What is VO2 max?
VO2 max is the maximal amount of oxygen an individual can utilize, typically over one minute during an intense, maximal effort. It reflects the maximal capacity of the individual to perform aerobic work. It is generally seen to be the gold standard measure for a person’s aerobic fitness. (1)
Oxygen is required by skeletal muscles for the aerobic metabolism of glucose and fatty acids, our bodies main sources of fuel. During aerobic metabolism of glucose, oxygen is used to produce 32 molecules of a substance called ATP. ATP is vital for the contraction of muscle fibres. During anaerobic metabolism (when no oxygen is present), only 2 molecules of ATP are produced. This is therefore only able to sustain exercise for very short periods of time. In addition, lactic acid is produced by anaerobic metabolism, which leads to muscle fatigue due to changes in pH. (2, 3)
An individual’s VO2 max is reliant on 2 factors. Firstly, the delivery of oxygen to the skeletal muscles. This involves the diffusion of oxygen from the air into the blood within the lungs, and the delivery of this oxygenated blood to the muscles by the heart and blood vessels. The second factor is the uptake of oxygen from the blood into the skeletal muscles, and the utilisation of this oxygen to create ATP.
When we exercise, as intensity increases, the oxygen demands of the skeletal muscles increases. Initially, the body can meet this demand, and oxygen consumption increases linearly. There comes a point however, when our bodies are not able to match the demand, and oxygen consumption then plateaus. This point is the VO2 max.
Why does it matter?
An individual’s VO2 max is a key indicator of athletic performance potential. A person cannot maintain exercise at an intensity above their VO2 max for prolonged periods of time. A high intensity exercise like a typical CrossFit workout is performed at around 80% of VO2 max. If one individual has a VO2 max of 80ml/kg/min and another has a VO2 max of 60ml/kg/min, then they will be carrying out this workout with an oxygen consumption of 64ml/kg/min and 48ml/kg/min respectively (80%). The individual who is capable of performing this exercise with a higher oxygen consumption will have the potential to produce more energy (more ATP).
One study of 23 recreational triathletes showed total performance times are significantly correlated with VO2 max values. (4)
A study of 13 male ultramarathon and 9 female marathon runners showed a significant association between VO2 max and increased athletic performance. Interestingly, for the men, the 5km times were more closely associated with VO2 max than the 84km times. After regression analysis, 98% of the total variance of performance times could be attributed to VO2 max alone. (5)
It is important to remember however, that although VO2 max gives us an indicator of maximal performance potential, this is not always correlated with actual performance. Individual effort, mindset and skill will also play an important role. I am sure we have all had times where it is the difficulty in the skill of a movement that governs our time in a workout, rather than our aerobic capacity.
What limits our VO2 max?
As discussed earlier, a number of different factors are needed for oxygen to be delivered from the air to the energy producing parts of a skeletal muscle cell. Do we need to train all these areas to improve VO2 max? Well the answer to this question seems to be no, as it appears that in humans at least, not all these factors are limiting.
We will start at the muscle. During maximal exercise, almost all the arterial oxygen which is delivered to skeletal muscle is extracted. The arterial oxygen content is about 200ml per litre. Venous content draining from maximally working muscles is 20-30ml per litre, so around 90% of the delivered oxygen is extracted. This implies that it is the oxygen supply, rather than the oxygen extraction by the muscles which is limiting VO2 max. Skeletal muscle capillary density is increased by training, which has been shown to increase VO2 max by increasing oxygen delivery to the muscle. (6)
The lungs are the site of oxygen diffusion from the air into the blood. At rest, a healthy individual with have an arterial oxygen saturation of more than 95%, so the lungs are efficient at oxygenating the blood. During exercise however, trained athletes are actually more likely to have a decrease in arterial oxygen saturations than untrained individuals. This is because trained athletes have a higher cardiac output (the amount of blood pumped by the heart per minute), so the blood travels more quickly through the lungs and therefore has less time to pick up oxygen. This effect can be reduced by increasing the oxygen content of inspired air.
Powers et al divided 20 subjects into 2 groups depending on their level of activity- low activity individuals with an average VO2 max 56ml/kg/min vs. high activity individuals with an average VO2 max 70ml/kg/min. They tested their VO2 max in room air (21% oxygen) and in a high oxygen environment (26% oxygen). The low activity group had higher oxygen saturations during exercise in both environments compared to the high activity group. VO2 max however did not increase significantly in the low activity group with extra oxygen, but it did increase significantly in the high activity group (from 70 to 74) in the high oxygen environment. This implies that in highly trained individuals, gas exchange in the lungs may contribute towards limitation of VO2 max. (7, 8)
VO2 max is shown to be strongly correlated to cardiac output. The cardiac output is calculated by heart rate multiplied by stroke volume (amount of blood pumped by the heart per beat). Given that maximum heart rate does not vary much from person to person, it follows that stroke volume is the main determinant of an individual’s cardiac output and therefore a major factor in determining their VO2 max.
Multiple studies have shown that improvements in VO2 max from training resulted mostly from increases in cardiac output. (9, 10)
Increasing the oxygen carrying capacity of the blood by increasing the number of red blood cells is another way to improve VO2 max. Blood doping is one example of this. Reinfusion of 900–1,350 mL blood has been shown to increase V̇O2 max by 4–9%. (11, 12)
This is also the rationale behind altitude training. Chronic exposure to a low oxygen environment stimulates the body to produce more red blood cells, thus increasing the oxygen carrying capacity of the blood. Levine et al showed a 5% increase in VO2 max and 9% increase in red cell mass following 10 weeks of ‘live high train low’ altitude training in competitive runners compared to control.
We can conclude therefore, that it is our cardiorespiratory system providing oxygen delivery to muscles that is the major limiting and determining factor in an individual’s VO2 max, and therefore their aerobic performance potential.
Would you like to read about how to train to improve VO2 max? This is a vast topic and I have only covered a brief overview here. Please comment with any questions or feedback below and let me know your suggestions for future topics that you would like me to cover!