Cardiovascular Drift and Maximal Oxygen Uptake during Running and Cycling in the Heat.

Introduction: Greater cardiovascular (CV) drift occurs during cycling compared to running in temperate conditions. CV drift also corresponds to proportional reductions in maximal oxygen uptake (V˙O2max) during heat stress. Whether exercise mode differentially affects CV drift—and accompanying declines in V˙O2max—during heat stress is uncertain. The purpose of this study was to test the hypothesis that a greater magnitude of CV drift, accompanied by a greater decrement in V˙O2max, occurs during cycling compared to running in hot conditions. Methods: 7 active men (mean ± SD; age = 25 ± 6 yr, percent body fat = 11.9% ± 2.4%) completed a control graded exercise test (GXT) on a cycle ergometer and treadmill. Then on separate, counterbalanced occasions they completed 15 or 45 min of cycling or running at 60% V˙O2max in 35°C, immediately followed by a GXT to measure V˙O2max (4 trials total). The separate 15- and 45-min trials were designed to measure CV drift and V˙O2max over the same time interval. Results: Heart rate increased 19% and 17% and stroke volume decreased 20% and 15% between 15 and 45 min during running and cycling, respectively, but modes were not different (all P > 0.05). Despite a 1.8°C larger core-to-skin thermal gradient during running, decrements in V˙O2peak were not different between exercise modes (95% CI for difference in change scores between 15 and 45 min: −0.2, 0.3). Conclusions: CV strain (indexed as CV drift) during prolonged exercise in the heat corresponds to reduced V˙O2max, irrespective of exercise mode or the thermal gradient. As such, the upward drift in heart rate associated with CV drift reflects increased relative metabolic intensity (%V˙O2max) during prolonged cycling or running in the heat. [ABSTRACT FROM AUTHOR]