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 (_ABS) and relative (_REL) to the acceleration at the individual´s maximum intensity, predicted from cardiorespiratory fitness. Results After initial increases, AvAcc_ABS and IG_ABS continuously declined beyond age 40–45, whereas AvAcc_REL and IG_REL increased until stabilising at age ~ 70 and declining at age ~ 60, respectively. Cardiorespiratory fitness constantly declined. Women had trivially higher AvAcc_ABS and moderately higher AvAcc_REL, but not IG_ABS and IG_REL, than men. Occupations involving at least moderate PA showed higher AvAcc_ABS and AvAcc_REL, but not IG_ABS and IG_REL 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_ABS and AvAcc_REL 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.