Individualized Relative-Intensity Physical Activity Accelerometer Cut Points

Abstract

Purpose Physical activity (PA) intensity is expressed as either absolute or relative intensity. Absolute intensity refers to the energy required to perform an activity. Relative intensity refers to a level of effort that takes into account how hard an individual is working relative to their maximum capacity. We sought to develop methods for obtaining individualized relative-intensity accelerometer cut points using data from a maximal graded exercise treadmill test (GXT) so that each individual has their own cut point. Methods A total of 2363 men and women 38 to 50 yr old from the CARDIA fitness study wore ActiGraph 7164 accelerometers during a maximal GXT and for seven consecutive days in 2005–2006. Using mixed-effects regression models, we regressed accelerometer counts on heart rate as a percentage of maximum (%HR_max) and on RPE. Based on these two models, we obtained a moderate-intensity (%HR_max = 64% or RPE = 12) count cut point that is specific to each participant. We applied these subject-specific cut points to the available CARDIA accelerometer data. Results Using RPE, the mean moderate-intensity accelerometer cut point was 4004 (SD = 1120) counts per minute. On average, cut points were higher for men (4189 counts per minute) versus women (3865 counts per minute) and were higher for Whites (4088 counts per minute) versus African Americans (3896 counts per minute). Cut points were correlated with body mass index (rho = −0.11) and GXT duration (rho = 0.33). Mean daily minutes of absolute- and relative-intensity moderate to vigorous PA were 34.1 (SD = 31.1) min·d⁻¹ and 9.1 (SD = 18.2) min·d⁻¹, respectively. RPE cut points were higher than those based on %HR_max. This is likely due to some participants ending the GXT before achieving their HR_max. Conclusions Accelerometer-based relative-intensity PA may be a useful measure of intensity relative to maximal capacity.