Physiological Associations, Within-Session Repeatability, and Exploratory Prediction of Peak Oxygen Uptake Using the Revised 3-Minute Seated Stepping Test in Healthy Young Japanese Adults

Background and Objectives: Cardiopulmonary exercise testing (CPET) is a reference standard for cardiorespiratory fitness but requires specialized equipment and trained personnel. The Revised 3-Minute Seated Stepping Test (r3MSST) requires minimal space and equipment. This study examined its physiological associations, within-session repeatability, and exploratory prediction of peak V˙O2 in healthy young adults. Methods: Fifty-seven healthy young adults completed the r3MSST and maximal CPET. Associations with CPET-derived variables were examined using Spearman’s rank correlations. A peak V˙O2 prediction model was developed using multiple regression and internally validated by bootstrap resampling. Repeatability was assessed using the intraclass correlation coefficient (ICC), standard error of measurement (SEM), minimal detectable change at the 95% confidence level (MDC95), and Bland–Altman analysis. Results: Peak V˙O2 during the r3MSST was strongly associated with CPET-derived peak V˙O2 (rs = 0.725, 95% CI: 0.567–0.831, p < 0.001). The model explained 85.8% of peak V˙O2 variance (R2 = 0.858); bootstrap validation yielded optimism-corrected R2 = 0.788, root mean square error (RMSE) = 269.4 mL/min, and calibration slope = 0.960. Repeatability was uncertain (ICC = 0.774, 95% CI: 0.231–0.911; SEM = 79.1 steps; MDC95 = 219.2 steps); Bland–Altman analysis showed a mean difference of −82.8 steps (95% limits of agreement, −250.5 to 84.9). Conclusions: This study provides preliminary evidence that physiological responses during the r3MSST are associated with CPET-derived measures of exercise capacity in healthy young adults; however, the association between step count and CPET-derived peak V˙O2 was weak, its incremental predictive contribution was limited, and repeatability showed a systematic difference between trials. Further studies should establish between-day reliability, external predictive validity, and applicability in clinical populations.