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 (Δ[HHb]) was measured with NIRS. The time constant (s) 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 O₂ 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; O₂ 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'δD[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 O₂ extraction was associated with the slowed VO₂ 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 O₂ deficit. This slowing in US compared to LS does not appear to be related to local O₂ availability.