• physical activity

More than 60 years of research have consistently shown that physical activity beneficially affects several health outcomes and reduces the risk of premature mortality. The Harvard Alumni Study, one of the first studies to suggest a dose–response association between physical activity and health, examined the association between ‘total’ physical activity (defined as strenuous sports, walking and stair climbing) and participation in strenuous sports and the risk of heart attack in about 17 000 men.1 ‘Total’ physical activity was categorised into groups based on energy expenditure in physical activity per week. The following were the important observations: (1) there was a substantial risk reduction when comparing the ‘inactive’ reference group expending <500 kcal per week in ‘total’ physical activity with the second group expending 500–999 kcal per week; (2) there was graded dose–response association, with declining risk up to about 3000 kcal per week of ‘total’ physical activity, beyond which the benefits plateaued; and (3) energy expenditure from strenuous sports appeared to have greater protective effect than ‘total’ volume of activity. However, the latter observation may partly be explained by more accurate reporting of sporting activities compared with physical activities of daily living of lower intensity (eg, stair climbing and walking). The dose–response curve from the landmark paper by Paffenbarger et al 1 was subsequently confirmed after accounting for confounding,2 and it has been replicated in women and ethnically diverse populations from low-income and middle-income countries.3–5

These previous studies assessed physical activity using different self-report instruments, which may be prone to cognitive biases (eg, recall bias) and which may result in overestimation of physical activity and underestimation of sedentary time. Due …