Correlations and 90% confidence limits (dashed line) between mean carbohydrate and fat oxidation rates measured during the high-intensity interval training sequence relative to the maximal oxygen uptake (VO2max) measured during the progressive exercise test in well-trained (WT, triangle) and recreationally trained (RT, circle) runners. Correlations and 90% confidence limits (dashed line) between mean carbohydrate and fat oxidation rates measured during the high-intensity interval training sequence relative to the maximal oxygen uptake (VO2max) measured during the progressive exercise test in well-trained (WT, triangle) and recreationally trained (RT, circle) runners. Ken J Hetlelid et al. BMJ Open Sport Exerc Med 2015;1:e000047 ©2015 by BMJ Publishing Group Ltd