Low VO2 Max Score on Fitness Tracker? Here’s what you should know

Low VO2 Max on fitness trackers

Do you pay the most attention to your Fitbit, Apple Watch, Garmin, or another wearable step count or total calories burned? Do you only review the miles or laps you ran, cycled, or swam?

If that’s all you notice on your wearable, you’re missing out on one of the most important metrics available, the VO2max score (also called cardio fitness score.)

This overlooked measurement is likely one of the most powerful predictors of your current health and fitness.

When you monitor your VO2 Max (cardio fitness level) over time, it’s a great way to see if you are getting more fit or going in the opposite direction and losing your fitness.

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Why your VO2max or cardio fitness score matters

More and more wearables adopt health metrics like cardio fitness scoring or VO2max scores into their workout and health apps.

Unfortunately, quite a lot of people actually have no idea what VO2 is and why it matters.

So let’s break it down!

In layman’s terms, your VO2max is a strong indicator of your overall cardiorespiratory fitness (cardio fitness, for short) because it shows how well your body uses oxygen during exercise.

VO2 Max stands for: V (volume)  + O2 (oxygen)  + Max (maximum)

It’s a measure of the maximum amount of oxygen your body can use when exercising or engaging in any form of physical and aerobic activity.

The higher your VO2max, the better your cardio fitness. That means your body uptakes oxygen efficiently, and you can run, cycle, or exercise intensely with ease.

Conversely, the lower your VO2max, the worse your cardio fitness. That means your body cannot take in and use oxygen very efficiently, so exercising strenuously is difficult.

What is VO2max or cardio fitness score?

Apple now uses the term cardio fitness to describe VO2 Max measurement

Wearables like Fitbit and Apple Watch use VO2max and cardio fitness interchangeably.

So you might see this scoring system listed as VO2max on one device and cardio fitness on another–they are the same thing!

Aerobic capacity test (VO2max), also known as the maximum oxygen consumption test, measures the maximum amount of oxygen a person (usually an athlete) can use during intense activity levels.

It is a good gauge of cardiovascular fitness and aerobic endurance levels.

In other words, VO2 max is defined as the maximum amount of oxygen your body can use while you’re exercising as hard as you can. 

A higher VO2 max indicates a higher level of cardiovascular fitness and endurance.

The belief is that a higher VO2max allows one to produce more energy, thereby performing more work.

This has made the VO2 max metric the “gold standard” measure of overall fitness for endurance trainers.

Because endurance athletes have trained their bodies to use oxygen more efficiently, more oxygen circulates to their muscles and allows them to work faster and harder. 

As a result, they generally have a higher VO2 max than the average person.

What factors influence a person’s VO2 max score?

Fitbit cardio fitness
Fitbit also uses the term cardio fitness to describe VO2 Max measurements

Many factors can influence VO2max, e.g., heredity, training, age, gender, and body composition. 

Structurally speaking, there are three components to the VO2 Max Puzzle.

  1. How much-oxygenated blood your heart can pump
  2. How much of it reaches your muscles
  3. And finally, how much of it is actually utilized by your muscles

Also important are a person’s lung capacity and heart volume.

  • The greater your lung capacity, the more oxygenated blood your heart can pump, and thus your higher VO2 number
  • The more oxygenated blood that can reach your muscles, the higher your VO2 max score, suggesting that capillary delivery is an important component
  • The better your muscles are at utilizing oxygen from your blood, the better your VO2 max reading

Cardio fitness score Fitbit

Good Cardio Fitness level score on Apple Watch
Cardio fitness or VO2 Max levels vary by age AND gender

Generally, VO2max declines with age (about 2% per year after age 30), and males typically have a greater oxygen consumption value than females.

What is the score range for cardio fitness and VO2max?

The average VO2max or cardio fitness score for untrained healthy men is around 37.9, and for untrained healthy women, the VO2 max is at 27.6 across all age groups.

