Background <p>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.</p> Methods <p>In the cross-sectional COmPLETE study, participants (<i>N</i> = 460, 48% women, age 55 [IQR 37, 71]) wore accelerometers for up to 14&#xa0;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.</p> Results <p>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.</p> Conclusions <p>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.</p> Trial registration <p>On clinicaltrials.gov (NCT03986892). Retrospectively registered 14 June 2019.</p>