Duration of "phase I" Vo_2p: A comparison of methods used in its estimation and the effects of varying moderate-intensity work rate

present study was designed to investigate whether absolute work rate (WR) affects Phase I pulmonary oxygen uptake (V_O2_p) duration during moderate-intensity (Mod) exercise and to compare two methods for estimating Phase I V_O2_p duration (P_I-Dur). Fourteen males (24 ± 5 yr) each completed 4-8 repetitions of Mod transitions fromW to 50, 70, 90, 110, and 130 W. P_I-Dur was identified by 1) a marked decrease in both respiratory exchange ratio and end-tidal partial pressure of O2 following exercise onset [i.e., visual inspection of three independent reviewers, and the average (Avg) of the two most similar values]; or 2) the intersection (time delay, TD) of the first and second components in a biexponential nonlinear regression of the entire V_O2_p response from exercise onset. P_I-Dur did not differ among WRs (P > 0.05), regardless of the estimation method used. No differences were detected between Avg and TD (time in s) at any of the five WRs (50 W, 21 ± 6 vs. 23 ± 10 s; 70 W, 23 ± 9 vs. 23 ± 7 s; 90 W, 24 ± 3 vs. 22 ± 5 s; 110 W, 23 ±6 vs. 22 ± 6 s; 130 W,± 6 vs. 21 ± 7 s; P > 0.05 for Avg and TD, respectively). Broad limits of agreement within Bland-Altman plots revealed relatively weak agreement among reviewers for individual estimation of P_I-Dur. A nonsignificant correlation coefficient (r = 0.13) and broad limits of agreement suggest disparity between individual Avg and TD estimates of PI-Dur. The present data do not support a role for Mod WR in determining P_I-Dur per se. Furthermore, this study illustrated a poor agreement of P_I-Dur estimates derived from two different, but accepted methods.