VO2 Max: The Key to Unlocking Your Aerobic Engine

What is VO2 Max?

Your VO₂ max is the maximum volume of oxygen that can be taken in via the lungs, transported via the bloodstream, and utilised by the muscles. It is a key performance indicator for endurance events because athletes with a high VO₂ max value are able to create more energy aerobically and without producing fatiguing metabolic byproducts such as lactic acid. Ultimately these athletes can sustain higher power outputs than their less fit counterparts. It has been reported that untrained individuals can improve their VO₂ max by 5-10% in 6 weeks, and as much as 20% in 6 months.

VO₂ max can be expressed as relative and absolute values. Relative values are expressed in ml/kg/min, and take into account an athlete’s body weight. This value is particularly important for weight bearing exercise such as running or cycling uphill, as the individual must overcome their bodyweight in order to gain forward momentum. This is why most elite marathon runners are extremely light, and the smaller cyclists excel in the mountains: they may not have the largest absolute power output, but they have the highest ratio of power/kilo or watts/kilo of body weight.

Absolute values are expressed in ml/min, and do not take into account an athlete’s body weight. This value is more important for non-weight bearing activities such as flat cycling, swimming and rowing, where body weight is supported and doesn’t have to overcome the forces of gravity to gain momentum. This is why Fabian Cancellara was so successful in flat cycling time trials – he was able to maintain high absolute power outputs throughout a race. Put him on a steep mountain, however, and he would drop off as he has a lot more body weight to haul up the hill than his smaller counterparts.

Factors Influencing VO2 Max

Several factors influence an athlete’s VO₂ max, most of which have some form of genetic limitation, but all of which can be improved through training. Assuming we ignore environmental conditions such barometric pressure and the partial pressure of oxygen in the air, there are three main factors to consider:

  • Cardiac Output: defined as the “amount of blood circulated around the body in one minute”. It is the combination of heart rate in beats per minute and stroke volume. Stroke volume is the amount of blood ejected from the heart each beat. Both heart rate and stroke volume have genetic limits, but stroke volume can be increased significantly through endurance training. An average athlete might have a stroke volume of 25ml/beat, whereas an elite will have 40ml/beat. At a heart rate of 150BPM, the average athlete is circulating 3,750ml of blood per minute (150x25ml), whereas the elite athlete is circulating a significantly higher 6,000ml of blood per minute (150x40ml). The higher an athlete’s cardiac output, the more blood (and therefore oxygen) that can be circulated to the working muscles to fuel aerobic performance.

  • Fibre type: will be discussed in detail in our fourth blog. Put simply, slow twitch (endurance) muscle fibres have a greater oxidative capacity than fast twitch (sprint) fibres. This is because they have a much greater mitochondrial density, capillary density and myoglobin content compared to fast twitch fibres. The importance of this will be explained below.

  • Arterial-Venous Oxygen difference (aVO₂diff): defined as the difference in oxygen concentration between the arterioles and venules after diffusion occurs at the capillaries. A quick lesson in physiology: Blood is sent from the heart up to the lungs to pick up oxygen (called ‘oxygenation’) and to let off carbon dioxide (called ‘deoxygenation’). This oxygenated blood then returns to the heart and is pumped out into our arteries. Arteries carry our oxygenated blood to smaller vessels called arterioles, and then into tiny vessels which surround our muscles called capillaries. Capillaries are the site at which oxygen is transported into the muscle (to be used for aerobic energy production), and carbon dioxide is transported out of the muscle. After this exchange, the blood in the capillaries now becomes deoxygenated blood (because oxygen has been transported inside the muscle) and travels through venules to the veins, and back to the heart. The process is then repeated.

Elite athletes are able to extract a high proportion of oxygen out of the blood stream compared to the average athlete. For example, both athletes might breathe in the same volume of air and have identical cardiac outputs, but the elite is able to extract more oxygen from the blood than his less fit counterpart. Why is this the case?

In an attempt to explain a complex process in simple terms, there are three main adaptations which will improve a person’s aVO₂diff: the number of mitochondria, capillary density, and myoglobin content of a muscle.

Mitochondria is the organelle in muscles which sole responsibility is to create aerobic energy: the more mitochondria we have, the more opportunities our muscles have to create energy.

Capillary density is the number of capillaries surrounding muscle sites. If we have more capillaries around our working muscles, we are able to increase the oxygen supply available for transport from in the bloodstream to inside the muscle.

Myoglobin is the middle man, and is a protein which is responsible for absorbing the oxygen from the capillaries and transporting it to the mitochondria to create aerobic energy.

