Relative and absolute intensity accelerometer metrics decipher the effects of age, sex, and occupation on physical activity.

Background: To investigate whether quantifying both the absolute and relative intensity of physical activity (PA) improves understanding of age, sex, and occupation-related differences in PA in healthy adults aged 20-89.

Methods: In the cross-sectional COmPLETE study, participants (N = 460, 48% women, age 55 [IQR 37, 71]) wore accelerometers for up to 14 days and underwent cardiopulmonary exercise testing. Average acceleration (AvAcc) and distribution of intensity (IG) of PA across the day were expressed in absolute terms (_) and relative (_) to the acceleration at the individual´s maximum intensity, predicted from cardiorespiratory fitness.

Results: After initial increases, AvAcc_ and IG_ continuously declined beyond age 40-45, whereas AvAcc_ and IG_ increased until stabilising at age ~ 70 and declining at age ~ 60, respectively. Cardiorespiratory fitness constantly declined. Women had trivially higher AvAcc_ and moderately higher AvAcc_, but not IG_ and IG, than men. Occupations involving at least moderate PA showed higher AvAcc_ and AvAcc_, but not IG_ and IG indicating longer periods of low-intensity PA, compared to sitting/standing occupations.

Conclusions: Distinct age trajectories of absolute and relative metrics as well as cardiorespiratory fitness suggest that the age-related decline in the latter preceded that of PA. Women's higher AvAcc_ and AvAcc_ relate to more low-intensity PA combined with lower cardiorespiratory fitness rather than more health-enhancing higher-intensity PA. Finally, the intensity profile of occupational PA may provide insight into why occupational PA appears to lack a prophylactic association with health. Quantifying both the absolute and relative intensity of accelerometer-assessed PA provides greater insight than either alone.

Trial Registration: On clinicaltrials.gov (NCT03986892). Retrospectively registered 14 June 2019.