understand everything about heart rate variability on sports watches

Appearing for several years on Garmin watches, heart rate variability, or HRV, is widely highlighted by the brand on its sports watches.

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The most recent models make it possible to obtain, every day, a “ VFC statute » supposed to indicate your current state of form or fatigue. But how is it calculated, what does it correspond to and what is it used for? This is what we will see in this complete file on the VFC status of Garmin watches.

What is heart rate variability?

Heart rate variability is the average difference between two heartbeats. In fact, the heart does not beat at a regular frequency, but with intervals which will differ slightly from one beat to the next. The difference between these differences is what we call heart rate variability, expressed in milliseconds (ms).

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For example, if a person has four heartbeats with intervals between each of 848 ms, 829 ms, 843 ms and 822 ms, the variability over this period will be 26 ms, or 848-822.

The advantage of connected watches is that they are able not only to analyze the heart rate itself, but also to precisely measure each heartbeat.

Therefore, simply based on its optical cardio sensor, the watch can also measure the user's HRV. To do this, it does not require an additional sensor like for blood oxygen saturation (SpO2).

Frequency variability is an excellent indicator of stress. To understand this, you need to know that the autonomic nervous system — which manages the body's automatic functions — is based on two parts: the sympathetic nervous system and the parasympathetic nervous system.

The evolution of HRV day after day, night after night, will allow us to see which part will gain the upper hand over the autonomic nervous system.

Why is my HRV decreasing?

If the HRV value drops, it is a sign of a prevalence of the sympathetic nervous system.

This is the part of the autonomic nervous system responsible for increasing heart rate, breathing capacity or sweating.

Generally, the sympathetic nervous system takes over during periods of stress in the body, whether it is a period of illness, mental stress, or alcohol consumption.

In my case, after two years of using a Garmin watch, I noticed that my HRV had systematically dropped when I was sick or highly stressed. In this way, this drop in HRV can be a good indicator to see that we are going to get sick or that we are under too much pressure.

When HRV drops, it can therefore be useful to take a little rest to recover both physically and mentally. It may also be completely normal, during peak training periods, to encourage the body to adapt before a deadline. Furthermore, after months of intense training, HRV can also drop during the sharpening period, when training is considerably reduced in the run-up to a competition.

Why is my HRV increasing?

Conversely, an increase in HRV is a sign of superiority of the parasympathetic nervous system.

If the sympathetic nervous system is the one that will manage reactions to stress and nervousness, the parasympathetic system is the one in charge of relaxation. It is this part of the autonomic nervous system that will reduce the heart rate, close the pupil or stimulate digestion.

Generally, an increase in HRV—and therefore a prevalence of the parasympathetic nervous system—results from a period of rest and recovery.

We will tend to favor a HRV which varies upwards, a sign of effective recovery. However, this still needs to be qualified. Indeed, an increasing HRV can also be a sign of overtraining with low-intensity activities. To restore balance, it is therefore better to increase the difficulty of a few training sessions.

What is “normal” heart rate variability?

Heart rate variability can be very different from one individual to another. Above all, it cannot fundamentally improve depending on its shape, upwards or downwards. Therefore, there is no HRV that can be described as normal, since it will depend above all on the individual.

What we will seek to measure with HRV, more than the value itself, is its evolution over time. At Garmin, for example, a HRV status considered as “ normal » means that the measurement remains stable over time, without an upward or downward peak.

It should be noted, however, that an individual's average HRV tends to decline with age. According to data from the connected bracelet brand Whoop, 50% of users aged 20 have a HRV between 60 and 115 ms, while 50% of those aged 60 have a HRV between 30 and 45 ms.

In absolute terms, it remains important to note that there is no such thing as a normal HRV. A very fit person may have a HRV of 39 ms while a highly sedentary person may have a HRV of 150 ms… and vice versa. In itself, HRV should therefore not be compared from one person to another.

What is HRV status at Garmin?

Since heart rate variability can vary greatly from one night to another and is highly individualized, it would not be very useful to measure it every night and give a raw value to the user.

HRV status smoothed over seven days

It is in this sense that Garmin goes a little further with its “ statut de VFC » which has two interests: first, it is smoothed over several days, then, it is contextualized.

Concretely, the Garmin HRV status is not the measurement of HRV during the last night, but a smoothed average over the previous seven nights. Enough to allow you to ignore a bad night so that the status is more representative of your general state of form at the moment. Each day, the average HRV throughout the night is obviously calculated – and accessible in the Garmin Connect data. But, for the HRV status, it is therefore an average of the seven previous nighttime averages which is calculated.

This means that you will have to wait a few days – in this case three weeks – after wearing a Garmin watch before seeing your first HRV status appear. Time for the watch to know you and calibrate your so-called “values”. normal ».

A contextualized VFC status

Garmin's HRV status also has the advantage of being accompanied by a small comment depending on where it is located in relation to a zone called ” normal » or « baseline ”, shown in gray.

This baseline will evolve over time and with the evolution of your HRV. It corresponds to what Garmin considers to be your optimal HRV zone, with a minimum value and a maximum value. Then, depending on where your HRV is in this baseline, several comments are associated, with a different shape and color):

• Average HRV in baseline: Balanced HRV (green circle)
• Average HRV above or just below baseline: Unbalanced HRV (orange square)
• Average HRV well below baseline: Low HRV (red triangle)

Once again, we are talking about the average HRV value smoothed over seven days here, and not the average HRV from the previous night. It can happen to have a low HRV overnight without it being a problem. However, it can become so if it is repeated over several days and that is when the status will change to “ Low HRV ».

Garmin's HRV status can be used as data in itself, to know if you need to recover or if you are in good shape. However, beyond the score itself, this status and HRV in general is used by Garmin in several of its indicators.

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Thus, HRV status is one of the main criteria taken into account by Garmin to establish the training readiness score. Alongside sleep quality, recovery time or training load, HRV status is one of four factors Garmin considers to assess whether you are fit to train or should rest.

In the same way, heart rate variability is essential for the analysis of the Garmin Body Battery. This index allows you to know if you are subjected to too much stress during the day and if you end it too tired. However, it is not the HRV status that is used, but the HRV itself, throughout the day, to assess periods of stress.

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Finally, for the evaluation of the training status also the HRV status is taken into account. With a HRV in the upper part of the “ baseline “, Garmin's algorithms will be more likely to qualify the training as ” productive “, even with a lower training load than usual. To achieve a “ pic » training too, a high HRV is preferable according to Garmin.

Garmin watches with HRV status

HRV status is offered on a large number of Garmin connected watches. We will find it both on models dedicated to fitness and running.

Here are the watches that allow you to measure HRV status from Garmin:

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