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Kinetics of O₂ and femoral artery blood flow during heavy-intensity, knee-extension exercise

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

Submitted 8 July 2004 ; accepted in final form 1 April 2005

It has been suggested that, during heavy-intensity exercise, O₂ delivery may limit oxygen uptake (O₂) kinetics; however, there are limited data regarding the relationship of blood flow and O₂ kinetics for heavy-intensity exercise. The purpose was to determine the exercise on-transient time course of femoral artery blood flow (_leg) in relation to O₂ during heavy-intensity, single-leg, knee-extension exercise. Five young subjects performed five to eight repeats of heavy-intensity exercise with measures of breath-by-breath pulmonary O₂ and Doppler ultrasound femoral artery mean blood velocity and vessel diameter. The phase 2 time frame for O₂ and _leg was isolated and fit with a monoexponent to characterize the amplitude and time course of the responses. Amplitude of the phase 3 response was also determined. The phase 2 time constant for O₂ of 29.0 s and time constant for _leg of 24.5 s were not different. The change () in O₂ response to the end of phase 2 of 0.317 l/min was accompanied by a _leg of 2.35 l/min, giving a _leg-to-O₂ ratio of 7.4. A slow-component O₂ of 0.098 l/min was accompanied by a further _leg increase of 0.72 l/min (_leg-to-O₂ ratio = 7.3). Thus the time course of _leg was similar to that of muscle O₂ (as measured by the phase 2 O₂ kinetics), and throughout the on-transient the amplitude of the _leg increase achieved (or exceeded) the _leg-to-O₂ ratio steady-state relationship (ratio 4.9). Additionally, the O₂ slow component was accompanied by a relatively large rise in _leg, with the increased O₂ delivery meeting the increased O₂. Thus, in heavy-intensity, single-leg, knee-extension exercise, the amplitude and kinetics of blood flow to the exercising limb appear to be closely linked to the O₂ kinetics.

Doppler blood flow; muscle oxygen uptake; oxygen uptake slow component

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