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

PURPOSE: This study quantified the metabolic demands (oxygen uptake (V̇O 2 )), power output adjustments, changes in the V̇O 2 /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% HR peak ). 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 (HR L = 77% HR peak ), middle (HR M = 86% HR peak ), and higher (HR H = 95% HR peak ) end of the vigorous intensity range. Time course of changes and patterns of responses were examined for V̇O 2 , power output, V̇O 2 /power output, and RPE for the composite and for each subject. RESULTS: Across the HR L (time to exhaustion ( Tlim ) = 56.3 ± 9.9 min), HR M (51.8 ± 13.5 min), and HR H (27.2 ± 17.7 min) trials, V̇O 2 and power output decreased quadratically ( P < 0.05) relative to the initial value from 10% to 100% of Tlim , whereas the V̇O 2 /power output increased quadratically from 20% to 100% Tlim , and RPE increased linearly from 50% to 100% Tlim . The V̇O 2 and RPE, collapsed across time, for HR L (54.3% ± 3.3% V̇O 2peak , 11 ± 1.5 RPE) were lower than HR M (64.9% ± 4.5% V̇O 2peak , 14 ± 1.7 RPE), and both were lower than HR H (80.1% ± 4.1% V̇O 2peak , 17 ± 1.4 RPE). None of the 12 subjects at HR L , 6 at HR M , and 7 at HR H were within the vigorous V̇O 2 range. CONCLUSIONS: The HR L was not sufficient to meet the desired metabolic intensity for vigorous exercise, whereas the middle to higher end of the range elicited a V̇O 2 within the prescribed range of only ~50%-60% of the subjects. This study indicated that exercise held constant at a percentage of HR peak cannot consistently be used to prescribe a desired metabolic stimulus.