Cross-validation of Non-exercise Estimated Cardiorespiratory Fitness: The NHANES Study

Background: Cardiorespiratory Fitness (CRF) is an independent health predictor of circulatory and respiratory systems and can be estimated using non-exercise equations. However, the accuracy of such equations in a national representative population is unknown. The objective of this study was to cross-validate 11 CRF equations developed by three different researchers using a United States representative population. Methods and Findings: The study included 2470 adult males and females from the National Health and Nutrition Examination Survey (1999 to 2004) with measured CRF (mCRF) available in terms of maximum oxygen consumption (VO2max). The relationships between non-exercise estimated CRFs and measured VO2max were analyzed by examining the Constant Error (CE), Standard Error of Estimate (SEE), correlation coefficient (r) and Root Of Mean Square Error (RMSE). The estimated CRFs from four equations for males and six equations for females were different from mCRF, with CE values ranging from -0.712 (Jurca2) to 0.457 mL/kg/min (Jackson/fat/2level) for males and from -3.722 (Rexhepi2014) to 1.166 mL/kg/min (Jackson/fat/2level) for females (P<0.05 for all). Moreover, SEE, r and RMSE values ranged from 0.036 to 0.079 mL/kg/min, 0.21 to 0.344 mL/kg/min and 2.172 to 2.657 mL/kg/min, respectively. Furthermore, the lowest RMSE values for males (Jackson/fat/5level) and females (Jurca2) represented 20.33% and 21.09% of the mean mCRFs, respectively. Conclusion: Among the 11 equations, Jackson/fat/5level for males and Jurca2 for females provided the most valid non-exercise equations to estimate CRF in a representative US population. Future studies are warranted to develop more accurate equations based on age, gender, race and health status.