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1 School of Kinesiology, The University of Western Ontario, London, Ontario, Canada; Department of Physiology and Pharmacology, The University of Western Ontario, London, Ontario, Canada
2 Department of Physiology and Pharmacology, The University of Western Ontario, London, Ontario, Canada; Canadian Centre for Activity and Aging, London, Ontario, Canada
3 School of Kinesiology, The University of Western Ontario, London, Ontario, Canada; Department of Physiology and Pharmacology, The University of Western Ontario, London, Ontario, Canada; Canadian Centre for Activity and Aging, London, Ontario, Canada
* To whom correspondence should be addressed. E-mail: jkowalch{at}uwo.ca.
Six male subjects (23 ± 4 yrs; mean ± SD) performed repetitions (6-8) of two-legged, moderate-intensity, knee-extension exercise during two separate protocols that included step transitions from 3W 
90% estimated lactate threshold (
L) performed as a single step (S3), and in two equal steps (S1, 3W ~ 45% L; S2, ~ 45% L ~ 90% L). The time constants (
) of pulmonary oxygen uptake (VO2), leg blood flow (LBF), heart rate (HR) and muscle deoxygenation (HHb) were greater (p<0.05) in S2 (VO2, ~ 52s; LBF, ~ 39s; HR, ~ 42s; HHb, ~ 33s) compared to S1 (VO2, ~ 24s; LBF, ~ 21s; HR, ~ 21s; HHb, ~ 16s), while the delay prior to an increase in HHb was reduced (p<0.05) in S2 (~ 14s) compared to S1 (~ 20s). The VO2 and HHb amplitudes were greater (p<0.05) in S2 compared to S1, while the LBF amplitude was similar in S2 and S1. Thus the slowed VO2 response in S2 compared to S1 is consistent with a mechanism whereby VO2 kinetics is limited, in part, by a slowed adaptation of blood flow and/or O2 transport when exercise was initiated from a baseline of moderate-intensity exercise.
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