The oxygen delivery response to acute hypoxia during incremental knee extension exercise differs in active and trained males.
BACKGROUND
It is well known that hypoxic exercise in healthy individuals increases limb blood flow, leg oxygen extraction and limb vascular conductance during knee extension exercise. However, the effect of hypoxia on cardiac output, and total vascular conductance is less clear. Furthermore, the oxygen delivery response to hypoxic exercise in well trained individuals is not well known. Therefore our aim was to determine the cardiac output (Doppler echocardiography), vascular conductance, limb blood flow (Doppler echocardiography) and muscle oxygenation response during hypoxic knee extension in normally active and endurance-trained males.


METHODS
Ten normally active and nine endurance-trained males (VO2max = 46.1 and 65.5 mL/kg/min, respectively) performed 2 leg knee extension at 25, 50, 75 and 100% of their maximum intensity in both normoxic and hypoxic conditions (FIO2 = 15%; randomized order). Results were analyzed with a 2-way mixed model ANOVA (group x intensity).


RESULTS
The main finding was that in normally active individuals hypoxic sub-maximal exercise (25 - 75% of maximum intensity) brought about a 3 fold increase in limb blood flow but decreased stroke volume compared to normoxia. In the trained group there were no significant changes in stroke volume, cardiac output and limb blood flow at sub-maximal intensities (compared to normoxia). During maximal intensity hypoxic exercise limb blood flow increased approximately 300 mL/min compared to maximal intensity normoxic exercise.


CONCLUSIONS
Cardiorespiratory fitness likely influences the oxygen delivery response to hypoxic exercise both at a systemic and limb level. The increase in limb blood flow during maximal exercise in hypoxia (both active and trained individuals) suggests a hypoxic stimulus that is not present in normoxic conditions.
Kennedy
Michael D
MD
Faculty of Rehabilitation Medicine, University of Alberta, Edmonton, Alberta, Canada.
Warburton
Darren Er
DE
Cardiovascular Physiology and Rehabilitation Laboratory, University of British Columbia, Vancouver, British Columbia, Canada.
Boliek
Carol A
CA
Faculty of Rehabilitation Medicine, University of Alberta, Edmonton, Alberta, Canada.
Esch
Ben Ta
BT
Cardiovascular Physiology and Rehabilitation Laboratory, University of British Columbia, Vancouver, British Columbia, Canada.
Scott
Jessica M
JM
Cardiovascular Physiology and Rehabilitation Laboratory, University of British Columbia, Vancouver, British Columbia, Canada.
Haykowsky
Mark J
MJ
Faculty of Rehabilitation Medicine, University of Alberta, Edmonton, Alberta, Canada.