Objectives While reliable performance diagnostics are crucial for guiding training and detecting small changes in swimming performance, the reliability of existing protocols remains insufficiently evaluated. Design Test-retest study. Methods Twenty-four swimmers and triathletes of both sexes completed a 20-s sprint to assess anaerobic power (i.e., maximal lactate accumulation rate [cLamax], peak lactate concentration [Lapeak]) and a 3-min step test (ST) to evaluate aerobic performance indicators (i.e., post-exercise peak oxygen uptake [VO2peak], cost of swimming [Cs], speed at lactate thresholds [vLT1, vLT2] and in the final step [vpeak], alongside corresponding heart rate [HR]) on three separate days (within 3 weeks). Reliability and agreement were examined using (among others) intraclass correlation coefficient (ICC 2,1), typical error (TE), and TE to smallest worthwhile change (SWC) ratio, indicating sensitivity. Results The sprint demonstrated moderate reliability for cLamax (ICC: 0.625-0.728) and Lapeak (ICC: 0.705-0.749), with high TEs of 11.8-16.8% and 9.7-12.0%, respectively. Although familiarization improved reliability, sensitivity remained inadequate (TE/SWC: =1.23). The ST exhibited good to excellent reliability (mixed sensitivity) across all parameters. Uncorrected post-exercise VO2peak (ICC: 0.829-0.963, TE: 3.8-7.1%, TE/SWC: 1.02-1.83) and Cs (ICC: 0.884-0.952, TE: 3.8-5.7%, TE/SWC: 0.83-1.17) aligned with HR-corrected values. vLT1, vLT2, and vpeak showed excellent reliability (ICC: 0.971-0.990, TE: 0.8-1.4%) with adequate sensitivity (TE/SWC: 0.60-1.14), whereas HR measures were less reliable (ICC: 0.828-0.951, TE: 1.2-3.5%) and insufficiently sensitive (TE/SWC=1.60). Conclusions The ST provides a reliable method for assessing aerobic performance, with high sensitivity for lactate-based training intensity markers and Cs, but limited sensitivity for VO2peak and HR. The 20-s sprint requires methodological refinement before practical application.
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