Effect of age on O 2 uptake kinetics and the adaptation of muscle deoxygenation at the onset of moderate-intensity cycling exercise

Phase 2 pulmonary O 2 uptake (V̇o 2 p ) kinetics are slowed with aging. To examine the effect of aging on the adaptation of V̇o 2 p 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 V̇o 2 p 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). V̇o 2 p data were filtered, interpolated to 1 s, and averaged to 5-s bins. HHb data were filtered and averaged to 5-s bins. V̇o 2 p data were fit with a monoexponential model for phase 2, 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 V̇o 2 p 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 V̇o 2 p (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 V̇o 2 p kinetics in O compared with Y adults was accompanied by a slower increase of local muscle blood flow and O 2 delivery discerned from a faster and greater muscle deoxygenation relative to V̇o 2 p in O.