Heart Rate Reserve and VO2 Reserve Are Not Interchangeable During Prolonged Exercise

Abstract Background Percentages of heart rate reserve (%HRR) and oxygen consumption reserve (%VO2R) provide equivalent intensities during incremental exercise. These percentages increase during prolonged exercise at a constant workload, and it is unclear if they increase to a similar degree. In this study, we tested whether %HRR and %VO2R maintain equivalency during prolonged exercise at a constant workload. Methods Fifteen males and 12 females participated in an incremental cycle ergometer test to determine maximum VO2. They then performed 2 prolonged exercise trials, 1 for 30 min at a vigorous-intensity workload corresponding with 60% VO2R, and 1 for 60 min at a moderate-intensity workload of 40% VO2R. HR and VO2 were measured continuously, and %HRR and %VO2R were reported every 5 min. A fan and consumption of cold water were used to minimize heat stress. Results Both %HRR and %VO2R increased during constant-workload exercise (P < 0.001), and %HRR increased significantly more than %VO2R (P < 0.001). Females exhibited greater %HRR than %VO2R beginning at 10 min into both trials (P < 0.05), while %HRR exceeded %VO2R for males beginning at 15 min of the 30-min trial (P < 0.05) and at 30 min of the 60-min trial (P < 0.05). HRR values for females were significantly greater than those for males at most time points (P < 0.05). Conclusion: During prolonged, constant-workload exercise, cardiovascular drift results in a greater increase in %HRR than %VO2R, and this is more pronounced in females than males.