Maximal oxygen uptake (VO2max) is the highest rate at which oxygen can be taken up, delivered to and utilised by the muscles during intensive exercise.

Alongside variables such as the lactate threshold and exercise economy, VO2max is a major performance factor in many fell and trail races, so seeking to improve it should be a primary focus of most structured training programs.

In this post, we’ll firstly break down the constituent parts that comprise the VO2max, before moving on to look at what kind of training is needed to signal the necessary adaptations for improvement.

Finally, we’ll present 4 workouts that have proven to be effective in our coaching of both professional and amateur athletes to improve the VO2max, where we’ll also include a little about the rationale and scientific basis behind the designs of each session.

Components of VO2max

The key components of the oxygen transport system can be broadly segmented into “central” factors and “peripheral” factors. Both are important to an athlete’s VO2max, and could represent a potential VO2max limiter.

“Central” parts of the system refer to the pulmonary diffusion of oxygen from the lungs to the blood and the transport of that oxygenated blood by the heart to the working muscles.
“Peripheral” refers to the parts of the system concerned with transfer of oxygen between the blood and the muscles and the oxidative capacity of the muscles themselves. This includes the capillaries (blood vessels to the muscle fibres) and the mitochondria within the muscle cells (the sites that produce energy aerobically).

There has been considerable debate among the scientific community as to which of these factors is the most significant contributor to the VO2max. In reality it depends a lot on an athlete’s genetics and training history, and without laboratory testing, it can be hard to know where the main limiters lie. So in most cases, it’s important to make sure you’re working and developing both peripheral and central factors. Let’s take a more in-depth look at each, starting with the central components of VO2max…

Central Factors

The central components at the beginning of the oxygen transport process start with oxygen being breathed in from the atmosphere and entering the lungs. It then diffuses into the blood ready to be sent to the muscles, where it is used to create energy.The diffusion of oxygen from the lungs to the blood is known as ‘pulmonary diffusion’ and is one key factor that influences VO2max.

Pulminary diffusion is most strongly impacted by the oxygen-carrying capacity of the blood. This is determined by total blood volume, the concentration of haemoglobin in the blood (which are the main vessels that carry oxygen in the blood). Conditions such as anaemia can reduce the oxygen-carrying capacity of the blood.

The transport of oxygenated blood to the muscles is facilitated by the beating of the heart muscle, where ‘cardiac output’ is a key determinant. This is the volume of blood pumped through the circulatory system in one minute, and is a product of both stroke volume (amount of blood output by the left ventricle of the heart in a single beat) and heart rate (beats per minute).

By achieving a higher maximal stroke volume, greater amounts of oxygenated blood can be transported to the working muscles.

Studies appear to show that when an untrained person commences endurance training, it is changes in stroke volume and that contribute the greatest to improvements in VO2max. Improvements in the oxygen-carrying capacity of the blood are also seen. In contrast, other factors impacting pulmonary diffusion and maximum heart rate do not appear to change considerably with training.

Once an athlete becomes more highly trained, and the heart’s stroke volume is close to its maximum potential, research suggests that cardiac output does not change appreciably, and it is then adaptations in peripheral factors that appear to allow VO2max to continue increasing.

Let’s look at the peripheral factors next…

Peripheral Factors

The peripheral factors that affect an athlete’s VO2max are principally the density of capillaries (the location of oxygen exchange between the blood and muscle fibres) and the quantity and function of the mitochondria in the muscle cells.

The good news is that both capillary and mitochondrial density can be greatly improved via a well-designed training program.

When a greater amount of capillaries are formed around the muscle fibres as a result of training, there is a greater surface area for diffusion of oxygen. Blood flow through the capillaries also slows down (since the capillaries are able to hold a higher volume of blood), meaning there is more time for the diffusion of oxygen to take place.

Mitochondria are the parts of the muscles where aerobic energy production takes place. For someone with a very low mitochondrial density, this could pose a limiter to VO2max due to a limit on the processing capacity of mitochondria themselves. However, this would be a very rare scenario, and it’s thought that the oxygen-processing capacity of the mitochondria typically far-exceeds the amount of oxygen that’s supplied to them.

Nevertheless, it has been shown that the density of mitochondria can still pose a limiter to VO2max. This is because mitochondrial density impacts the rate of O2 extraction from the capillaries. In other words, a higher mitochondrial density can lead to better O2 extraction from muscle capillaries.

