Purpose: Given the growing evidence of the importance of durability in endurance performance, this study aimed to investigate the reliability of a triathlon-specific test protocol for assessing physiological determinants of fatigued running performance, that is, after strenuous cycling in well-trained triathletes of both sexes. Methods: On 2 occasions, 10 triathletes (5 females; maximal oxygen uptake [VO2peak] 60.4 [6.0] mL·min-1·kg-1) completed an incremental cycling test (~2 W·kg-1, +20 W, 3 min) to exhaustion followed by a constant-workload segment at 85% of power at lactate threshold 2 (LT2) to reach 20 kJ·kg-1 (males) or 15 kJ·kg-1 (females) of total mechanical work. Subsequently, they performed an incremental running test to exhaustion (2.8 m·s-1, +0.4 m·s-1, 5 min + 30-s rest) to assess fatigued VO2peak, oxygen cost, and lactate thresholds. Energy expenditure and carbohydrate/fat oxidation were calculated during all submaximal work rates. Results: All diagnostic determinants of fatigued running performance showed good to excellent reliability (intraclass correlation coefficient [ICC] >= .845), except for fractional utilization of VO2peak at LT2 (ICC = .193), along with small measurement errors (typical error <= 3.6%) for all determinants. In contrast to the excellent reliability of energy expenditure during constant-workload cycling (ICC = .940, typical error = 3.4%), substrate oxidation demonstrated limited reliability during constant cycling and submaximal fatigued running (ICC = .560-.928, typical error = 6.3%-19.6%). Conclusion: The triathlon-specific test protocol demonstrated high reliability for measures of fatigued running performance, comparable to fresh measurements in previous reliability studies, making it suitable for regular evaluation of triathletes.
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