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1 Canadian Centre for Activity and Aging, The University of Western Ontario, London, Ontario, Canada; School of Kinesiology, The University of Western Ontario, London, Ontario, Canada
2 Canadian Centre for Activity and Aging, The University of Western Ontario, London, Ontario, Canada; School of Kinesiology, The University of Western Ontario, London, Ontario, Canada; Department of Physiology and Pharmacology, The University of Western Ontario, London, Ontario, Canada
* To whom correspondence should be addressed. E-mail: dpaterso{at}uwo.ca.
The purpose was to examine the adaptation of pulmonary O2 uptake (VO2p) and deoxygenation of the vastus lateralis muscle at the onset of heavy-intensity constant-load cycling exercise in young (Y; 24 ± 4 yrs; n=5; mean ± SD) and older (O; 68 ± 3 yrs; n=6) adults. Subjects performed repeated transitions on 4 separate days from 20 W to a work rate corresponding to heavy-intensity (43) exercise. VO2 was measured breath-by-breath. The concentration changes in oxy-(HbO2), deoxy-(HHb) and total hemoglobin/myoglobin (Hbtot) were determined by near-infrared spectroscopy (NIRS; Hamamatsu NIRO-300). VO2p data were filtered, interpolated to 1 s, and averaged to 5 s bins. HHb-NIRS data were filtered and averaged to 5 s bins. A mono-exponential model was used to fit Vs O2p (phase 2, 
VO2p) and HHb (following the time delay (TD) from exercise onset to the start of an increase in HHb) data. The VO2p 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 HHb following TD was faster (p<0.05) in O (8 ± 2 s) than Y (14 ± 2 s). The slower VO2p kinetics and faster muscle deoxygenation in older compared to young adults during heavy-intensity exercise implies that the kinetics of muscle perfusion are slowed relatively more than those of VO2p in older adults. This suggests that the slowed VO2 kinetics in older adults may be a consequence of a slower adaptation of local muscle blood flow relative to that in young adults.
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