Recalibrated FRIEND equation for peak oxygen pulse is accurate in endurance athletes: the NOODLE study

Peak oxygen pulse (O₂P_peak) is an important index of cardiorespiratory fitness (CRF). The FRIEND database is a global source of reference values for CRF. However, no reference equation is tailored for endurance athletes (EA) to predict O₂P_peak. Here, we adjusted the well-established FRIEND equation for O₂P_peak to the characteristics of the EA population. 32 (34.0%) female EA and 62 (66.0%) male well-trained EA underwent maximal cardiopulmonary exercise test on a treadmill. V̇O_2max was 4.5 ± 0.5 L min^-1 in males and 3.1 ± 0.4 L min^-1 in females. O₂P_peak was 23.6 ± 2.8 mL beat^-1 and 16.4 ± 2.0 mL beat^-1 for males and females, respectively. Firstly, we externally validated the original FRIEND equation. Secondly, using multiple linear regression, we adjusted the FRIEND equation for O₂P_peak to the population of EA. The original FRIEND equation underestimated O₂P_peak for 2.9 ± 2.9 mL beat^-1 (P < .001) in males and 2.2 ± 2.1 mL beat^-1 (P < .001) in females. The updated equation was 1.36 + 1.07 (23.2 · 0.09 · age - 6.6 [if female]). The new equation explained 62% of the variance and significantly predicted O₂P_peak (R² = 0.62, β = 0.78, P < .001). The error of the EA-adjusted model was 0.1 ± 2.9 mL beat^-1 (P = .82) and 0.2 ± 2.1 mL beat^-1 (P = .65) for males and females respectively. Recalibration of the original FRIEND equation significantly enhances its accuracy among EA. The error of the EA-adjusted model was negligible. A new recalibrated equation should be used to predict O₂P_peak in the population of EA.