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

¹Canadian Centre for Activity and Aging, ²School of Kinesiology, and ³Department of Physiology, The University of Western Ontario, London, Ontario, Canada N6A 3K7

Submitted 4 November 2003 ; accepted in final form 4 March 2004

Phase 2 pulmonary O₂ uptake (O_2p) kinetics are slowed with aging. To examine the effect of aging on the adaptation of O_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 O_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). O_2p data were filtered, interpolated to 1 s, and averaged to 5-s bins. HHb data were filtered and averaged to 5-s bins. O_2p 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 O_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 O_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 O_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 O_2p in O.

near-infrared spectroscopy; muscle O₂ utilization

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