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

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.