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The matching of muscle O2 delivery to O2 utilization can be inferred from the adjustments in muscle deoxygenation (Δ[HHb]) and pulmonary O2 uptake (V̇o2p). This study examined the adjustments of V̇o2p and Δ[HHb] during ramp incremental (RI) and constant-load (CL) exercise in adult males. Ten young adults (YA; age: 25 ± 5 yr) and nine older adults (OA; age: 70 ± 3 yr) completed two RI tests and six CL step transitions to a work rate (WR) corresponding to 1) 80% of the estimated lactate threshold (same relative WR) and 2) 50 W (same absolute WR). V̇o2p was measured breath by breath, and Δ[HHb] of the vastus lateralis was measured using near-infrared spectroscopy. Δ[HHb]-WR profiles were normalized from baseline (0%) to peak Δ[HHb] (100%) and fit using a sigmoid function. The sigmoid slope ( d) was greater ( P < 0.05) in OA (0.027 ± 0.01%/W) compared with YA (0.017 ± 0.01%/W), and the c/ d value (a value corresponding to 50% of the amplitude) was smaller ( P < 0.05) for OA (133 ± 40 W) than for YA (195 ± 51 W). No age-related differences in the sigmoid parameters were reported when WR was expressed as a percentage of peak WR. V̇o2p kinetics compared with Δ[HHb] kinetics for the 50-W transition were similar between YA and OA; however, Δ[HHb] kinetics during the transition to 80% of the lactate threshold were faster than V̇o2p kinetics in both groups. The greater reliance on O2 extraction displayed in OA during RI exercise suggests a lower O2 delivery-to-O2 utilization relationship at a given absolute WR compared with YA.

This study examined the separate and combined effects of acute hypoxia (Hypo) and heavy-intensity "priming" exercise (Hvy) on pulmonary O(2) uptake (Vo(2p)) kinetics during moderate-intensity exercise (Mod). Breath-by-breath Vo(2p) and near-infrared spectroscopy-derived muscle deoxygenation {deoxyhemoglobin concentration [HHb]} were monitored continuously in 10 men (23 ± 4 yr) during repetitions of a Mod 1-Hvy-Mod 2 protocol, where each of the 6-min (Mod or Hvy) leg-cycling bouts was separated by 6 min at 20 W. Subjects were exposed to Hypo [fraction of inspired O(2) (Fi(O(2))) = 15%, Mod 2 + Hypo] or "sham" (Fi(O(2)) = 20.9%, Mod 2-N) 2 min following Hvy in half of these repetitions; Mod was also performed in Hypo without Hvy (Mod 1 + Hypo). On-transient Vo(2p) and [HHb] responses were modeled as a monoexponential. Data were scaled to a relative percentage of the response (0-100%), the signals were time-aligned, and the individual [HHb]-to-Vo(2) ratio was calculated. Compared with control (Mod 1), τVo(2p) and the O(2) deficit (26 ± 7 s and 638 ± 144 ml, respectively) were reduced (P0.05) in Mod 2-N (20 ± 5 s and 529 ± 196 ml) and increased (P0.05) in Mod 1 + Hypo (34 ± 14 s and 783 ± 184 ml); in Mod 2 + Hypo, τVo(2p) was increased (30 ± 8 s, P0.05), yet O(2) deficit was unaffected (643 ± 193 ml, P0.05). The modest "overshoot" in the [HHb]-to-Vo(2) ratio (reflecting an O(2) delivery-to-utilization mismatch) in Mod 1 (1.06 ± 0.04) was abolished in Mod 2-N (1.00 ± 0.05), persisted in Mod 2 + Hypo (1.09 ± 0.07), and tended to increase in Mod 1 + Hypo (1.10 ± 0.09, P = 0.13). The present data do not support an "O(2) delivery-independent" speeding of τVo(2p) following Hvy (or Hvy + Hypo); rather, this study suggests that local muscle O(2) delivery likely governs the rate of adjustment of Vo(2) at τVo(2p) greater than ∼20 s.

The relationship between the adjustment of muscle deoxygenation (Δ[HHb]) and phase II VO2p during moderate-intensity exercise was examined before (Mod 1) and after (Mod 2) a bout of heavy-intensity “priming” exercise. Moderate intensity VO2p and Δ[HHb] kinetics were determined in 18 young males (26 ± 3 yr). VO2p was measured breath-by-breath. Changes in Δ[HHb] of the vastus lateralis muscle were measured by near-infrared spectroscopy. VO2p and Δ[HHb] response profiles were fit using a monoexponential model, and scaled to a relative % of the response (0–100%). The Δ[HHb]/V̇o2 ratio for each individual (reflecting the local matching of O2 delivery to O2 utilization) was calculated as the average Δ[HHb]/V̇o2 response from 20 s to 120 s during the exercise on-transient. Phase II τVO2p was reduced in Mod 2 compared with Mod 1 ( P < 0.05). The effective τ′Δ[HHb] remained the same in Mod 1 and Mod 2 ( P > 0.05). During Mod 1, there was an “overshoot” in the Δ[HHb]/V̇o2 ratio (1.08; P < 0.05) that was not present during Mod 2 (1.01; P > 0.05). There was a positive correlation between the reduction in the Δ[HHb]/V̇o2 ratio and the smaller τVO2p from Mod 1 to Mod 2 ( r = 0.78; P < 0.05). This study showed that a smaller τVO2p during a moderate bout of exercise subsequent to a heavy-intensity priming exercise was associated with improved microvascular O2 delivery during the on-transient of exercise, as suggested by a smaller Δ[HHb]/V̇o2 ratio.