Overall, capillary density is probably the greatest peripheral VO2max limiter in most athletes. In contrast, mitochondrial density seems to play a bigger role in improving things like substrate utilisation, the location of the first and second lactate thresholds, and endurance. This is because it's thought that a higher mitochondrial density and efficiency allows for the oxygen-processing demand to be shared across a greater number of mitochondrial sites, allowing for more energy to be produced via the less efficient process of fat oxidation, rather than carbohydrate oxidation (thus resulting in lowered lactate levels for a given power output, and better glycogen sparing).

VO2max as a System

It’s important to understand that VO2max is essentially dependent on a complex system of processes, including pulmonary diffusion, cardiac output, the oxygen-carrying capacity of the blood, capillary density, and mitochondrial density and function.

The rate-limiting factor in one individual may be quite different from the rate-limiting factor in another.

We can think of this like a factory. In one instance, the output of the factory might be limited by the supply of raw materials to the factory. In another instance, the factory may be limited by staffing shortages, and in another instance, by faulty machinery.

It’s the same with VO2max. One person may be limited by cardiac output, whereas another may be limited by capillary density. The types of training that will help these athletes overcome their limitations may not be the same, and this is why training interventions should always be individualised, and you should not trust a coach who simply prescribes training based on what’s worked for them!

VO2max Adaptations

Now that we know a bit more about the components that make up the VO2max, we can get to the practical training details of eliciting positive adaptations.

We’ll start by looking at methods to improve the cardiac output firstly…

Central Adaptations

Since it’s clear that the delivery of large volumes of oxygenated blood via a high cardiac output is critical to running performance, we need to ensure that training targets improvements in this area throughout the majority of the training cycle.

By and large, a key goal when designing workouts to elicit these improvements is to achieve heart rates close to maximal levels and maintain around 90% of the maximum heart rate for the longest duration possible.

The reason we need to raise and hold the heart rate at close to these maximal levels is to facilitate adaptations in stroke volume via:

Improvement in the volume and wall thickness of the left ventricle
Greater stretch from this increased volume, resulting in greater elastic recoil

We essentially want to ‘stretch’ the heart muscle by filling it with lots of blood, so that it can increase in capacity and improve its contractile strength to deliver more blood with each beat.

Since heart rate tracks closely to VO2, we can use heart rate during a workout as a good indication of what % of VO2max is reached during high intensity efforts and whether this is sufficiently high to stimulate the desired response.

Training to spend large amounts of training time around 90% heart rate max can be applied as one sustained effort or through the use of interval training (i.e. alternating bouts of work and rest), where the latter is the most common method used. In addition, there’s considerable build up of lactate and associated fatiguing metabolites with these efforts, so the rest periods become important to allow the clearance of these fatiguing metabolites before the next effort.

Peripheral Adaptations

Training to elicit improvements in capillary density as well as mitochondrial content and function involve a few different training methods.

Longer duration, lower intensity training is arguably one of the best training methods to build mitochondrial and capillary density, since it allows for a high number of muscle contractions over a long period of time, with relatively minimal stress. Muscle contractions play a key role in the signalling of mitochondrial biogenesis and capillary development. This signal is largely independent of running intensity, so the signal is similar whether running at a lower or a higher speed.

In addition to large amounts of lower intensity training, very high intensity workouts (e.g. 4-7x 30 second all-out sprints) have also been shown to improve mitochondrial growth and function. These sessions can be mentally and physically very taxing though, so should only be used relatively sparingly, and we often only employ this type of session at key times of the year.

Finally, building greater capillary and mitochondrial density within and around higher-power muscle fibres - i.e. Type IIa and IIx (fast twitch) fibres - involves designing workouts that sufficiently activate the target fibres.

Type I or so-called ‘slow twitch’ muscle fibres need very little stimulation for this activation to occur compared to fast twitch fibres (especially the Type IIx fibres, which require a load that both the Type I and Type IIa fibres cannot wholly carry). Adaptations in capillary and mitochondrial density are therefore stimulated by running at intensities that are high enough to stimulate the target fibres. For example, a low-intensity run would be appropriate to activate Type I fibres, whereas 5-10 minute efforts at or slightly above threshold would be more appropriate for activating Type IIa fibres. Activating Type IIx fibres requires very hard efforts that can only be sustained for a couple of minutes or less.