(2). The study’s VO2 max results were based on CPX (cardiopulmonary exercise testing) of the adults.

VO2 Max by sex and age as per Mayo Clinic
Source: Mayo Clinic (See Reference # 4)

VO2 Max Norms for Men

Age Very Poor Poor Fair Good Excellent Superior
13-19 Under 35.0 35.0-38.3 38.4-45.1 45.2-50.9 51.0-55.9 Over 55.9
20-29 Under 33.0 33.0-36.4 36.5-42.4 42.5-46.4 46.5-52.4 Over 52.4
30-39 Under 31.5 31.5-35.4 35.5-40.9 41.0-44.9 45.0-49.4 Over 49.4
40-49 Under 30.2 30.2-33.5 33.6-38.9 39.0-43.7 43.8-48.0 Over 48.0
50-59 Under 26.1 26.1-30.9 31.0-35.7 35.8-40.9 41.0-45.3 Over 45.3
60 Under 20.5 20.5-26.0 26.1-32.2 32.3-36.4 36.5-44.2 Over 44.2

VO2 Max Norms for Women

Age Very Poor Poor Fair Good Excellent Superior
13-19 Under 25.0 25.0-30.9 31.0-34.9 35.0-38.9 39.0-41.9 Over 41.9
20-29 Under 23.6 23.6-28.9 29.0-32.9 33.0-36.9 37.0-41.0 Over 41.0
30-39 Under 22.8 22.8-26.9 27.0-31.4 31.5-35.6 35.7-40.0 Over 40.0
40-49 Under 21.0 21.0-24.4 24.5-28.9 29.0-32.8 32.9-36.9 Over 36.9
50-59 Under 20.2 20.2-22.7 22.8-26.9 27.0-31.4 31.5-35.7 Over 35.7
60 Under 17.5 17.5-20.1 20.2-24.4 24.5-30.2 30.3-31.4 Over 31.4

 

Now, compare these average numbers with that of world-class elite athletes, and you see a dramatic difference.

Elite Olympic athlete VO2 Max score
Data from Hawley and Burke’s “Peak Performance: Training and Nutritional Strategies for Sport.”)

Now that we have covered some of the basics around VO2 max scores let’s look into how some of today’s wearable fitness trackers monitor cardio fitness using VO2max.

How do fitness trackers measure VO2max scores in 2020?

Most fitness trackers and wearables in the market today use the optical heart rate sensor(s) to measure heart rate data and calculate your VO2 max score. This process is also known as photoplethysmography. heart sensor on Fitbit InspireThe accuracy of the VO2max function on a fitness tracker or smartwatch is a product of how good the optical heart rate sensor is on the unit, along with how the tracker calculates VO2max.

Garmin Optical Heart Rate Sensors  

Fitness trackers such as Garmin smartwatches emit green light through the skin at the user’s wrist and then use a photodiode to detect changes in reflectivity that can be used to calculate heart rate. Garmin VO2 Max and Heart Rate sensors

The arteries’ blood flow rate varies with the heart’s contraction (systole) and relaxation (diastole) in a periodic pattern.

During contraction, the density of hemoglobin increases, and blood absorbs more green light.

And during relaxation, the density of hemoglobin is lower, and blood reflects more green light.

Apple Watch Heart Rate Sensors

Apple Watch uses green LED lights paired with light-sensitive photodiodes to detect the amount of blood flowing through your wrist at any given moment.Apple Watch Heart Rate sensors and VO2 Max

The optical heart sensor can also use infrared light.

This mode is what Apple Watch uses when it measures your heart rate in the background and for heart rate notifications. 

Apple Watch uses green LED lights to measure your heart rate during workouts and Breathe sessions and to calculate walking average and Heart Rate Variability (HRV). sensors on Apple watch for blood oxygen and heart

Apple Watch Series 4 or later also has built-in electrodes in the Digital Crown and the back of the Apple Watch, measuring the electrical signals across your heart when used with the Heart Rate app or the ECG app.

