Abstract
BackgroundIt 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.MethodsTen 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 × intensity).ResultsThe 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.ConclusionCardiorespiratory 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.
Highlights
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
There was a significant interaction amongst FIO2 condition, fitness and intensity for stroke volume and total vascular conductance which revealed decreases at rest and 25% relative intensity in the active group (Figure 2B and 2C respectively)
Rest and Sub-maximal Exercise These results indicate that a hypoxic exercise stimulus elicits a limb blood flow increase compared to normoxia, at sub-maximal intensities (25%: + 378 mL/min, 50%: + 480 mL/min, 75%: + 210 mL/min; Figure 3A) in active males but not necessarily in trained males (Figure 3A)
Summary
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. A number of investigators have reported that during hypoxic sub-maximal knee extension (KE) exercise an increase in limb blood flow, leg oxygen extraction and enhanced limb vascular conductance occurs compared to the equivalent work rate in normoxia [1,2,3,4]. Limited research has examined the comparative cardiovascular response of highly trained athletes to active individuals during acute hypoxic maximal exercise [10,11] This is surprising considering the number of mass athletic competitions held at altitude (trail running races, cross country skiing races, mountain bike races) that include both average and elite participants. The increasing practice of altitude exposure in the form of tents for home use and live high – train low training methods purported to improve athletic performance [12,13,14,15], necessitates the need for more research examining the cardiovascular changes to acute hypoxia in well trained and lesser trained participants
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