Differences in exercise limb blood flow and muscle deoxygenation with age: contributions to O2 uptake kinetics

The adjustments of pulmonary oxygen uptake ((V) over dot O-2p), limb blood flow (LBF) and muscle deoxygenation (Delta HHb) were examined during transitions to moderate-intensity, knee-extension exercise in seven older (OA; 71 +/- 7 year) and seven young (YA; 26 +/- 3 year) men. YA and OA performed repeated step transitions from an active baseline (3 W; 100 g) to a similar relative intensity of similar to 80% estimated lactate threshold (theta(L)), and YA also performed the same absolute work rate as the OA (24 W, 800 g). Breath-by-breath (V) over dot O-2p, femoral artery LBF (Doppler ultrasound) and muscle HHb (near-infrared spectroscopy) were measured. Phase 2(V) over dot O-2p, LBF, and Delta HHb data were fit with a mono-exponential model. tau(V) over dot O-2p was greater in OA (58 +/- 21 s) than YA(80%) (31 +/- 9 s) and YA(24W) (29 +/- 11 s). The increase in LBF per increase in (V) over dot O-2p was not different between groups (5.3-5.8 L min(-1)/L min(-1)); however, the tau LBF was greater in OA (44 +/- 19 s) than YA24W (18 +/- 7 s). The overall adjustment in Delta HHb (tau'DHHb) was not different between OA and YA, but was faster than tau(V) over dot O-2p in OA. This faster tau'Delta HHb than tau(V) over dot O-2p resulted in an overshoot of the normalized Delta HHb/Delta(V) over dot O-2p response relative to the steady state level that was significantly greater in OA compared with YA suggesting that the adjustment of microvascular blood flow is slowed in OA thereby requiring a greater reliance on O-2 extraction during the transition to exercise.