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HRV and VO2Max, an explanation and device comparison

Spend less than ten minutes on any fitness forum and HRV and VO2Max are traded like they are the most important indicators of health, required to be as high as possible. Not so, and what are they even?

To reliably measure VO₂Max, specialized equipment is still absolutely necessary. VO₂ max is the maximum (max) rate (V) of oxygen (O₂) your body can use during exercise, which is best detected by measuring oxygen intake and exhale.

HRV is a bit of a mysterious box of something. Our heart beats in more or less steady intervals, known as “sinus rhythm” due to being triggered (in healthy people) by the sinus node in the right atrium of the heart. To accommodate small fluctuations in needed blood in the body, this rhythm can change ever so slightly between beats, and of course speed up if even more is needed. Pulse frequency is known as chronotropy, while the pressure exerted by the heart with each beat is known as inotropy.

Both are somewhat predictive of exercise results and exercise strength. A high(ish) HRV shows, that parasympathetic (rest and digest) and sympathetic (fight and flight) systems of the body work well. Bad sleep, stress, drugs, alcohol, and some other factors shift this towards a continuous sympathetic system, meaning less influence of the parasympathetic system and thus shorter HRV.

However, a HRV over 0.12 seconds isn’t a sign of a balanced system, it’s a potentially fatal state of the heart called sinus arrhythmia. There’s also no absolute number how high or low HRV should be, but as a rule of thumb, a higher HRV means better restedness and less systemic stress.


So, HRV is a decent indicator of recovery and stress reduction. Striving to improve it, if even ever so slightly, can help with physical and mental ailments or, even better, show us what is working, and what not.

To clinically measure HRV, we use a so-called “Holter” monitor, a portable ECG. I strapped myself into one for four days and measured mine. At the same time, I was wearing both an Apple Watch Ultra and Garmin Fenix 7 Pro Solar. To exclude side differences, I wore either watch on each arm for two days.

To complicate matters, there are multiple ways to measure HRV.

Fig 1: Sinus Rhythm

This is, how an ECG looks on a monitor. It depicts the movement of the electrical charge through the heart, with the P being the “sinus” pacemaker, and the QRS complex actually showing the contraction of the chambers (very loosely explained).

All measurements focus on this R peak, and the distance between two of them, shown as “RR Interval” above. How to calculate a score from this is, however, dependent on how long the measurement was, and how it was obtained. Apple Watch and Garmin use SDNN and multiple rMSSD respectively. SDNN is a long term measurement of variability, taking samples or continuously monitoring heart beats, then calculating a mean and a median variability. rMSSD takes a two to five minute reading and uses this to determine HRV right now.

Both methods (there are dozens more) have their place. Third party apps like HRV4Training actually extract raw heart beat data from Apple Health and apply rMSSD calculations, because that’s what athletes are most interested in.

On to the data. Over four days, I experienced little change in my day-to-day HRV. HRV changes with small things, such as a short meditation (Me, meditate? What’s next? Vegan latte and goat yoga?) or drinking a glass of water. Taking 96 readings per day, however, can get a decent average for the current status of HRV.

HolterApple Watch NativeHealth Data -> rMSSDGarmin
Table 1.: HRV accuracy

As we can see, my Apple Watch underestimated my HRV greatly, while Garmin’s approach to SDNN seems to have come to a similar conclusion as the Holter, which uses rMSSD on demand.

After discussing these findings, I was told to turn on “Afib detection” on my Apple Watch, a feature used to detect the absence of atrial impulses, and to do a morning “Mindfulness” session, because it would force the watch into rMSSD. During the sessions I did not meditate, I just let them run alongside my morning weather forecast watching.

HolterApple Watch NativeHealth Data -> rMSSDGarmin
Table 2.: HRV accuracy with “tricks”

This greatly improved reliability of the Apple Watch HRV data, at the cost of a morning routine and about one hour less battery on my Apple Watch Ultra.


To measure VO₂Max, both devices require a lead in time of multiple weeks running and exercising. This, to be honest, is the reason this post is so delayed, I had to wear both individually for three weeks while doing trail runs and hikes, before I could use the data provided.

Subsequently, I also visited our physiolab and exercised while wearing a mask to capture and calculate my oxygen use.

LaboratoryApple Watch NativeGarmin
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Table 3.: VO₂Max

Here, results were much closer to the lab standard, with the Garmin slightly closer. This isn’t surprising, Garmin uses software optimized for athletic pursuits and has a slightly better breath cadence detection than Apple. In both cases, and I am surprised they came that close, VO₂Max is a best guess, not a real measurement. It is taken by observing breathing patterns and blood oxygen under exercise, not O₂ content in your breath.


Both devices are equally capable of providing a basic picture of HRV and VO₂Max. Garmin is a little closer, but given the more or less “best guess” nature of both devices, neither should be taken as gospel.

HRV isn’t a fitness parameter and more a sign that we’re “in balance” and stress is low. Rather than aiming for exact numbers, aim to increase HRV to a point where you feel genuinely calm and ready to exercise. Too high is not a good thing, either, and stress is a natural and important part of our physiology, so don’t get discouraged.

VO₂Max is a good determinant of cardiovascular and pulmonary training status. Here, too, however, it’s not the final number that counts but bringing it up as you train is. Since muscle mass and physiology differ between genders and age groups, not to mention the impact many medications may have on your VO₂Max, it makes no sense printing a table here. Just go out, run or hike, and if your VO₂Max goes up over the coming months, know that all is well. If it doesn’t, that doesn’t mean all isn’t well, it just means, well, nothing. You’re running, right? That’s what matters.

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