VO2max Workouts

Below are 4 effective and scientifically-based workout designs that can be used to stimulate adaptations in both the central and peripheral factors presented above and positively alter the aerobic capacity:

2 Minute Intervals

These relatively short duration intervals use a high intensity target to stimulate a rapid heart rate response, improving stroke volume as well as the mitochondrial function (efficiency) within the muscle cells.

They should be completed as a set of 7-10x 2-min efforts at the maximum pace you can sustain consistently over the whole set of efforts, taking 2-mins of very light jogging between. Be sure to implement a thorough warm-up before beginning the efforts, which should include some short bursts of high-intensity running, which helps to prime your aerobic system so that it responds faster at the start of the main set of efforts, allowing you to spend more time running close to VO2max.

Once you’re familiar with this session, you can increase oxygen uptake and heart rate more rapidly, by performing a ‘hard start’ at the beginning of each effort, meaning you begin with a ~20-second hard effort, before settling into a slightly slower pace that’s sustainable for the rest of the effort.

6-8 Minute Intervals

These longer efforts can, in some cases, allow more time to be spent running close to VO2max, for a lower subjective effort. This is because this session makes use of the so-called ‘VO2 slow component’, where oxygen consumption slowly rises towards VO2max across each effort. Having a relatively high volume of high-intensity running, these efforts are particularly useful for developing mitochondrial and capillary density around fast twitch muscle fibres, and since heart rate will remain high, it’s also good for improving cardiac output.

After a thorough warm-up (as described above), this session should be completed as 4x 6-8 minute efforts, at a pace or intensity that’s just slightly above threshold. You should see your heart rate drifting up to at least 90% of your max heart rate within each effort. Take roughly 3-4 minutes of very light jogging to recover between each effort.

Microburst Interval Blocks

Microburst intervals using a 2:1 work-rest ratio (e.g. 30’ on/15’ off as shown above, or the popular 40’ on/20’ off) raise HR rapidly due to a combination of the high intensity “on” work bouts and short “off” recovery bouts, which facilitate the “drifting” upwards of the heart rate towards maximal levels throughout each block. The inclusion of “micro-recovery” in each block allows for reasonably long duration blocks to be achieved, leading to greater total time spend at a high % of heart rate max.

Some very recent research also suggests that microburst intervals are also an effective design for improving mitochondrial efficiency, and they activate a high percentage of muscle fibres so will be beneficial for developing capillaries around higher-power muscle fibres.

After a good warm-up as described above, this session should be completed as 3 blocks of 9-16x 30-seconds ‘hard’ running and 15-seconds ‘easy’ jogging. Using a 7-Zone System, the ‘hard’ efforts should be performed in Zone 6. In practice, this intensity is hard to gauge, because heart rate will be slow to respond initially. However, you should aim to pace each 30-second effort at a consistent pace that you can sustain for the whole three blocks of efforts, and the first few efforts shouldn’t feel too taxing (around an 8/10). Across each block of microbursts, you should see your heart rate drifting up at least to 85% of your max heart rate.

Take 3-5 minutes of gentle jogging between each block of microbursts.

Long Duration, Lower Intensity Run

Runs lasting between 1-hour to 3-hours or more are really beneficial in stimulating peripheral adaptations, as well as contributing to some extent towards central factors.

We recommend keeping these long runs largely to a ‘Zone 2’ intensity, which should be performed between 75-85% of threshold heart rate, and should feel like a 3-4/10 intensity. This will minimise the stress caused by these sessions, while also activating a meaningful proportion of muscle fibres.

To make sure you’re training at the right intensity, you can perform a talk test as described here. As a general rule of thumb, your breathing should remain ‘conversational’ throughout, meaning you can speak in full, unbroken sentences.

References

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Raleigh, J. P., Giles, M. D., Islam, H., Nelms, M., Bentley, R. F., Jones, J. H., ... & Tschakovsky, M. E. (2018). Contribution of central and peripheral adaptations to changes in maximal oxygen uptake following 4 weeks of sprint interval training. Applied Physiology, Nutrition, and Metabolism, 43(10), 1059-1068.

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How To Increase VO2max As A Runner