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J Appl Physiol 99: 1822-1834, 2005. First published July 21, 2005; doi:10.1152/japplphysiol.01183.2004
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Kinetics of O2 uptake, leg blood flow, and muscle deoxygenation are slowed in the upper compared with lower region of the moderate-intensity exercise domain

Shelley L. MacPhee,1,2 J. Kevin Shoemaker,2,3 Donald H. Paterson,1,2 and John M. Kowalchuk1,2,3

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

Submitted 21 October 2004 ; accepted in final form 18 July 2005

Six male subjects [23 yr (SD 4)] performed repetitions (6–8) of two-legged, moderate-intensity, knee-extension exercise during two separate protocols that included step transitions from 3 W to 90% estimated lactate threshold ({theta}L) performed as a single step (S3) and in two equal steps (S1, 3 W to ~45% {theta}L; S2, ~45% {theta}L to ~90% {theta}L). The time constants ({tau}) of pulmonary oxygen uptake (O2), leg blood flow (LBF), heart rate (HR), and muscle deoxygenation (HHb) were greater (P < 0.05) in S2 ({tau}O2, ~52 s; {tau}LBF, ~ 39 s; {tau}HR, ~42 s; {tau}HHb, ~33 s) compared with S1 ({tau}O2, ~24 s; {tau}LBF, ~21 s; {tau}HR, ~21 s; {tau}HHb, ~16 s), while the delay before an increase in HHb was reduced (P < 0.05) in S2 (~14 s) compared with S1 (~20 s). The O2 and HHb amplitudes were greater (P < 0.05) in S2 compared with S1, whereas the LBF amplitude was similar in S2 and S1. Thus the slowed O2 response in S2 compared with S1 is consistent with a mechanism whereby O2 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.

oxygen uptake kinetics; femoral arterial blood flow kinetics; Doppler ultrasound; knee-extension exercise; near-infrared spectroscopy


Address for reprint requests and other correspondence: J. M. Kowalchuk, Canadian Centre for Activity and Aging, School of Kinesiology, 3M Centre, The Univ. of Western Ontario, London, Ontario, Canada N6A 3K7 (e-mail: jkowalch{at}uwo.ca)




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