Individualized Relative Intensity Physical Activity Accelerometer Cut-points.

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 2363 men and women aged 38 to 50 years 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 (%HRmax) and on rating of perceived exertion (RPE). Based on these two models, we obtained a moderate intensity (%HRmax=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 (cts/min). On average, cut-points were higher for men (4189 cts/min) versus women (3865 cts/min), and were higher for Whites (4088 cts/min) versus African Americans (3896 cts/min). Cut-points were correlated with BMI (rho=-0.11) and GXT duration (rho=0.33). Mean daily minutes of absolute and relative intensity moderate-to-vigorous PA (MVPA) were 34.1 (SD=31.1) min/day and 9.1 (SD=18.2) min/day, respectively. RPE cut-points were higher than those based on %HRmax. This is likely due to some participants ending the GXT prior to achieving their maximum heart rate. CONCLUSIONS Accelerometer-based relative intensity PA may be a useful measure of intensity relative to maximal capacity.