If I were to ask you how strong are you, you’d probably answer with the weight you can bench
press or maybe how many pushups you can do.
But asking someone to quantify their cardiorespiratory fitness isn’t that straightforward.
Maybe you know your mile time but that’s technically a measure of speed, not physiology.
That’s why exercise scientists use VO2, the volume of oxygen consumed by your body
per minute.
And aside from letting us put a number to our cardio fitness, it’s also one of the
strongest predictors of heart disease and death that we have.
If you’re new here, welcome, my name is Patrick and I have a master’s degree in
exercise physiology which means that for the duration of grad school I ate, slept, and
breathed VO2 — pun very intended.
I would need to know it both to understand research, but also for practical reasons.
Like when designing rehab programs for patients coming off of heart surgery, their VO2 influenced
how intense their exercise should and could be.
And if you’re at all interested in the field of exercise science, you’ll see this metric
all the time, so in this video, you’re going to learn what VO2 is, how to calculate it,
and why it’s so important.
VO2 is measured in milliliters per kilogram per minute.
That’s milliliters of oxygen, not blood, consumed per minute per kilogram of body mass.
This is technically relative VO2 which is useful for comparing different sized bodies,
but you could also measure absolute VO2 by leaving out body weight.
Some folks call it just oxygen consumption, which is fine for now.
But what’s actually consuming oxygen when we measure VO2?
Well, mostly the mitochondria in our body’s tissues.
The reason you know mitochondria as the powerhouse (or powerstation of the cell) is because it
uses oxygen to make lots of energy.
Even then, different people can have more or less efficient mitochondria from each other,
your mitochondria become less efficient over time, and different organs have different
amounts of mitochondria and different rates of oxygen consumption.
Like our hearts consume 11% of our oxygen while only making up about sixth tenths of
a percent of our mass.
Our brains consume 20% of our oxygen while only making up 2 percent of our body weight.
On the other hand your muscles are about half of your body weight but only consume 27% of
your total VO2 at rest.
But not surprisingly, when you exercise, your muscles need more oxygen so your overall VO2
goes up.
Right, our mitochondria need oxygen, our muscles have lots of mitochondria.
They’re about to get busy, so let’s feed emm.
To actually calculate this number at exercise and at rest, we’ll use something called
the Fick Equation, VO2 equals cardiac output times the arteriovenous oxygen difference.
I’ll explain.
Imagine systemic circulation like an all you can eat sushi restaurant with a conveyor belt
that delivers sushi once every minute.
The hungry patron represents the body’s oxygen-hungry tissues, the plates are red
blood cells which carry sushi, or oxygen, through the conveyor belt which represents
blood vessels.
If we wanted to measure how much our patron ate at any given time, we could count the
sushi on this side of the belt and again on this side of the belt, the difference should
be the amount of sushi eaten.
If our patron isn’t very hungry, lots of sushi comes back to us in the kitchen, but
sometimes, he eats a lot and very little comes back to us.
We can see the same thing in a real human body.
We’d have to use special tools to do this, but we could compare the amount of oxygen
in our arteries to the oxygen in our veins.
If we subtract the difference, that should be the amount taken up by the tissues in systemic
circulation.
This is called the a-v O2 difference: the difference in oxygen content between our arterial
and venous blood.
In a resting body, that’s somewhere around a 4mL difference of Oxygen per 100mL of blood
and gets closer to a 16mL difference at max exercise.
That’s because our tissues, mostly skeletal muscles, consume more oxygen from circulation
during exercise, leaving less oxygen in the veins, increasing the a-vO2 difference.
In our sushi example, this would be like if our patron ate more sushi, leaving less sushi
on this side of the belt.
Which begs the question, how are we going to adjust to feed our hungry customer?
The first thing you as a sushi chef do is send out more plates with each wave.
These plates can only hold a limited number of sushi, so overstuffing the plates isn’t
an option.
If our patron is still hungry, not only will you keep the plates coming, but you’ll increase
the speed of the conveyor belt.
In our bodies, something similar happens.
When our tissues get hungrier, we increase stroke volume, this is the amount of blood
pumped out of the left ventricle with each pump, and we increase heart rate, heart beats
per minute.
For most non-athletic folks, the stroke volume plateaus around 120 beats per minute, so if
we want to keep increasing our workload, we have to increase heart rate.
Eventually, we reach a point where heart rate plateaus too.
At that point, we’re delivering as much oxygen as we can to our tissues — this is
called our VO2max.
These variables go back into the Fick Equation.
Cardiac output, or Q, can be calculated by multiplying stroke volume and heart rate.
Multiply that by the AVO2 difference we found earlier, tidy up some of our units, and we’ve
got absolute VO2 as milliliters of oxygen consumed per minute.
Divide that by body weight in kilograms and that’s relative Vo2, milliliters per kilogram
per minute.
So conceptually, that’s what VO2 is — how much oxygen our bodies consume.
Here’s how much blood we pump out every minute and here’s how much oxygen we took
from it.
But we still have a big question to tackle: why do we care?
Why would you ever use this number in real life?
The first reason is that VO2max is the best and most representative measure of cardiorespiratory
fitness we have.
If you had a physical education course or played a team sport before, you might’ve
done what’s called a Cooper Test, a 12 minute run for distance.
You probably did one at the beginning and one at the end of a training program or semester
of PE.
Now, there are multiple tests that use different scales and percentiles to rank fitness, which
makes sense.
We’d totally expect fitter people to perform better on those tests, and they’re certainly
useful for testing a lot of people and saving time and money.
And these can be good estimates for someone’s VO2 but these kinds of tests don’t give
us any direct physiological measurements.
On the other hand, if we did measure the physiology that goes into transporting oxygen from your
lungs to your muscles and then being used for work, we’d have nearly too many options
to choose from.
We could narrow in on how well the ventricles work, how well your blood vessels transport
blood, or how well your muscle cells extract oxygen to feed their mitochondria.
But the thing that usually limits aerobic performance is the ability to get oxygen to
your tissue, and VO2 is the most direct measurement of that.
The specifics of how and why we test VO2 the way we do is pretty technical and math heavy,
but if you want a separate video about that, let me know in the comments.
The second reason we care about VO2 is because a low VO2 is a very strong predictor of heart
disease.
It’s probably not surprising that fitness and heart disease are related, but it’s
also independent of other risk factors like obesity, or type 2 diabetes or high blood
pressure.
Multiple meta-analyses show how fitness can be a more powerful predictor of mortality
risk than some of the other typical risk factors like smoking, obesity, or high cholesterol.
And this has been shown across sexes, heart disease conditions and other comorbidities.
And luckily, VO2 is a modifiable risk factor.
Different studies have different results, but improving your VO2 by 1 metabolic equivalent,
or MET, or 3.5mL/kg/min improved chance of survival by 10-25 percent, which is a really
good payoff for an attainable fitness improvement.
So cardiorespiratory fitness isn’t just associated with heart disease, it’s one
of the strongest predictors we have.
Cool, where do we go from here?
That’s your call!
In the comments, I want you to decide the next video topic related to exercise physiology.
I could make something about VO2 measurement and graded exercise testing, I could do a
video about aerobic exercise adaptations and what happens to your VO2 when you train.
But it’s up to you, let me know in the comments.
And while you’re down there, subscribe and hit the bell so you get notified when I post
a new video.
That really helps me out and keeps the channel going.
Have fun, be good.
I’ll see you next time.