Influence of phase I duration on phase II V̇O₂ kinetics parameter estimates in older and young adults

Older adults (O) may have a longer phase I pulmonary O₂ uptake kinetics (V̇o_2p) than young adults (Y); this may affect parameter estimates of phase II V̇o₂p. Therefore, we sought to: 1) experimentally estimate the duration of phase I V̇o_2p (EE phase I) in O and Y infjects during moderate-intensity exercise transitions; 2) examine the effects of selected phase I durations (i.e., different start times for modeling phase II) on parameter estimates of the phase II V̇o_2p response; and 3) thereby determine whether slower phase II kinetics in O infjects represent a physiological difference or a by-product of fitting strategy. V̇o_2p was measured breath-by-breath in 19 O (68 ± 6 yr; mean ± SD) and 19 Y (24 ± 5 yr) using a volume turbine and mass spectrometer. Phase IV̇o_2p was longer in O (31 ± 4 s) than Y (20 ± 7 s) (P < 0.05). In O, phase II τV̇o_2p was larger (P < 0.05) when fitting started at 15 s (49 ± 12 s) compared with fits starting at the individual EE phase I (43 ± 12 s), 25 s (42 ± 10 s), 35 s (42 ± 12 s), and 45 s (45 ± 15 s). In Y, τV̇o_2p was not affected by the time at which phase II V̇o_2p fitting started (τV̇o_2p = 31 ± 7 s, 29 ± 9 s, 30 ± 10 s, 32 ± 11 s, and 30 ± 8 s for fittings starting at 15 s, 25 s, 35 s, 45 s, and EE phase I, respectively). Fitting from EE phase I, 25 s, or 35 s resulted in the smallest CI τV̇o_2p in both O and Y. Thus, fitting phase II V̇o_2p from (but not constrained to) 25 s or 35 s provides consistent estimates of V̇o_2p kinetics parameters in Y and O, despite the longer phase I V̇o_2p in O.