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

Low VO2 Max on fitness trackers

One in five Americans own a fitness device today and the $31 billion wearable industry keeps adding on metrics to its popular products.

Many fitness trackers and smartwatches now have the ability to measure your VO2 max scores. It is one of the metrics that has become increasingly popular over the past few years for fitness enthusiasts. Should you really be concerned about a low VO2 max score from your Apple Watch or Garmin or Fitbit device?

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First things first…

What is VO2 Max score?

Aerobic capacity test (VO2 Max) also known as the maximum oxygen consumption test measures the maximum amount of oxygen 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. ‘V’ stands for Volume and ‘O2’ represents Oxygen.

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?

There are many factors that can influence VO2max, e.g. heredity, training, age, gender, and body composition.

Structurally speaking, there are three components to the VO2 Max Puzzle. How much-oxygenated blood your heart can pump, How much of it reaches your muscles, and finally how much of it is actually utilized by your muscles.

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, thus 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.

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 VO2 max?

The average VO2 max 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 VO2 max results in the study 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

AgeVery PoorPoorFairGoodExcellentSuperior
13-19Under 35.035.0-38.338.4-45.145.2-50.951.0-55.9Over 55.9
20-29Under 33.033.0-36.436.5-42.442.5-46.446.5-52.4Over 52.4
30-39Under 31.531.5-35.435.5-40.941.0-44.945.0-49.4Over 49.4
40-49Under 30.230.2-33.533.6-38.939.0-43.743.8-48.0Over 48.0
50-59Under 26.126.1-30.931.0-35.735.8-40.941.0-45.3Over 45.3
60Under 20.520.5-26.026.1-32.232.3-36.436.5-44.2Over 44.2

VO2 Max Norms for Women

AgeVery PoorPoorFairGoodExcellentSuperior
13-19Under 25.025.0-30.931.0-34.935.0-38.939.0-41.9Over 41.9
20-29Under 23.623.6-28.929.0-32.933.0-36.937.0-41.0Over 41.0
30-39Under 22.822.8-26.927.0-31.431.5-35.635.7-40.0Over 40.0
40-49Under 21.021.0-24.424.5-28.929.0-32.832.9-36.9Over 36.9
50-59Under 20.220.2-22.722.8-26.927.0-31.431.5-35.7Over 35.7
60Under 17.517.5-20.120.2-24.424.5-30.230.3-31.4Over 31.4

 

Now, compare these average numbers now with that of world-class elite athletes and you will 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 and measure VO2 Max.

How do fitness trackers measure VO2 max scores in 2020?

Most fitness trackers and wearables in the market today use the optical heart rate sensor to measure heart rate data and calculate your VO2 max score. This process is also known as photoplethysmography.

Accuracy of VO2 max on a fitness tracker or smartwatch is a function of how good the optical heart rate sensor is on the unit along with how the tracker calculates VO2 max.

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 rate of blood flow in arteries 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. 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).

Apple Watch Series 4 or later also has built-in electrodes in the Digital Crown and the back of Apple Watch, which can measure 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 in a similar way 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 along with optical heart rate 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 as opposed to 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 a variety of calculations to compute out the VO2 max score.

Calculating VO2 max from Heart Rate data on Fitness Trackers

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

There are other calculations that 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 in order 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

In the case of Garmin, its devices leverage FirstBeat Analytics algorithms to do this.  They use a proprietary calculation to determine 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 automatically recognize the best data for making the calculation, meaning stops and starts are excluded along with any other interference that occurs along the way.

Apple Watch VO2 Max calculation

In the case of Apple Watch, the 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, it’s important that you 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 really interesting.

Motion can affect the heart rate sensor. Rhythmic movements, such as running or cycling, give better results compared to 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 result in 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. It also depends if the VO2 max calculation was done when you were moving or when you were stationary.

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 people with 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. Only Outdoor Walk and Outdoor Run using the Apple Watch can detect and record VO2 max scores on Apple’s health app.

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

Why is my VO2 Max score on 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 made any change to his exercise routine but noticed that the VO2 max was spiking 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 find out. 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 make sure 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 the basis of 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, it is very much limited by your genetics.

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 basis of 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 on a regular basis do see the 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, it could 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.

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