Adaptation of pulmonary O2 uptake kinetics and muscle deoxygenation at the onset of heavy-intensity exercise in young and older adults

The purpose was to examine the adaptation of pulmonary O-2 uptake ((V) over dot O-2p) and deoxygenation of the vastus lateralis muscle at the onset of heavy-intensity, constant-load cycling exercise in young (Y; 24 +/- 4 yr; mean +/- SD; n = 5) and older (O; 68 +/- 3 yr; n = 6) adults. Subjects performed repeated transitions on 4 separate days from 20 W to a work rate corresponding to heavy-intensity exercise. (V) over dot O-2p was measured breath by breath. The concentration changes in oxyhemoglobin, deoxyhemoglobin (HHb), and total hemoglobin/ myoglobin were determined by near-infrared spectroscopy (Hamamatsu NIRO-300). (V) over dot O-2p data were filtered, interpolated to 1 s, and averaged to 5-s bins. HHb-near-infrared spectroscopy data were filtered and averaged to 5-s bins. A monoexponential model was used to fit (V) over dot O-2p [ phase 2, time constant (tau) of (V) over dot O-2p] and HHb [ following the time delay (TD) from exercise onset to the start of an increase in HHb] data. The tau(V) over dot O-2p was slower ( P < 0.001) in O ( 49 +/- 8 s) than Y (29 +/- 4 s). The HHb TD was similar in O (8 +/- 3 s) and Y (7 +/- 1 s); however, the tau HHb following TD was faster ( P < 0.05) in O ( 8 +/- 2 s) than Y ( 14 +/- 2 s). The slower (V) over dot O-2p kinetics and faster muscle deoxygenation in O compared with Y during heavy-intensity exercise imply that the kinetics of muscle perfusion are slowed relatively more than those of (V) over dot O-2p in O. This suggests that the slowed (V) over dot O-2p kinetics in O may be a consequence of a slower adaptation of local muscle blood flow relative to that in Y.