Metabolic and Perceptual Responses to Constant Heart Rate Exercise at Vigorous Intensities in Women

Abstract

Purpose This study quantified the metabolic demands (oxygen uptake [V̇O2]), power output adjustments, changes in the V̇O2/power output ratio, and perceptual responses (rating of perceived exertion [RPE]) during constant heart rate (HR) exercise performed within the vigorous intensity range (77-95% HRpeak). Methods Twelve women (mean ± SD age: 22 ± 4 years) performed a graded exercise test to exhaustion to determine peak parameters and 3, randomly ordered, constant HR trials to exhaustion or for 60 min at the lower (HRL = 77% HRpeak), middle (HRM = 86% HRpeak), and higher (HRH = 95% HRpeak) end of the vigorous intensity range. Time course of changes and patterns of responses were examined for V̇O2, power output, V̇O2/power output, and RPE for the composite and for each subject. Results Across the HRL (time to exhaustion [Tlim] = 56.3 ± 9.9 min), HRM (51.8 ± 13.5 min), and HRH (27.2 ± 17.7 min) trials, V̇O2 and power output decreased quadratically (p < 0.05) relative to the initial value from 10-100% of Tlim, while the V̇O2/power output increased quadratically from 20-100% Tlim, and RPE increased linearly from 50-100% Tlim. The V̇O2 and RPE, collapsed across time, for HRL (54.3 ± 3.3% V̇O2peak, 11 ± 1.5 RPE) were lower than HRM (64.9 ± 4.5% V̇O2peak, 14 ± 1.7 RPE) and both were lower than HRH (80.1 ± 4.1% V̇O2peak, 17 ± 1.4 RPE). None of the 12 subjects at HRL, 6 at HRM, and 7 at HRH were within the vigorous V̇O2 range. Conclusions The HRL was not sufficient to meet the desired metabolic intensity for vigorous exercise, while the middle to higher end of the range elicited a V̇O2 within the prescribed range of only ~50-60% of the subjects. This study indicated that exercise held constant at a percentage of HRpeak cannot consistently be used to prescribe a desired metabolic stimulus.