When you place your finger on the Digital Crown, it creates a closed circuit between your heart and both arms, capturing the electrical impulses across your chest. 

Apple Watch measures heart rate between 30 – 210 bpm (beats per minute).

Fitbit and other Budget Trackers 

The Fitbit optical heart rate sensors function similarly, using LED green light and multicolor photodiodes. Fitbit can measure heart rate between 30 – 220 bpm.Fitbit heart rate sensors and VO2 Max

Budget Fitness trackers such as MorePro or LetsCom use Nordic 52832 as the main CPU and optical heart rates sensors such as Silicon labs  Si1142 or the PD70-01C-TR7 sensor.

These sensors usually have a single green LED to provide a low power dynamic heart rate monitoring instead of the advanced sensor collections in Apple Watch, Garmin, or Fitbit.

Once the optical heart rate sensors on the back of your smartwatch or fitness tracker have calculated the heart rate data, they can then use various calculations to compute the VO2 max score.

Calculating VO2max from heart rate data on fitness trackers & smartwatches

You can manually calculate your VO2 max using a standardized formula once you know your maximum heart rate and resting heart rate from the heart rate data.

Calculate VO2 Max using your resting heart rate and age:

VO2 max = 15.3 x (MHR/RHR)

MHR = Maximum heart rate (beats/minute) calculated using age = 208 – (0.7 x age)

RHR = Resting heart rate (beats/minute) = number of heart beats in 20 seconds x 3

Other calculations help with the estimated VO2 max. You can read up on them if you like on this page from Wikipedia. 

Although the algorithms used by Fitness trackers use similar computations, they do look at many other factors to improve the accuracy of VO2 max calculation.

For instance, FirstBeat Analytics, used by Garmin, calculates VO2 max based on the type of activity (running, walking speed), time spent on waiting for traffic lights to turn et al.)

Garmin VO2 Max Calculation Excellent VO2 max score in Garmin Connect app

In the case of Garmin, its devices leverage FirstBeat Analytics algorithms to do this.  They use a proprietary calculation to determine the VO2 max score.

The algorithm behind Firstbeat Analytics’ VO2max calculation learns from you over time. The more you use your device, the more reliable your VO2max estimate becomes.

It is also specifically designed to recognize the best data for making the calculation automatically. Meaning stops and starts are excluded, along with any other interference that occurs along the way.

Apple Watch VO2 Max calculation

In the Apple Watch case, the cardio fitness (VO2 max scores) are not visible directly on the watch. It can be found using the Health App on your iPhone in the Respiratory section.

The calculation uses Heart rate data and your pace to calculate VO2 max. This is supposed to be an estimate.

Since the basic component used in VO2 Max calculation by fitness trackers is your heart rate data, you must understand the limitations.

Your heart rate data can be different depending upon the type of motion during which it is calculated.

If you are interested in how Apple Watch calculates VO2 max, you will find the details in this Apple Patent document fascinating.

Motion can affect the heart rate sensor. Rhythmic movements, such as running or cycling, give better results than irregular movements, like tennis or boxing.

Fitbit Trackers VO2 max calculation

Fitbit trackers similarly compute an estimated VO2 Max and term it as ‘Cardio Fitness Score.’

In the case of Fitbit, they calculate the VO2 max using your resting heart rate (RHR) and your user profile.

Some of these calculations that are derived from base optical heart rate sensor data can be complex. For example, look at how Whoop calculates Heart Rate Variability (HRV) below in simple terms.Whoop heart rate variability calculations

Even assumptions around Heart rate data can vary from one fitness device or fitness app maker to another.

For example, users who use the Strava app with Whoop noticed different heart rate readings.

Here is the answer from Whoop on how it treats heart rate data from Strava:

My max and average Heart Rate are different in Strava and WHOOP – why?

While the underlying heart rate date used for both is the same, Strava does some additional processing that can slight differences.

In particular, Strava will filter out GPS and associated HR data for times when you’re not moving, which can result in different average and max heart rates than shown in WHOOP.

