The relationship between oxygen uptake reserve and heart rate reserve is affected by intensity and duration during aerobic exercise at constant work rate.

The relationship between the percentage of heart rate reserve (%HRR) and percentage of oxygen uptake reserve (%VO₂R) has been recommended for prescribing aerobic exercise intensity. However, this relationship was derived from progressive maximal exercise testing data, and the stability of the relationship during prolonged exercise at a constant work rate has not been established. The main aim of this study was to investigate the stability of the %VO₂R-%HRR relationship during prolonged treadmill exercise bouts performed at 3 different constant work rates. Twenty-eight men performed 4 exercise tests: (i) a ramp-incremental maximal exercise test to determine maximal heart rate (HR(max)) and maximal oxygen uptake (VO₂(max)) and (ii) three 40-min exercise bouts at 60%, 70%, and 80% VO₂R. HR and VO₂ significantly increased over time and were influenced by exercise intensity (p < 0.001 and p = 0.004, respectively). A 1:1 relationship between %HRR and %VO₂R, and between %HRR and %VO₂(max), was not observed, with mean differences of 8% (t = 5.2, p < 0.001) and 6% (t = 4.8, p < 0.001), respectively. The VO₂ values predicted from the ACSM running equation were all significantly higher than the observed VO₂ values (p < 0.001 for all comparisons), whereas a difference for HR was observed only for the tenth min of exercise at 80% VO₂R (p = 0.041). In conclusion, the main finding of this study was that the %HRR-%VO₂R relationship determined by linear regression, obtained from progressive maximal exercise testing, did not apply to prolonged treadmill running performed at 3 work rates.