Adjustments of pulmonary O ₂ uptake and muscle deoxygenation during ramp incremental exercise and constant-load moderate-intensity exercise in young and older adults

The matching of muscle O ₂ delivery to O ₂ utilization can be inferred from the adjustments in muscle deoxygenation (Δ[HHb]) and pulmonary O ₂ uptake (VO _2p). This study examined the adjustments of V O _2p 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). V O _2p 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.V O _2p 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 V O _2p kinetics in both groups. The greater reliance on O ₂ extraction displayed in OA during RI exercise suggests a lower O ₂ delivery-to-O ₂ utilization relationship at a given absolute WR compared with YA.