VO2 max (S1W5) Flashcards
How and why do we standardise VO2 max?
- Divide VO2max by body mass of the individual
- Allows us to compare VO2max of people of different body masses (relative VO2max)
Why use VO2 max as a measure for aerobic capacity?
- VO2max is an overall descriptor of the systems required to work to process O2
Explain how VO2 max and aerobic capacity links to the systems
Pulmonary ventilation
- taking up O2 to absorb it into the body
- once in the bloodstream, we need good O2-carrying capacity so O2 can reach tissues
Peripheral blood flow
- need a strong heart to distribute blood full of O2
- peripheral blood flow - O2 extraction - need to carry blood to the muscle cells
Aerobic metabolism
- need a high mitochondria count
- O2 extraction capacity will be smaller with less mitochondria and aerobic enzymes
Aerobic contribution vs exercise duration
- Short sprints use ATP-CP systems, long distance running uses aerobic glycolysis and lipolysis
- All energy systems are always active at the same time, just to different extents depending on exercise intensity
Aerobic contribution vs exercise duration: tap and tank analogy
Tanks
- Each tank contributes to overall ATP turnover
- Size of tanks - fat tank is enormous
- Can walk for 1000km if not constrained by anything else based on fat alone
Taps
- Energy per time from PC system is large (large tap) - lots of energy coming through at the same time
- Tap for the fat is small - released over a long period of time
The maths behind oxygen uptake (VO2): Fick equation
VO2 = HR x SV x (a-v)O2 difference
- a-v = artery-mixed venous
The maths behind oxygen uptake (VO2): Fick equation: central portion
- HR x SV
- Faster the heart beats (HR), the more blood/O2 goes into circulation
- Stroke volume x HR - how many litres of blood go out every minute? (cardiac output) - doesn’t tell us about O2 uptake
- The central CV system - heart and cardiac output
The maths behind oxygen uptake (VO2): Fick equation: peripheral portion ((a-v)O2 difference)
- How much O2 in the blood is being extracted by the muscle
- The difference between O2 in the arteries and veins
- The difference is the O2 that has been extracted by active tissue (muscle)
- ‘Ability of the muscles to extract O2’
The maths behind oxygen uptake (VO2): Fick equation: arterio-mixed venous
- Measuring O2 content in a vein coming from legs - blood came from every muscle in the leg but not all would have been active
- Blood comes back from active and inactive tissue, so it’s mixed-venous
Training adaptations at muscle level
- Determined by amount of blood supply to the muscle
- Capillary number
- Capillary recruitment
- Mitochondria size and number
- Oxidative enzymes
- Myoglobin
SNS activity
- Stress response
- Adrenaline can be measured - stressed = lots of adrenaline - leads to higher contractility and blood being supplied to muscles (not to intestines, etc)
What limits VO2 max? / What does VO2 max depend on?
VO2 depends on . . .
- a good cardio-vascular system and high cardiac output
- well-developed oxidative capacity in skeletal muscle (e.g. mitochondria, oxidative enzymes etc)
- oxygen delivery is the limiting factor
Evidence as to what limits VO2 max: acute exercise study
- isolated muscle can use more O2 than the heart can deliver
- e.g. one vs two legged exercise - O2 extraction by quads during one-legged exercise is 2-3 times higher than that measured in the same muscle group during whole body exercise
- implies a single leg can do more O2 extraction than compared to both legs being active - if body could supply more O2 to both legs we would be stronger
- therefore shows that blood supply/oxygen delivery is the limiting factor of VO2 max
Evidence as to what limits VO2 max: longitudinal training studies (Basset and Howley, 2000)
- muscle biopsy to measure mitochondrial enzymes and measure VO2 max
- 6-8 weeks training period then remeasured these
- 2.2 fold increase in mitochondrial enzymes but only ~20-40% increase in VO2 max
- enzymes increase more than VO2 max
- if the legs improved their aerobic capacity by a factor of 2.2, VO2 max should go up by 2.2 but this doesn’t happen
- therefore shows that blood supply/oxygen delivery is the limiting factor of VO2 max
Evidence as to what limits VO2 max: training study with manipulation of blood volume
- reducing blood volume through phlebotomy after training (to pre-training levels) reduces both cardiac output and VO2 max to pre-training levels (Montero et al, 2015)
- taking away the blood takes away the positive adaptations of blood delivery - suggests central factors limit VO2 max, not peripheral
central portion: stroke volume
- oxygen delivery, and thus VO2 max is partially limited by stroke volume
- can’t train maximum heart rate - stroke volume is trainable so is the principle limiter of VO2 max
