UC San Diego

Background: Regular physical activity (PA) reduces the risk of new chronic diseases, slows the progression of prevalent chronic diseases, and promotes other health benefits. An older individual’s level of energy expenditure while performing an activity (absolute intensity) may be discordant with their level of exertion relative to their maximal possible effort which expresses the very same amount of absolute effort as a percent of the individual’s maximum capacity (relative intensity). VO2max (maximal oxygen uptake (mL/kg/min)), the gold standard measurement of cardiorespiratory fitness, can be used to normalize absolute activity; if the VO2max of an individual is known and their instantaneous oxygen uptake (VO2) can be measured, ((VO2 / VO2max)*100) gives the percent of their maximal exercise capacity (i.e., their relative intensity). Methods: Aim 1 assessed the performance of previously published VO2max prediction equations in relation to measured VO2max and recalibrated the equations. Aim 2 developed new machine-learned (ML) VO2max prediction algorithms. In the final aim, daily hours spent in light intensity and moderate-to-vigorous PA (MVPA) were calculated on the absolute scale (accelerometer-measured PA) and relative scale (accelerometer-estimated VO2 / predicted VO2max using Aim 2’s algorithms). The associations between absolute and relative intensity PA, total mortality, and incident major cardiovascular disease (CVD) were quantified. Results: In Aim 1, the prediction equations yielded root mean squared error (RMSE) values ranging from 4.2 to 20.4 mL/kg/min and from 3.9-4.2 mL/kg/min after recalibration. The newly developed ML algorithms in Aim 2 yielded RMSE values ranging from 2.9-4.4 mL/kg/min. In Aim 3, on each PA measurement scale (relative and absolute), an increase in light intensity and MPVA reduced the risk of both outcomes. On the absolute scale, MVPA was more strongly associated with both outcomes than light intensity, but the opposite occurred on the relative scale. Conclusion: The PA intensity paradigm should keep shifting towards recommendation of more movement, regardless of intensity, and placing greater emphasis on relative light intensity (37% - 46% of maximal capacity) as modifiable behavioral targets that are more easily achieved, reduce risks of death and CVD, and promote healthy aging.