By the Numbers: What is Vo2 Max?

VO2 Max has been a lab measurement discussed for decades in regards to the world’s top-performing athletes. It is synonymous in endurance sports to relate one’s athletic potential to their VO2 Max measurement. Is this metric the end all be all? If you don’t possess the aerobic capacity of Olympic cross country skiers are you doomed for athletic mediocracy? 

There is no doubt that some individuals won the genetic lottery when it comes to endurance sports, but this is not solely based on their VO2 max. There are many factors at play when it comes to an individual’s endurance potential aside from VO2 max values. Let’s take a further look. 

VO2 Max

What is it? In science terms: it is your body’s maximum capacity to transport and consume oxygen to produce energy. This value has two measurements: relative and absolute. The absolute value is measured in liters of oxygen per minute, while the relative is measured in milliliters of oxygen per kilogram of body weight per minute. 

The Fick equation below is used to calculate VO2:

VO2 = 𝗤 x (a-vO2 diff)

Q = Cardiac Output

a-vO2diff = arteriovenous oxygen difference

Cardiac Output is the product of stroke volume (SV) multiplied by heart rate (HR) and is measured in liters per minute. From this, we can observe that any increase in SV or HR, and our cardiac output will also increase…therefore a large component of increasing one’s VO2

The Arteriovenous Difference represents the amount of O2 (oxygen) that is taken up from 100mL of blood by the tissues, during 1 systemic circuit trip. This will show the oxygen content in your blood between the arterial and venous blood. Simply put: we are comparing the blood leaving your heart to the blood that is returning to the heart. Why would there be a difference? 

Once an individual begins to exercise, there will be an increased demand from the tissues to supply them with oxygen. Thus, we will begin to see a decrease in the oxygen supply returning to the heart. To sum it up: seeing an increase in an athlete’s a-vO2 diff during exercise is due to an increase in oxygen for energy production.  

All this science! A lot goes into understanding what constitutes an individual’s VO2 and having an understanding of the variables is important when it comes to you as an athlete looking to increase this value. The same logic that we used for cardiac output also applies to the arteriovenous difference….if we increase this value, we also increase our VO2 Max. Easy, right? 

VO2 Max Absolute and Relative

Cycling is an endurance sport, aka an aerobic sport. You need oxygen! Your absolute VO2 max represents your “physiological ceiling” for utilizing oxygen. Your relative VO2 max is more a reflection of endurance ability and allows us to compare different-sized individuals. As an example, losing weight will increase your relative VO2 max, but that does not mean your absolute value has increased. Although the absolute value stays the same, you will likely be riding/climbing/running faster. This is why the relative value is the one most frequently referenced.

VO2 Max measurements are calculated in a laboratory setting. VO2 max tests are sport-specific. In regards to a cycling-specific test, keep in mind that if you were to perform a running or cross-country skiing VO2 test you will likely see a higher value since more muscles are recruited, which increases the consumption of oxygen. What this test shows is the ability of your cardiovascular system to deliver as well as your muscles to utilize oxygen, not simply how much oxygen you are able to breathe in. 

This value represents an individual’s cardiorespiratory “capacity.” Therefore it is commonly thought of as your endurance engine, the bigger the engine, the better you will be. But is this completely true? 

If you tested yourself in a lab today, and you received a value for your absolute as 52 mL O2/kg/min, and a relative value as 3 L O2/min, what can you do with this information? How do these numbers improve your training and racing?? Perhaps the answer is that for many of us just knowing these values, doesn’t really matter. 

So then what does matter? 

What is a Watt?

Well, a watt is a unit of power of course! People like to say a Watt is a Watt is a Watt.  While a Watt might always be 1-joule of energy per second, how you make the 1-joule, and what you can do with it, are entirely individualized, and dependent upon a few factors. So wait…not just VO2?

When we are racing, what matters most is the ability to get from the starting line to the finish line faster than the others in the field. How do we do this? It is not from a VO2 value alone, and you know this too. It takes endurance, bike handling skills, the ability to maintain energy levels, and power….lots of power. 

Power is a great tool to look at when it comes to VO2 and application. When testing VO2 what we should be looking at is the power we produce at this given value, and maybe most importantly, the length of time we can hold this power. If we are able to produce 350 watts at our VO2 Max, but can only sustain that for 1 minute, that may not help us that much if we need to be able to sustain that power for say 3-5 minutes in an upcoming race. This is where Maximum Aerobic Power enters. MAP, or maximum aerobic power, is correlated with your power at VO2 Max. Not only is it the length of time you can sustain a certain power at VO2 Max, but how increasing this MAP affects your ability to hold sustained power for long periods of time. What is your ability to maintain a percentage of your MAP for a length of time? How does this even relate to your own training and racing? Well, let’s take two athletes: If Athlete A can hold 80% of their VO2 Max for 10 minutes, and Athlete B can maintain 85% of their VO2 Max for that duration, the second rider is going to finish faster. 

