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 (H RM) 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 mor nings and two afternoons, with one morning and one afternoon session o n the same day. Measurements were made while subjects were supine, sit ting, 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 determin e the relation between HR and VO2 in the sedentary and active HR range s, and four EE values (HRM-EE) based on the four calibration sessions were calculated for each subject. The four group mean HRM-EE values we re nearly identical (CV: 1.1%). The regression equations generated fro m the four calibration sessions did not differ significantly for the g roup as a whole, but for some subjects there were significant differen ces among sessions in the slope of the active regression equation (P = 0.005). Intraindividual CVs for HRM-EE were generally < 10%, but rang ed from 0.1% to 24.7%. In general, within an individual, HR was more v ariable than was VO2, and intraindividual variability in EE was associ ated with intraindividual variability in the flex HR and sedentary HR range. HRM is appropriate for assessment of EE for a group; however, c aution is recommended when HRM is used for individual determinations o f EE.