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What is a VO2 max test?

VO2 max CO2
Peter Stuart
28 Apr 2015

Is VO2 max a true measure of your potential and does it provide data you can train with? Cyclist takes a test to find out.

There’s an almost perverse obsession with numbers in cycling, be they power, weight, cadence or heart rate. Arguably, sitting on a pedestal as the king of physiological data, though, is VO2 max.

Often viewed as the ultimate metric of fitness, the VO2 max figures attributed to top endurance athletes are worn like medals and have been used to explain why the likes of Lance Armstrong (VO2 max of 84.0) could sprint up Ventoux, or how Miguel Indurain (88.0) could cruise like a moped at 55kmh.

But the numbers don’t tell the whole story.

‘VO2 max measures how efficiently you can get oxygen in through your lungs,’ says David Dixon, senior lecturer in exercise physiology at the University of East London.

‘It’s the amount of oxygen you can breathe in millilitres per minute, usually expressed in proportion to your bodyweight.’

VO2 max essentially tells you how much oxygen your body is asking for, and how much your lungs are able to deliver – but it doesn’t always follow that the athlete with the highest score wins the race.

To learn more, I find myself in Dixon’s laboratory, pedalling gently aboard an SRM power meter-equipped static bike with a mask strapped to my face.

Against the clock

VO2 max exertion

The VO2 max test is fairly simple. I am to perform a straightforward ramp test, where the resistance will increase every minute until exhaustion. At that point my oxygen intake and CO2 output will be measured to determine how much oxygen I was processing during the test.

Firstly, though, Dixon administers a test to measure my lung capacity. Although lung capacity and VO2 max tend to be similar, the two are not proportional.

A lung capacity (‘forced vital capacity’ or FVC, to be specific) of six litres would be likely to match up to a VO2 max of six litres per minute, but they indicate very different things. 

FVC is tested by forcefully exhaling into measuring equipment after taking the biggest in-breath possible. After doing this three times, Dixon determines my FVC is 6.38 litres, slightly above average for my age, height and weight.

That suggests my lungs can take in relatively large amounts of oxygen, but it doesn’t mean my VO2 max will be high unless my body has the right conditioning to make use of that oxygen.

VO2 max heart rate

Before the test, a tiny sample of blood must be taken to determine the levels of lactic acid in my system, with a simple prick to the finger.

My blood pressure, height and weight are also measured, as well as my oxygen efficiency at rest. Based on this data, I will have to fulfil five separate criteria once the VO2 max assessment is complete that will confirm it’s been a true maximal test.

‘We need a lactate over eight, a heart rate within 10 beats of your maximal, an RPE [rate of perceived exertion] of 19 or above, a ratio of respiratory exchange above 1.03, and a plateau in the efficiency of your oxygen,’ says Dixon.

These criteria ensure that should I be too weak-willed to push myself to the maximum, the figures will reveal that my body is capable of more.

After warming up, the test starts at 100 watts, and is set to increase at 20-watt intervals every minute. Ideally, the test should last between six and 12 minutes, meaning Dixon has projected I will reach exhaustion around the 300 watt mark.

As the intensity creeps up slowly, the lab assistant asks me to point to my current pain rating, a score out of 20 to demonstrate my RPE.

For now, it’s somewhere around the 6 mark, where the scale begins, illustrating low effort. The first half of the test is painless. From minute to minute I point to the low numbers of the RPE chart, and keep going.

A graph is forming on Dixon’s computer, based on the readings from my breath analysis and heart rate monitor, which is plotting my body’s slow journey to exhaustion.

VO2 max threshold

I’m up to 300 watts. Already the test is nearing the intended climax, but it’s clear that it will go beyond 12 minutes before I reach my maximum. I have no idea where I will end up, as I rarely train with power readings, although I know that on my weekly roller sessions I can stick around 320 watts for half an hour if I really bury myself.

At around 360 watts I start to get anxious about the ordeal I’m about to dive into.

Intersecting lines

The crucial trade-off in the lungs and muscles under extreme exercise is the balance between oxygen and carbon dioxide.

In normal conditions you breathe in and absorb a certain volume of oxygen and exhale a lesser volume of CO2. Normally this ratio of CO2 to oxygen (respiratory exchange) sits at around 0.7.

During exercise, this gradually rises towards a ratio of 1.0, and in the VO2 max test, it will exceed 1.0, as the amount of CO2 you exhale begins to exceed the oxygen you take in.

 ‘The volume of oxygen we absorb depends on the activity and its intensity,’ says Dixon. ‘So when we put you into intense exercise we’re getting towards the maximum efficiency of the body to get that oxygen in. Once we reach that maximum, we then use other mechanisms to function – anaerobic systems – and therefore we fatigue a lot quicker. That’s why we give up.’

VO2 max test

According to the raw data, it’s at 360 watts that my CO2 to oxygen ratio tips over 1.0.

I’m not aware that my body is slipping into oxygen debt, but my RPE pain rating has crept up to 16 and I don’t have much energy to point at the chart any more.

As 380 watts comes, I have to focus heavily on my pedalling motion and efficiency to keep my cadence at 90. When the 400 watt mark arrives, my pedalling becomes more erratic, my hands are slipping from the sweat and my legs feel torn to shreds.

I manage to keep going as 420 watts rolls by, but I’m really struggling. For me, this is uncharted territory.

When 440 watts arrives, I give a final effort but I simply can’t turn the cranks any more. The SRM power meter has determined that I can’t continue at this intensity, and the resistance suddenly disappears in an unceremonious way.

I collapse over the bars, gulping in breaths, but at least I’m not vomiting or fainting, as I’d been told I might. The mask is removed from my face, to my great relief, and my finger is once again pierced with a needle to extract blood for lactate testing.

Density of data

VO2 max watts

With power, heart rate and a vast ocean of oxygen and CO2 figures, the 20 minutes I’ve just spent on the bike have produced enough data to entertain even the most obsessive mind for weeks.

At first Dixon tells me that he saw a VO2 max figure of 75ml/min/kg as I reached my peak output, but more analysis is required to provide the exact figure as an average over the last minute.

As the data begins to settle, Dixon lets me know that in the last minute I consumed 5.23 litres of oxygen. At my bodyweight of 72kg that gives me a VO2 max score of 72.6. But did I achieve the five criteria of a VO2 max test? 

‘You achieved four and a half of them. The one that’s really hard to see is the plateau in your oxygen consumption. We were seeing the beginnings of a plateau, but then you gave up so you were more or less there,’ says Dixon.

VO2 max lactic acid

Annoyingly, I feel as though if I had known the test was coming to an end, I could have pushed that bit harder, squeezing for that plateau and scoring a slightly higher test score. Dixon laughs off the suggestion, though. The figure isn’t really open to a better score through better execution of the test.

But what does all this mean? While my VO2 max apparently puts me in the sphere of an ‘elite athlete’, my results at local races would suggest that isn’t the case.

‘It’s not so much the VO2 max that determines your performance,’ says Dixon. ‘It’s the ability to sustain high intensity within that. You could have a VO2 max of 70 but if your high-intensity threshold is only 60% of that you’d be slower than someone with a VO2 max of 60 who’s able to sustain 80% of that.

VO2 max

‘On the back of this test we’d sit down and set out your goals depending on what type of racing you’re doing. You have your VO2 max figure, but probably more important is a lactic threshold test – working out your threshold and how we can improve that,’ says Dixon. 

So while I may relish the opportunity for bragging rights about a healthy set of lungs, the VO2 max is only really a tool for improving the only result that really matters – the result on the road.

Read our guide to the best bike rollers and how to create a cycling training plan

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