Purpose: This study aimed to assess (1) the concurrent validity between critical power (CP) and the corrected power output at the respiratory compensation point (PORCP) and between W' and the quantity of energy depleted above PORCP (W'mod-SRS) in a modified Step-Ramp-Step (mod-SRS) protocol; (2) the test-retest reliability of endurance variables derived from a mod-SRS protocol. Methods: Twelve trained male triathletes (31 ± 6 yr, 67.9 ± 7.1 mL min-1 kg-1) performed (1) an SRS protocol, (2) 4-5 severe-intensity constant work rate trials, (3) two mod-SRS protocols (test/retest), and (4) a heavy-intensity constant work rate trial. CP and W' were computed using a "best individual fit" approach. PORCP was corrected appropriately, while W'mod-SRS was computed as the energy depleted above PORCP. Test-retest reliability was assessed with the intraclass correlation coefficient (ICC), the standard error of measurement (SEM), and the coefficient of variation (CV%). Results: CP was higher than PORCP in both trials (bias = 19 ± 19 W and 17 ± 20 W, P = 0.019 and 0.045). W' and W'mod-SRS were similar during both trials (bias = -2.8 ± 7.9 and -5.4 ± 8.8 kJ, P = 0.735 and 0.173). Test-retest reliability was moderate for W'mod-SRS (ICC = 0.66, SEM = 3.0 kJ, CV = 11.6%) and high for VO2peak, gas exchange threshold (GET), RCP, and their associated PO values (ICCs = 0.85-0.95, SEMs = 68-184 mL min-1, and 7-8 W, CVs = 2.1%-3.7%). Conclusions: The mod-SRS protocol reliably measures VO2peak, gas exchange threshold, and RCP. Corrected PORCP systematically underestimated CP, which consequently impacts the computation of W' using a mod-SRS protocol. The design of the protocol may allow a more complete depletion of energy available above RCP while still eliciting VO2peak.
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