So the way various fitness trackers and apps compute max heart rate data can be different, resulting in variability of VO2 max readings.

Are VO2 Max Calculations accurate on fitness trackers?

The relationship between heart rate and VO2 max is not precise, and it varies from one individual to another.

For example, it depends upon factors when the heart rate was measured. And it’s also dependent on whether you were moving or stationary when making the VO2 max calculation.

So Apple’s predicted VO2 max, or for that matter, Garmin or any other fitness tracker, may not be very accurate.

Research has shown that Fitbit and other wearable trackers have accuracy issues when measuring heart rate and related metrics for darker skin tone.Inaccuracy of fitness trackers for heart rate

Some budget fitness trackers have only added SpO2 functionality as more of a marketing ploy.

Without any research-based trials or transparency around their SpO2 estimation process, these trackers just use ‘Spo2’ as a marketing gimmick.

Unlike the measurements that you get at a doctor’s office by wearing a face mask and running or walking on a treadmill (Cardiopulmonary exercise testing), the estimate on fitness trackers is squarely dependent upon your Apple Watch’s heart rate sensor, your pace during workout activity and your profile.

And while that’s pretty good, it is not infallible.

Why is my VO2 max readings missing on Apple Health?

Unlike many fitness trackers that randomly compute your VO2 max based on heart rate data, Apple Watch’s approach is a little more strict.

If you can’t locate any VO2 max readings in the Health app, you have not used the right workouts.

Outdoor Walk and Outdoor Run using the Apple Watch can detect and record cardio fitness levels via VO2 max scores on Apple’s health app.

VO2 max relates to maximal exertion. When running, your workout has to be intense enough to generate a reading. And you’ll need to sustain it for at least 20 minutes.

Why is my VO2 Max score on my fitness tracker so low?

Now that you understand the background of how the VO2 max is calculated, the lower readings displayed on your fitness tracker or smartwatch can be due to many reasons.

The estimated VO2 max from fitness trackers gives you a general direction around your aerobic capacity and maximum oxygen consumption levels.

A free online tool from the University of Kansas Medical center allows you to benchmark your VO2 max and obtain a percentile reading.

Enter your Age, Sex, and VO2 max, and it reads out how you compare with others in the same age group. Use this tool to get a performance benchmark for your VO2 Max. 

Typically a good fitness tracker should show you small increases in VO2 max as you progress through weeks of training trying to build up your endurance, but most of the time, the VO2 max readings are pretty much within the range of 100 basis points for most average athletes.

Heart Rate is indeed an important variable. What was your HR when you started and was it different from the last time you ran?

If it’s lower, your watch sees a lower HR for the activity, and so you’re now “fitter” and have a better VO2 max for that particular run. That’s the problem with the ‘estimated or calculated’ VO2 max on a tracker.

VO2 max does not take into account temperature, humidity, or even elevation. To expand, if you’re run route takes you up a hill, and so you naturally slow. You’re now less fit – HR vs. Pace.

Here are a few examples that you can see where VO2 max readings stay almost the same inspire of different training routines of varying degrees of intensity.

No change in VO2 Max data inspire of exercising

Here is another example of a user who has been regularly working out without any noticeable change in recorded VO2 max.No change in VO2 Max on Apple Health

Here’s another example from a user who had not changed his exercise routine but noticed that the VO2 max spiked up.

Increase in VO2 max without incremental exercise

This is a small sample, and we are taking the user’s words towards our inferences, but it does show that there could be problems with how the VO2 estimation works on various fitness trackers.

Other reasons why your VO2 max is showing low could be because of the following key points:

Software Updates on the fitness tracker

Most of the fitness trackers and smartwatches go through various software upgrades.

Sometimes, it is possible that the manufacturer made adjustments to the calculations, and you notice a drop in your VO2 max.

Since there is not much transparency around how they calculate the score, it is hard to determine. Your low VO2 max reading could simply be a result of the changes in calculations.

