Effect of age on O2 uptake kinetics and the adaptation of muscle deoxygenation at the onset of moderate-intensity cycling exercise.

Phase 2 pulmonary O2 up take (VO_2p) kinetics are slowed with aging. To examine the effect of aging on the adaptation of VO_2p, and deoxygenation of the vastus lateralis muscle at the onset of moderate-intensity constant-load cycling exercise, young (Y) (n = 6; 25 ± 3 yr) and older (O) (n = 6; 68 ± 3 yr) adults performed repeated transitions from 20 W to work rates corresponding to moderate-intensity (80% estimated lactate threshold) exercise. Breath-by-breath VO_2p was measured by mass spectrometer and volume turbine. Deoxy (HHb)-, oxy-, and total Hb and/or myoglobin were determined by near-infrared spectroscopy (Hamamatsu NIRO-300). VO_2p data were filtered, interpolated to 1 s, and averaged to 5-s bins. HHb data were filtered and averaged to 5-s bins. VO_2p data were fit with a monoexponential model for phase2, and HHb data were analyzed to determine the time delay from exercise onset to the start of an increase in HHb and thereafter were fit with a single-component exponential model. The phase 2 time constant for VO_2p was slower (P < 0.01) in O (Y: 26 ± 7 s, O: 42 ± 9 s), whereas the delay before an increase in HHb (Y: 12 ± 2 s; O: 11 ± 1 s) and the time constant for HHb after the time delay (Y: 13 ± 10 s; O: 9 ± 3 s) were similar in Y and O. However, the increase in HHb for a given increase in VO_2p (Y: 7 ± 2 µM·l^-1·min^-1; O: 13 ± 4 µM·l^-1·min^-1) was greater (P < 0.01) in O compared with Y. The slower VO_2p kinetics in O compared with Y adults was accompanied by a slower increase of local muscle blood flow and O₂ delivery discerned from a faster and greater muscle deoxygenation relative to VO_2p in O. [ABSTRACT FROM AUTHOR]