(39) RESPONSES TO CARDIORESPIRATORY ENDURANCE EXERCISE ANCHORED TO A VIGOROUS HEART RATE INTENSITY

Exercise prescription to improve cardiorespiratory endurance (CE) is often based on percentages of the reserve or maximum heart rate (HR_max) or oxygen consumption rate (V̇O_2max) that reflect vigorous intensities (77-95% HR_max, 64-90%V̇O_2max). Exercise anchored to heart rate (HR) requires adjustments in the power output (P) that result in decreases in the metabolic rate (V̇O₂). The magnitude of changes in the P and V̇O₂ for exercise anchored to a vigorous intensity HR have not been determined. Examining these responses relative to the moderate, heavy, and severe exercise intensity domains will provide further insights into the physiological demands of the current CE exercise recommendations.

Purpose: This study examined the P and metabolic (V̇O₂) responses to exercise at a constant HR in the middle (86% HR_max) of the vigorous intensity range and quantified the time spent in the severe ( >respiratory compensation point [RCP]), heavy (< RCP, >ventilatory threshold [VT]), and moderate (< VT) exercise intensity domains.

Methods: Nine women (mean±SD, age=22.8±4.0yrs, body mass=64.4±6.3 kg, hheight=167.7±6.2 cm) performed a graded exercise test on a cycle ergometer to determineV̇O_2max, HR_max, peak power output (PPO), VT, and RCP. On a separate day, the subjects performed a constant HR trial at 86% HR_max to exhaustion or up to 60 min (time to exhaustion [T_Lim]=54.8±13.1 min, range=26.3-63.7min). V̇O₂ and P were recorded and normalized to their respective values at PPO in 10% intervals of T_Lim. One-way repeated measures ANOVAs, post-hoc t-tests with a Bonferroni corrected alpha of p< 0.005, and polynomial regression analyses were used to examine the time course of changes and patterns of responses (linear, quadratic) for V̇O₂ and P (p< 0.05).

Results: During the trial at 86% HR_max (159.6±8.9 b±min^-1) HR was maintained within ±2 b±min^-1 of the designated HR. There were negative, quadratic decreases in V̇O₂ (R²=0.978) and P (R² =0.984) that reflected significant decreases, relative to the initial (10%T_Lim) value from 20% to 100% of T_Lim. The V̇O₂decreased from 72±6% to 60.8±8% of V̇O_2max from the initial 10% to 100% of T_Lim. The mean V̇O₂ responses fell below the recommended V̇O₂ range (64-90% V̇O_2max) for vigorous exercise at 40% of T_Lim. The P decreased from 57±7% to 44±7% of PPO from the initial 10% to 100% of T_Lim. The subjects spent an average of 51.1±16.5, 3.5±4.0, and 0.2±0.6 min in the moderate, heavy, and severe intensity domains, respectively.

Conclusions: There was a decrease in the metabolic cost that tracked P for exercise anchored at 86% HR_max. The decreases in P resulted in the majority of the time spent in the moderate intensity domain and the relative V̇O₂ was below the recommend range for vigorous exercise for 60% of the trial. Thus, for some subjects, the metabolic intensity for exercise anchored to 86% HR_max may be too low to maximize improvements in CE fitness. PRACTICAL APPLICATIONS: Researchers and practitioners should consider the dissociation between HR and V̇O₂ responses during exercise anchored to HR to ensure the desired metabolic stimulus is met when examining or prescribing exercise to improve CE.


Acknowledgements: "NONE"