BACKGROUND: To improve cardiorespiratory fitness, exercise prescription is often based on percentages of the maximum heart rate (HRmax) or oxygen consumption rate (VO2max) that reflect vigorous intensities (77-95%HRmax, 64-90%VO2max). Adjustments in power output (P) are required during constant HR exercise which results in metabolic rate decreases. This study examined the metabolic (VO2) and P responses during constant HR trials (77, 86, and 95% HRmax) within the vigorous intensity range. METHODS: Fourteen men (mean±SD, age=25.6±4.5yrs) performed a cycle ergometer graded exercise test to determine VO2max, HRmax, and peak power output (PP). Responses were recorded during three constant HR trials at 77%HRmax (138.9±8.4 b·min-1; time to exhaustion [TLim]=56.0±10.1 min, range=25.33-60.0min), 86%HRmax (155.3±9.5 b·min-1; TLim=44.2±19.0 min, range=13.7-60.0min), and 95%HRmax (171.1±10.2 b·min-1; TLim=9.7±7.5 min, range=2.0-26.7min) to exhaustion or up to 60 min. VO2 and power (P) were normalized to their respective values at PP in 10% intervals of TLim. Two-way repeated measures ANOVAs and t-tests with a Bonferroni corrected alpha were used for comparisons across time (p<0.005) and among HR trial intensities (p<0.017) for VO2 and P. Mean values for VO2 across time were qualitatively compared to the corresponding vigorous ranges for each HR trial. RESULTS: There were no time by intensity interactions, but there were main effects for time and intensity for VO2 and P (p<0.05). The VO2 (77%HRmax=56.7±3.8, 86%HRmax=68.1±4.3, 95% HRmax=91.1±3.3%VO2max) and P (77%HRmax=50.4±6.4, 86%HRmax=60.4±7.0, 95% HRmax=77.5±5.7%PP) responses were significantly lower for 77%HRmax relative to 86%HRmax and 95%HRmax, and 86%HRmax was lower than 95%HRmax. Across time, both VO2 and P significantly decreased relative to the initial values from 10%-100%TLim. The mean VO2 responses fell below the recommended VO2 range for vigorous exercise at 10% of TLim for 77%HRmax, and at 90% of TLim for 86%HRmax. The mean VO2 responses remained above the recommended VO2 range for vigorous exercise until 70% of TLim for 95%HRmax. CONCULSIONS: Only the 86%HRmax trial resulted in a vigorous VO2 that was sustained for at least 20 min to meet the current guidelines. To ensure the required metabolic stimulus is met, practitioners and researchers should consider the dissociation between HR and VO2 responses during constant HR exercise.

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