The UK Biobank used several methods to measure fitness and physical activity in middle- and older-aged adult participants. Fitness was measured using a risk-stratified individualised submaximal bike ergometer test, resulting in 22 different test protocols. Physical activity was assessed using questionnaires and also objectively in a subsample of 100,000 volunteers using a wrist-worn accelerometer worn for 7 days of free-living.

We conducted a study to compare measures of fitness and physical activity from the UK Biobank with gold-standard measures. Fitness estimates from the bike test were compared with respiratory gas measurements from a maximal exercise test. Energy expenditure estimates from self-report and wrist-worn accelerometry were compared with energy expenditure as measured by the doubly labelled water method. The latter also provided an opportunity to validate energy intake estimates from a new online 24-hour dietary recall method.

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