Your ability to not only maintain high power at your VO2 Max but also your ability to sustain a high percentage of your VO2 max for extended periods of time is what really matters when it comes down to these metrics. 

There’s More at Play

While VO2 and FTP are metrics that many endurance athletes like to throw around, there are many other factors at play when it comes to an individual’s performance. 

For example, you have the efficiency and economy of every individual athlete. This is reflected as your Gross Economy (GE.) By dividing the mechanical energy put into the bike (watts) by the total energy expended to produce that work, we can measure your Gross Efficiency (GE) on a bike. Higher efficiency means you can either put out more watts for the same amount of energy or put out equal watts for less energy. You can either go faster for the same amount of time or the same speed for longer. 

Cycling economy is another factor in the equation of performance. It is similar to Gross Economy but does not take into account fuel utilization (fats/carbs.) Which fuel source your body is using does matter, as you produce less energy per liter of oxygen with fat compared to carbohydrates. As your body switches from fat to carbohydrate, you are creating more power for the same amount of oxygen, another reason why GE increases as intensity goes up.

Finally, we have speed economy (SE.) This can increase with 0 change in your fitness levels. How do you change this? Equipment and aerodynamics! Remember all those times you didn’t stay tucked?? Free speed was being thrown away! 

Hopefully, this helps you see how many different factors can influence performance, and being great at one thing, doesn’t guarantee you will be the fastest rider on the road.

So if you don’t have a lab to test your VO2 or economy, where do you go?

SYSTM 4DP

4DP is a comprehensive power test that measures not just FTP, but Neuromuscular Power (5-second power), Maximal Aerobic Power (5-minute power), and Anaerobic Capacity (1-minute power). It is called Full Frontal and it reveals what you are capable of across a range of efforts, as well as the important relationships between the different ways of producing power on the bike. Take, for example, Maximal Aerobic Power (MAP) and FTP.

Roughly equivalent to your power at VO2 Max, MAP is determined in Full Frontal by an all-out, 5-minute effort. In physiological terms, it represents the upper limit of your body’s ability to use oxygen to produce power. More than that, MAP also acts as a ceiling on your FTP. In other words, your FTP will always be below and constrained by your MAP.  While everyone’s MAP to FTP ratio is different, there are limits to how close—or far apart—they can be. When reasonably fresh, your MAP can’t be lower than 115% of your FTP.  You can do as much tempo and threshold work as you want, but until you increase your MAP, your FTP simply won’t go up.

On the other end of the spectrum, there are athletes with MAP values as high as 140% of their FTP.  Just as the ceiling can be too low, it can also be too high. Once your MAP reaches a certain level relative to FTP, it won’t increase until you establish a good foundation of endurance. It’s not just the numbers in isolation, but the relationship between them that’s important.

Two athletes can have the same FTP, but differing MAP values. If both athletes want to improve their sustained power, they will not necessarily take the same road to get there. If athlete A has a MAP value of 140% of their FTP, and athlete B has a MAP value of 115% of their FTP, then athlete B will need to work on increasing their MAP value in order to increase their aerobic (FTP) ceiling. A power test that measures just FTP would not be able to reveal the key difference between these athletes. 

While it is good and well to get into a lab and have your VO2 measured, it is not the end all be all value in terms of endurance capacity. It is just a value, and just knowing this metric has no implication on your performance if you don’t know how to use this in training. FIguring out your power at VO2 is a much more informative value when it comes to implementation. 

Knowing how you produce power across a range of durations and intensities is the first step in developing an effective training strategy tailored to your unique physiology. Accurate, consistent power measurement during training, coupled with a solid understanding of where you are, is the key to getting where you want to be. 

Mac Cassin is the Chief Cycling Physiologist at Wahoo Sports Science. He holds a degree in Integrative Physiology from the University of Colorado-Boulder and has won multiple National Championships. The experience of juggling athletic goals with collegiate and career responsibilities has taught Mac that peak performance is achievable even for those who cannot focus exclusively on training. While concentrating on exercise physiology in an academic setting, Mac competed at the World Championships, Pan American Championships and World Cups on both the road and track.
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