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Purpose: This study quantified the metabolic demands (oxygen uptake (...O2)), power output adjustments, changes in the ...O2/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 yr) performed a graded exercise test to exhaustion to determine peak parameters, and three 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 ...O2, power output, ...O2/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, ...O2 and power output decreased quadratically (P < 0.05) relative to the initial value from 10% to 100% of Tlim, whereas the ...O2/power output increased quadratically from 20% to 100% Tlim and RPE increased linearly from 50% to 100% Tlim. The ...O2 and RPE, collapsed across time, for HRL (54.3% ±3.3% ...O2peak, 11 ± 1.5 RPE) were lower than HRM (64.9% ± 4.5% ...O2peak, 14 ± 1.7 RPE), and both were lower than HRH (80.1% ± 4.1% ...O2peak, 17 ± 1.4 RPE). None of the 12 subjects at HRL, 6 at HRM, and 7 at HRH were within the vigorous ...O2 range. Conclusions: The HRL was not sufficient to meet the desired metabolic intensity for vigorous exercise, whereas the middle to higher end of the range elicited a ...O2 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. [ABSTRACT FROM AUTHOR]