Many reports describe using a supramaximal verification phase - exercising at a power output higher than the highest power output recorded during an incremental cardiopulmonary test - to validate VO2max. The impact of verification phases on estimating the proportion of individuals who increased VO2peak in response to high-intensity interval training (HIIT) remains an underexplored area in the individual response literature. This analysis investigated the influence of same-day and separate-day verification phases during repeated measurements (incremental tests – INCR1 and INCR2; incremental tests + supramaximal verification phases – INCR1+ and INCR2+) of VO2peak on typical error (TE) and the proportion of individuals classified as responders (i.e. the response rate) following four weeks of HIIT (n=25) or a no-exercise control period (n=9). Incorporation of supramaximal verification consistently reduced the standard deviation of individual response, typical error, and confidence interval widths. However, variances were statistically similar across all groups (p>0.05). Response rates increased when incorporating either one (INCR1 to INCR1+; 24% to 48%, p=0.07) or two (INCR2 to INCR2+; 28% to 48%, p=0.063) supramaximal verification phase(s). However, response rates remained unchanged when either zero-based thresholds or smallest worthwhile difference response thresholds were used (50% and 90% confidence intervals, all p>0.05). Supramaximal verification phases reduced random variability in VO2peak response to HIIT. Compared with separate-day testing (INCR2 and INCR2+), the incorporation of a same-day verification (INCR1+) reduced CI widths the most. Researchers should consider using a same-day verification phase to reduce uncertainty and better estimate VO2peak response rate to HIIT
