Introduction: Exercise intensity is often regulated in hot conditions by maintaining a constant target heart rate (HR) to counteract increased physiological strain and thereby avoid premature fatigue. It is unknown, however, whether the HR-percent maximal oxygen uptake (%VO2max) relationship is maintained during prolonged exercise in the heat when the rise in HR concomitant with cardiovascular drift (CV drift) is eliminated by lowering exercise intensity. The purpose of this study was to determine if VO2max is reduced when exercise intensity and absolute VO2 are lowered by a magnitude sufficient to reduce CV drift and maintain constant HR during prolonged exercise in the heat, and thereby examine if the HR-%VO2max relationship is preserved.
Methods: Seven men cycled at 60% VO2max in 35 degrees C for 15 min (one trial) and 45 min (two trials) while HR rose over time (HRvar) or remained constant (HRcon). VO2max was measured immediately after the 15 and 45 min trials to correspond with the same time interval in which CV drift occurred.
Results: Power output decreased 37%, VO2 decreased 24%, and VO2max decreased 7.5% from 15 to 45 min in HRcon, while HR remained the same. In HRvar, HR increased 13%, SV decreased 10%, and VO2max decreased 15%.
Discussion: %VO2max was decreased from approximately 60% to 50% to hold HR constant in these conditions, so the HR-%VO2max relationship was not preserved in the absence of CV drift. Attenuating CV drift by lowering exercise intensity only partially eliminated the reduction in VO2max after prolonged exercise in the heat.