Agreement Between Same-Day and Separate-Day Assessment of Critical Power, W′, and Peak Oxygen Uptake in Recreational Cyclists and Triathletes

Objective: The purpose of this exploratory study was to examine mean differences, reliability, and agreement between CP and W′ derived from a 3MAT performed after an ICT and values obtained during a standalone 3MAT performed on a separate day. Methods: Ten recreationally trained cyclists and triathletes completed four laboratory visits, including an ICT followed by a 3MAT after 30 min of recovery (same-day condition) and a standalone 3MAT performed on a separate day. CP and W′ were derived from both conditions and compared using paired t-tests and Bland–Altman agreement methods. Results: No systematic differences, but limited individual-level agreement were observed between same-day and separate-day conditions for CP (mean difference: 1.72 ± 36.5 W) or W′ (mean difference: 0.99 ± 5.9 kJ), with trivial to small effect sizes. Peak VO2 values were also not significantly different across testing conditions (p = 0.483). However, Bland–Altman analysis revealed wide limits of agreement for both CP (−69.8 to 73.3 W) and W′ (−10.7 to 12.7 kJ), indicating substantial variability at the individual level. Conclusions: Although same-day testing does not introduce systematic bias in CP or W′ estimation, the wide limits of agreement suggest that these protocols are not interchangeable for individual monitoring. Same-day testing may be appropriate for group-level assessments but should be used with caution when applied to individual athlete monitoring.