Quantifying the relative intensity of free-living physical activity: differences across age, association with mortality and clinical interpretation—an observational study

To describe age-related differences in the absolute and relative intensity of physical activity (PA) and associations with mortality. UK Biobank participants with accelerometer-assessed PA (mg) and fitness data (N=11 463; age: 43-76 years) were included. The intensity distribution of PA was expressed in absolute and relative terms. The outcome was mortality. PA volume (average acceleration) and absolute intensity were lower with increasing age (~-0.03 to -0.04 SD of mean value across all ages per year; p<0.001) but differences in relative intensity by age were markedly smaller in women (-0.003 SD; p<0.184) and men (-0.012 SD; p<0.001). Absolute intensity was higher in men, but relative intensity higher in women (p<0.001). Over a median (IQR) follow-up of 8.1 (7.5-8.6) years, 121 (2.4 per 1000-person-years) deaths occurred in women and 203 (5.0 per 1000-person-years) in men. Lower risk of mortality was observed for increasing absolute or relative intensity in women, but for absolute intensity only in men. In men, the lowest risk (HR 0.62, 95% CI 0.43, 0.91) was observed in those with high absolute intensity (80th centile), but low relative intensity (20th centile). Conversely, in women, the lowest risk was associated with high levels (80th centile) of both absolute and relative intensity (HR 0.59, 95% CI 0.41, 0.86). Absolute PA intensity dropped with age, while relative intensity was fairly stable. Associations between PA intensity and mortality suggest that prescribing intensity in absolute terms appears appropriate for men, while either absolute or relative terms may be appropriate for women.