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1 Canadian Centre for Activity and Aging, London, Ontario, Canada; School of Kinesiology, The University of Western Ontario, London, Ontario, Canada
2 Canadian Centre for Activity and Aging, 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 temporal relationship between the kinetics of phase 2 pulmonary O2 uptake (VO2p) and deoxygenation of the vastus lateralis muscle was examined during moderate-intensity constant-load leg cycling exercise. Young adults (5 male, 6 female; 23 ± 3 yr; mean ± SD) performed repeated transitions on 3 separate days from 20 W to a work rate corresponding to moderate-intensity (80% ventilatory threshold) exercise. Breath-by-breath VO2p was measured by mass spectrometer and volume turbine. 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. 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 (30 ± 8 s) was slower (p<0.01) than the HHb (10 ± 3 s). The delay prior to an increase in HHb was 13 ± 2 s. This delay may reflect either a delay in activation of muscle O2 consumption "metabolic inertia" and/or an early close matching of local muscle blood flow and muscle O2 consumption, or a reduced volume of Hb in the field off-setting an increased deoxygenation. Thereafter, the kinetics of local muscle deoxygenation assessed by NIRS were faster than those of phase 2 VO2p suggesting that the adaptation of muscle O2 consumption during the on-transient of exercise is accomplished by an increase in O2 extraction and a widening of the a-vO2 difference, with the rate of increased local perfusion and the increase in O2 delivery not closely matched to the metabolic rate of the tissue.
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