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Developing a new curvilinear allometric model to improve the fit and validity of the 20-m shuttle run test as a predictor of cardiorespiratory fitness in adults and youthBackground and Objectives: Doubts have been raised concerning the validity of the 20m shuttle run test (20mSRT) as a predictor of cardiorespiratory fitness (CRF) in youth based on Léger’s equation/model. An alternative allometric model has been published recently that is thought to provide, not only a superior fit (criterion validity) but also a more biologically and physiologically interpretable model (construct validity). The purposes of this study were to explore whether allometry can provide a more valid predictor of CRF using 20mSRT compared with Léger’s equation/model. Methods: We fitted and compared Léger’s original model and an alternative allometric model using two cross-sectional datasets (youth, n=306; adult n=105) that contained measurements of CRF (V ̇O2peak /V ̇O2max) and 20mSRT performance. Quality-of-fit was assessed using explained variance (R2) and Bland and Altman’s limits of agreement. Results: The allometric models provided superior fits for the youth (explained variance R2=71.9%) and adult (R2=77.7%) datasets compared with Léger’s equation using their original fixed (R2=35.2%) or re-estimated parameter models (R2=65.9%), confirming that the allometric models demonstrate acceptable criterion validity. However, the allometric models also identified a non-linear “J-shaped” increase in energy cost (V ̇O2peak/V ̇O2max) with faster final shuttle-run speeds, (fitted speed exponent =1.52; 95% CI 1.38 to 1.65). Conclusion: Not only do allometric models provide more accurate predictions of CRF (V ̇O2peak/V ̇O2max; ml.kg-1.min-1) for both youth and adults (evidence of criterion validity), the “J-shaped” rise in energy demand with increasing final shuttle-run speed also provides evidence of construct validity, resulting in a more plausible, physiologically sound and interpretable model.