What Limits VO2max

• What is VO2max?

  ability of body to take up, transport, and utilize O2; a failure to increase VO2 despite an increase in metabolic demands
• What are the components of VO2?

  convection

  • VO2 = Q x a-vO2difference

  diffusion

  • VO2 = DMO2 x (PO2capillary - PO2mitochondria)
• Fick's Principle:

  • ability to deliver oxygenated blood and extract it at the muscle
• Convention

  • cardiovascular system and oxygen delivery
  • VO2 = Q x a-vO2difference
  • Q = HR x SV
  • a-vO2difference = (CaO2 - CvO2)
• Possible sites of VO2max limitation:

  1. pulmonary function
  2. cardiac output
  3. O2 carry capacity of blood
  4. muscle diffusion
  5. skeletal muscle metabolic factors
• 1 Pulmonary Function (VE):

  • does reaching a maximal limit of ventilation determine VO2max
  • breathing with hose causes an increase in VT
  • increase VT = increase VD
  • because VD increases, VE must increase too to maintain VA
  • therefore, there was extra space, but it was not used
  • therefore, something else is preventing maximum rate; not due to the lungs!
• 1 Pulmonary Diffusion (%SaO2):

  • does the ability of the lungs to transport inspired O2 from alveoli to pulmonary capillaries limit VO2max
  • saturation of trained athletes = no change before after after
  • saturation of highly trained athletes = decrease in saturation during maximum exercise
  • inside gas cylinder, very little change with trained, but highly trained increased VO2max
  • therefore, pulmonary diffusion can limit VO2max in highly trained athletes
• 1 Pulmonary Diffusion:

  • people with gas exchange problems de-saturate quickly and cannot exchange gas
  • inspire O2 = increase VO2 = improve diffusion into blood
  • 17% increase in VO2max when they breathe high O2 compared to room air
  • therefore, capacity of lungs can be a limiting factor to VO2max
• 2 Cardiac Output:

  • does heart's ability to raise cardiac output limit VO2max
  • testing VO2 in quads with whole body exercise vs single leg
  • cardiac output was less with single leg
  • increase flow = increase VO2max
  • therefore, the heart is a limiting factor
• 2 Cardiac Output:

  • big part of force = stretch on heart to increase contraction and blood flow
  • removing dog's pericardium
  • cardiac output increased and therefore increased VO2max after the pericardium was removed
  • therefore, cardiac output limits VO2max
• 2 Cardiac Output:

  • reducing HR via beta-blockade = decrease VO2max
  • beta-blockade blocks NE at SA node
  • if we lower HR, will that impair VO2max in untrained people
  • beta-blockade lowers HR and SV = lowers Q = lowers VO2max
  • therefore, beta-blockade is a limiting factor for VO2max
• 3 O2 Carrying Capacity of Blood (Hb):

  • does the amount of O2 we carry determine VO2max
  • concentration of Hb determines CaO2
  • increase Hb = increase CaO2
  • study reinfused formed elements into participants
  • this increase Hb content = increase VO2max during incremental exercise = increase endurance performance
  • therefore, CaO2 influences VO2max
• 4 Muscle Diffusion:

  1. distance between capillary and mitochondria
  2. contact area
  3. pressure gradient
• 4 Muscle Diffusion:

  • is there a limit to the amount of O2 we can extract from arterial blood
  • CaO2 does not change much
  • CvO2 does change
  • CvO2 falls as intensity increases
  • a-vO2difference widens
  • there is always O2 in blood, even during maximum exercise
  • therefore, muscle O2 diffusion capacity does not limit VO2max because there is always O2 in the blood
• 4 Muscle Diffusion:

  • PO2 is lower higher up in atmosphere
  • the lower the number of molecules in the gas you are breathing = lower PO2
  • decrease driving pressure (PO2) = decrease VO2max
  • therefore, PO2 influences VO2max; impairs ability of muscle to use O2
• 5 Skeletal Muscle Metabolic Factors:

  • is their a maximal ability of muscle to utilize O2 for ATP resynthesis
  • VO2max related to mitochondria --> more mitochondria = more O2 = higher VO2max
  • VO2max of whole muscle vs piece of muscle --> the same in untrained; VO2max increases in both conditions for trained (but higher for piece of muscle)
• Test yourself:

  • took blood out of people
  • blood volume decrease and Q/VO2max decrease
  • decrease blood volume = less pumped/min = less Hb = less O2/less blood moved