Cardiorespiratory fitness levels among US adults 20-49 years of age: Findings from the 1999-2004 National Health and Nutrition Examination Survey - Wang, C-Y; Haskell, W L; Farrell, S W; Lamonte, M J; Blair, S N; Curtin, L R; Hughes, J P; Burt, VL (Am J Epidemiol, 2010)