Oxygen is breathed in via the lungs, pumped around the body via the heart, and arrives at muscles. In order to maximise the amount of oxygen our muscles take up, we must first surround the muscles with oxygenated blood (ie have lots of capillaries around the muscle). Then we must have an adequate number of myoglobin to absorb the oxygen from the blood, bring it into the muscle, and transport it to the mitochondria. Lastly, we must have enough mitochondria to actually utilise this oxygen to produce aerobic energy. Try to think of mitochondria as a bunch of turnstiles: if there are 100 oxygen molecules trying to pass through 10 turnstiles, it will take a long time to get all 100 molecules through. If, through aerobic training, we increase the number of turnstiles we have to 20, it will take half the time for 100 oxygen molecules to pass through. The body is very good at regulating itself, and won't take up more oxygen than it can use. In order to absorb more oxygen from the bloodstream, we need to increase our oxygen transport (capillary density/myoglobin content) and utilisation (number, size and surface area of mitochondria) capacity.

Source: Powers, S. & Howley, E.  Exercise Physiology: Theory and Application to Fitness and Performance. (McGraw - Hill Educations, 2012) Ch19 Fig 19.6.

Source: Powers, S. & Howley, E. Exercise Physiology: Theory and Application to Fitness and Performance. (McGraw - Hill Educations, 2012) Ch19 Fig 19.6.

Training to improve your VO₂ Max.

There are two primary methods to improve your VO₂ max: High Volume Training (HVT) and High Intensity Training (HIT).

HVT: best completed in the absence of fatiguing metabolic byproducts such as lactic acid. The starting point of this zone, according to the American College of Sports Medicine, is at 56% of your VO₂ max. The top end of this zone is the point just before you begin to produce lactate above that of resting levels (also termed "aerobic threshold"). This is a very individual point and can only be accurately determined by completing a VO₂ max test and analysing VE/VO2 data, or by directly measuring lactate during the test. This is your “long-slow distance” training zone, which has been reported to take up 75% of total volume in the typical elite endurance runner. This work is best completed continuously without any rest periods.

HIT: refers to completing interval work above your threshold. This zone is characterised by a heart rate/power/pace just above your anaerobic threshold (or functional threshold power, maximal lactate steady state: they all mean the same thing) to the intensity at which you reach 100% of your VO₂ max. Aiming to maintain 95% VO₂ max or above has been reported as being the most beneficial intensity to improve aerobic adaptations using this training method.

The ideal work:rest ratio for this is 1:1, with the work period always being between 2-3 minutes. Very advanced athletes may consider dropping the rest down to 2 minutes, but going any lower will likely change the session focus from a VO₂ max session into a threshold session.

The goal here is to accumulate 10 minutes “time at VO₂ max”. This will generally equate to around 15-20 minutes of ‘work’, as it will likely take 60 seconds or so to actually reach your VO₂ max zone intensity at the beginning of each interval. An example of this type of training would be 5 sets of 3 minute runs at 95% of velocity at VO₂ max with 3 minutes of passive recovery/easy walking. It has been reported that HIT training takes up 10-15% of the training volume in elite endurance runners. The left over 10-15% is devoted to recovery and tapering.

Both HVT and HIT sessions both improve cardiac and skeletal metabolic (mitochondrial oxidative capacity, fat oxidative capacity, glucose transport capacity) functions, but possibly due to different pathways. They also both improve capillary density, myoglobin content, and stroke volume as discussed above.

Some Things to Consider:

  • VO₂ max is an important indicator for successful endurance performance. As the length of activity becomes longer (over 20 minutes), the other factors discussed in our previous blog contribute more significantly to performance.

  • 75% of HVT and 10-15% of HIT was reported among elite endurance runners, who run well in excess of 100km/week. Cycling, swimming, rowing sports etc provide much less muscle damage to the body, so you should consider whether your body can handle more than only 10-15% HIT.

  • Luckily for us, mitochondria, capillary density, myoglobin and stroke volume are all improved through completing the same training method (whether HVT or HIT), so you don’t need to worry about ‘focusing’ on either aspect: they are improved simultaneously.

  • Unfortunately, however, your individualised training zones cannot be accurately determined by finding a “percentage of maximum”. The only zone this is applicable to is the beginning of your zone 2 endurance (which occurs at 56% VO₂ max), the others are determined by your unique physiological data gained from completing a VO₂ max test.

Next week we will focus on discussing, educating and improving everything related to lactate threshold, so we can functionally maintain a high percentage of VO₂ max...

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Written by Luke McIlroy – Director of Sport Science at METS Performance Consulting