METABOLIC RESPONSES DURING ENDURANCE EXERCISE ANCHORED TO VIGOROUS HEART RATES

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). Power output (P) adjustments during constant HR exercise are required, resulting in metabolic rate decreases. PURPOSE: Examine 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 ± 4.5 yrs) 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 ± 8.4b·min-1; time to exhaustion [TLim] = 56.0 ± 10.1 min, range = 25.33-60.0 min), 86%HRmax (155.3 ± 9.5b·min-1; TLim = 44.2 ± 19.0 min, range = 13.7-60.0 min), and 95%HRmax (171.1 ± 10.2b·min-1; TLim = 9.7 ± 7.5 min, range = 2.0-26.7 min) to exhaustion or up to 60 min. VO2 and 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 intensities (p < 0.017) for VO2 and P. Mean VO2 values 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). VO2 (77%HRmax = 56.7 ± 3.8, 86%HRmax = 68.1 ± 4.3, 95% HRmax = 91.1 ± 3.3%V̇O2max) 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, VO2 and P significantly decreased relative to the initial values from 10%-100%TLim. Mean VO2 responses fell below the recommended VO2 range for vigorous exercise at 10%TLim for 77%HRmax, and at 90%TLim for 86%HRmax. 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 met the current guidelines, sustaining vigorous VO2 for at least 20 min. To ensure the required metabolic stimulus is met, researchers should consider the dissociation between HR and VO2 responses during constant HR exercise.