Wrong user profile Information

Your user profile (particularly weight, sex, and age information) plays an important role in the calculations.

If you see an unusual or abnormal VO2, Max, check your user profile information on your fitness tracker to ensure that you have entered the correct information.

Check the heart rate sensor on the back of the fitness tracker?

Is it scratched? Is it dirty?

These could impact the quality of your heart rate readings.

Since HR readings are based on the calculation of VO2 max, your low or abnormal VO2 max could be due to a malfunctioning heart rate sensor.

Check with your doctor and get a real VO2 Max test.

Should you be concerned about the low VO2 max? For most folks, the answer is probably no.

If you see that your VO2 max is consistently staying below the average acceptable score ( 37.9 for men and 27.6 for women), you can reach out to your doctor and have them perform a real VO2 max test at their office. 

Can you increase your VO2 max score on your Fitness tracker?

Honestly, there is not much that you can do. Although many people claim that your VO2 max is trainable, your genetics are the biggest determinant…and something you cannot change.

There’s only 5-15% potential improvement (3). Not a whole lot of wiggle room there. If you come into the lab and get tested with a  VO2 max of 50 ml/kg/min, the best you can hope for with “all the training in the world” is an improvement to ~57 ml/kg/min. 

A study that looked at VO2 max values in twins (9) concluded that genetic factors explained 72-74% of the difference in VO2 max and, even when “sports participation” was factored in, genetic factors continued to explain between 57% and 63% of the variance in VO2 max.

Presumably, studies like this formed the belief that VO2 max was largely genetic and could only be increased by a small amount through training.

That being said, most users training for endurance runs or other high-intensity routines regularly do see a 5 – 15% improvement in VO2 Max over a period of time.

Logging a 20-minute, high-intensity outdoor run workout once a week will help to increase your VO2 max.

How does VO2 Max influence other related metrics?

When your VO2 max goes up, your resting heart rate goes down. 

Lower is better with Resting Heart Rate. For most people, a resting heart rate between 60 and 100 beats per minute is considered normal.

It’s affected negatively by stress, hormones, and medication. Getting into better shape can not only lower your resting heart rate, but it could also help save your life.

Those with a high VO2 Max will likely also have a lower resting heart rate, lower blood pressure, and lower risk for chronic diseases, as these are also positively correlated with being fit.

By exercising regularly using HIIT or an endurance run, you should be able to see some improvements in your VO2 max score and your overall health.

References:

1.https://well.blogs.nytimes.com/2010/07/19/a-carb-boost-without-the-carbs/

2.https://www.mayoclinicproceedings.org/article/S0025-6196(15)00642-4/pdf

3. https://simplifaster.com/articles/how-trainable-is-vo2-max/

4.https://www8.garmin.com/manuals/webhelp/vivosmart4/EN-US/GUID-1FBCCD9E-19E1-4E4C-BD60-1793B5B97EB3.html

5 https://www.garmin.com.sg/minisite/garmin-technology/health-science/heart-rate/

6.https://support.apple.com/en-us/HT204666

7.https://help.fitbit.com/articles/en_US/Help_article/1565.htm

8.https://www.silabs.com/sensors/biometric/si1143-44

9.Fagard, R., Bielen, E. and Amery, A. “Heritability of aerobic power and anaerobic energy generation during exercise.” Journal of Applied Physiology. 1991; 70(1): 357-362.

I am a technologist with years of experience with Apple and wearOS products and have a Bachelor’s degree in Computer Science. In my day job, I advise fortune 500 companies with their digital transformation strategies and also consult with numerous digital health startups in an advisory capacity. I'm VERY interested in exploring the digital health&fitness-tech evolution and keep a close eye on patents, FDA approvals, strategic partnerships and developments happening in the wearables and digital health sector. When I'm not writing or presenting, I run with my Apple Watch or Fossil Gen 5 LTE and keep a close eye on my HRV and other recovery metrics.

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