Pulmonary O₂ uptake and muscle deoxygenation kinetics are slowed in the upper compared with lower region of the moderate-intensity exercise domain in older men.

This study sought to determine the effect of the pre-transition work rate (WR) and WR transition magnitude on the adjustment of pulmonary oxygen uptake (VO(2p) kinetics) in older men. Seven men (69 ± 5 years; mean ± SD) each performed 4-6 cycling transitions from 20 W to either a WR corresponding to 90% estimated lactate threshold (full step, FS) or 2 equal-step transitions (lower step, LS; upper step, US) to the same end-exercise WR as in FS. Gas exchange was analysed breath-by-breath and muscle deoxygenation (∆[HHb]) was measured with NIRS. The time constant (τ) for VO(2p) was greater in US (53 ± 17 s) and FS (44 ± 11 s) compared to LS (37 ± 9 s); τVO(2p) for US also trended (p = 0.05) towards being greater than FS. The VO(2p) gain in US (9.97 ± 0.41 mL/min/W) was greater than LS (9.06 ± 1.17; p = 0.06) and FS (9.13 ± 0.54; p < 0.05). The O(2) deficit was greater in US (0.25 ± 0.08 L) than LS (0.19 ± 0.06 L); yet the 'accumulated O(2) deficit' (0.44 ± 0.13 L; O(2) deficit from LS + US) was similar to that of FS (0.42 ± 0.13 L; p = 0.38). The effective Δ[HHb] response time (τ'∆[HHb]) for US (36 ± 12 s) was greater than LS (27 ± 6 s; p = 0.07) and FS (26 ± 4 s; p < 0.05), suggesting that the slowed adjustment of muscle O(2) extraction was associated with the slowed VO(2) kinetics of the US. Despite already slowed VO(2p) kinetics, older men exhibit further slowing when small WR transitions are initiated from an elevated pre-transition WR, yet this results in no cumulative impact on O(2) deficit. This slowing in US compared to LS does not appear to be related to local O(2) availability.