The Adaptation of Oxygen Uptake, Limb Blood Flow, and Muscle Deoxygentation at the Onset of Moderate-Intensity Knee-Extension Exercise in Young and Old Adults

Abstract

The slowed pulmonary O2 uptake (VO2p) kinetics (and muscle O2 consumption) of older adults may be limited by convective O2 delivery to the exercising limb. However, the effects of age on the kinetics of femoral artery blood flow (LBF) remain unknown. PURPOSE: To determine the effect of age on VO2p, LBF and muscle deoxygenation kinetics during moderate-intensity exercise. METHODS: 5 young (Y; 25 ± 2 yrs) and 5 old (O; 70 ± 6 yrs) male subjects performed repeated (n = 4) step-transitions (6 min) from an active baseline (3 W) to moderate-intensity (80% θL), alternate-leg, knee-extension (KE) exercise. VO2p was measured breath-by-breath by mass spectrometer and volume turbine. LBF was measured via Doppler ultrasound at the femoral artery. Deoxygenated hemoglobin/myoglobin (ΔHHb) of the vastus lateralis muscle was measured continuously by near-infrared spectroscopy. The VO2p, LBF and HHb data were filtered, time-aligned and ensemble-averaged to provide a single response for each subject, and then further averaged into 10 s (VO2p, LBF) or 5 s (ΔHHb) bins. VO2p data were fit with a monoexponential function from the phase 1- phase 2 interface to the end of exercise and LBF was fit from the onset to the end of exercise. Following a delay (HHbTD), HHb data were fit with a monoexponential function to 180 s to assess the adaptation of muscle deoxygenation during the adaptation period of VO2p. RESULTS: Phase 2 τVO2p was greater in O (45 ± 12 s) compared to Y (30 ± 5 s) during the on-transient of KE exercise. The magnitude of the steady-state increase in LBF (ΔLBF/Δ VO2p; O: 5.5 ± 1.3; Y: 5.3 ± 0.3) and the rate of increase of LBF (τLBF; O: 39 ± 19 s; Y: 29 ± 12 s) were not different between O and Y. HHbTD (O: 17 ± 4 s; Y: 17 ± 3 s), τHHb (O: 15 ± 8 s; Y: 10 ± 6 s) and the effective τHHb (HHbTD + τHHb; O: 32 ± 9 s; Y: 27 ± 5 s) were not different between groups. CONCLUSIONS: These results demonstrate a slower adaptation of VO2p in older subjects during moderate-intensity KE exercise but no significant age-related differences in the adaptation of bulk limb blood flow or muscle deoxygenation. The slower adaptation of muscle O2 consumption in O compared to Y and a similar adaptation of HHb (representing a faster contribution of HHb relative to the rise in VO2p in O compared to Y) would suggest that an impairment in microvascular flow/O2 delivery but not bulk limb blood flow contributes to the slower VO2p kinetics in O compared to Y.(Supported by NSERC)