Purpose: This study aimed to investigate oxygen/energy cost (OC/EC) of running and substrate utilization before and after strenuous cycling in well-trained junior triathletes and the relevance of changes in these variables for fatigued running performance. Methods: Nineteen junior squad triathletes (4 female, 15 male; 17.2 [1.8] y; maximal oxygen uptake (VO2peak) 61.4 [5.1] mL·kg-1·min-1) completed 3 submaximal running steps (2.8 m·s-1, +0.4 m·s-1, and 5 min) under fresh conditions, followed by an incremental cycling test (~2 W·kg-1, +20 W, and 3 min) to exhaustion. Afterward, they performed another incremental running test to exhaustion under fatigued conditions for VO2peak, OC, and time-to-exhaustion assessment. During both runs, OC, EC, and carbohydrate/fat oxidation (CHO/FO) were assessed. Results: Contrary to trivial/small average changes in OC (210 [15] to 209 [14] mL·kg-1·km-1, P = .71) and EC (4.75 [0.33] to 4.59 [0.29] kJ·kg-1·km-1, P = .02), CHO decreased (2.96 [0.68] to 2.08 [0.68] g·min-1) while FO increased (0.15 [0.13] to 0.48 [0.22] g·min-1) significantly from fresh to fatigued running (P < .001). Besides VO2peak (r = .68, P = .002), the changes in CHO (r = -.60, P = .01) and FO (r = .67, P = .003) were significantly correlated with fatigued time to exhaustion (1715 [172] s). Multiple regression and commonality analysis identified VO2peak, OC, and the change in FO as the best model for time to exhaustion (R2 = 88%). Conclusion: Despite trivial/small changes in OC/EC from fresh to fatigued running, a pronounced shift in substrate utilization from CHO to FO was evident in junior triathletes, which was also associated with fatigued running performance.
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