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1 Canadian Centre for Activity and Aging, The University of Western Ontraio, 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 Ontraio, 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.
Phase 2 pulmonary O2 uptake (VO2p) kinetics are slowed with aging. To examine the effect of aging on the adaptation of VO2p and deoxygenation of the vastus lateralis muscle at the onset of moderate-intensity constant-load cycling exercise, young (Y) (n=6; 25 ± 3 yrs) and older (O) (n=6; 68 ± 3 yrs) adults performed repeated transitions from 20 W to work rates corresponding to moderate-intensity (80% 
L; MOD) exercise. Breath-by-breath VO2p was measured by mass spectrometer and volume turbine. Deoxy-(HHb), oxy-(HbO2), and total Hb/Mb (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 data were filtered and averaged to 5 s bins. VO2p data were fit with a mono-exponential 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 VO2p was slower (p<0.01) in O (Y: 26 ± 7 s; O: 42 ± 9 s), whereas the delay prior to an increase in HHb (Y: 12 ± 2 s; O: 11 ± 1 s) and the HHb following 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 VO2p (Y: 7 ± 2 µM/L/min; O: 13 ± 4 µM/L/min) was greater (p<0.01) in O compared to Y. The slower VO2p kinetics in O compared to 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 VO2p in O.
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