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Objectives: To examine the relations between maximal and submaximal indices of aerobic fitness and off road cycling performance in a homogeneous group of high level mountain bikers. Methods: 12 internationally competitive mountain bikers completed the study. Maximum oxygen uptake (V̇O2max), peak power output (PPO), power output (PO), and oxygen uptake (V̇O2) at first (VT) and second (RCT) ventilatory thresholds were measured in the laboratory, and correlated with race time during a cross country circuit race. Results: The only physiological indices of aerobic fitness correlated with off road cycling performance were PO and V̇O2 at RCT when normalised to body mass (r=20.63 and r=20.66, respectively; p,0.05). VT, V̇O2max, and PPO were not correlated to performance in this homogeneous group of high level mountain bikers. Conclusions: The results of this study suggest that submaximal indices of aerobic fitness such as PO and V̇O2 at RCT are more important determinants of off road cycling performance than maximal indices such as PPO and V̇O2max. This study confirms the importance of body mass for mountain biking performance. As aerobic fitness explained only 40 % of the variance, other physiological and technical factors should be investigated, as they may be important determinants of cross country performance among elite mountai