of 7.5). Her total MET-mins score from work is 4,500 MET-mins weekly (120 minutes x 5 days a week x 7.5). Based on time use survey results by age and sex, Yolanda’s average weekly MET-mins from household chores amounts to 165 MET-mins. In an average week, Yolanda’s activity measures 6,165 MET-mins in total.

In comparison, Julianne, aged 70, is retired and reports taking hour-long leisurely strolls once a week (MET intensity of 3.5). Based on time use survey results, Julianne’s average weekly activity from household chores amounts to 237 MET-mins. Julianne’s total activity in an average week amounts to 447 MET-mins in total.

Yolanda and Julianne’s different levels of weekly activity mean they are exposed to different levels of additional risk attributable to inactivity. Yolanda’s activity categorises her in the high activity level category (greater than 4,200 weekly MET-mins) while Julianne would be in the sedentary activity level category (below 600 weekly MET-mins). Yolanda would have minimal additional risk of developing diseases linked to inactivity. However, Julianne would be at additional risk of developing inactivity-related disease.

In assessing scenarios where additional MET-mins are used to explain improvements in activity, it is the total number of additional MET-mins over a week that is important in determining health risk benefits. For example,
15 minutes of moderate-intensity activity, 5 days a week, amounts to 75 minutes in total. These 75 minutes could be distributed over any number of days to achieve the same benefit.