Between-day and within-day variability in the relation between heart rate and oxygen consumption: effect on the estimation of energy expenditure by heart-rate monitoring

Estimation of energy expenditure (EE) by heart-rate (HR) monitoring (HRM) assumes that the relation between HR and oxygen consumption (VO2) is stable between days and within a day. To evaluate this assumption, 12 healthy subjects underwent an HR-VO2 calibration session on two mornings and two afternoons, with one morning and one afternoon session on the same day. Measurements were made while subjects were supine, sitting, standing while shifting body weight side-to-side, and walking at four intensities. Subjects wore an HR monitor during waking hours on another day (15.1 +/- 1.5 h). Regression analysis was used to determine the relation between HR and VO2 in the sedentary and active HR ranges, and four EE values (HRM-EE) based on the four calibration sessions were calculated for each subject. The four group mean HRM-EE values were nearly identical (CV: 1.1%). The regression equations generated from the four calibration sessions did not differ significantly for the group as a whole, but for some subjects there were significant differences among sessions in the slope of the active regression equation (P = 0.005). Intraindividual CVs for HRM-EE were generally < 10%, but ranged from 0.1% to 24.7%. In general, within an individual, HR was more variable than was VO2, and intraindividual variability in EE was associated with intraindividual variability in the flex HR and sedentary HR range. HRM is appropriate for assessment of EE for a group; however, caution is recommended when HRM is used for individual determinations of EE.