Heart Rate Variability

The ultimate guide to the body’s speedometer!

Heart rate variability is a biomarker that measures the body’s physiological load. Simply explained, we can think of the measurement as a speedometer for the body. We can use the measure both to follow when we are stressed and as a tool for mental training.

What is Heart Rate Variability?

To explain heart rate variability, it is easiest to start with a concept many people recognize, your heart rate! Heart rate is a measure of how many beats per minute your heart beats. Monitoring your heart rate is a common way to optimize physical fitness training. For example, by comparing your current heart rate with your maximum heart rate, you can control your physical load by training in a specific heart rate zone.

The heart is an incredible muscle that beats over 100,000 times a day. Its main purpose is to pump blood and supply the body with oxygen and nutrition. A common misconception is that the heart works like a clock and that a heart rate of 60 means that the heart beats exactly one beat per second.

In reality, the heart’s stroke length varies from beat to beat. This means that one stroke may take 1.5 seconds while the next only takes 0.8 seconds. How much this stroke length varies is called heart rate variability. When we look at recorded heartbeats, we immediately see how the stroke length varies. In the image below, you see an ECG signal where the arrows show how the length of the beats varies:

Your heart rate varies in sync with your breathing

By checking your pulse on your neck with two fingers and taking a couple of deep breaths, you can actually feel this variation yourself. When you inhale, the pulse increases (stroke length decreases) and when you exhale, the pulse decreases (stroke length increases). Cool huh! In medical terms, this is called respiratory sinus arrhythmia and is completely normal and a sign of health.

The frequency variation occurs because the body is a machine that likes to be efficient. When you breathe in fresh oxygen, it wants to quickly oxygenate your blood by increasing blood flow through the lungs. Therefore, the heart pumps a little extra when you inhale. When you exhale, there is no new oxygen to pick up and the heart rate drops. As a consequence, the heart rate varies with breathing, which in a graph looks like this:

However, it is not only breathing that affects the heart rate. The body’s autonomic nervous system (ANS) also has great power over how the heart behaves. If you expose the body to an unexpected stressor, the brain and ANS will quickly signal the body to get ready to flee or fight.

The heart rate will then increase while the variation between the beats decreases. Instead of beating irregularly with a variable stroke length, the heart begins to tick more like a clock. When the perceived danger is over, the heart rate will decrease and become more irregular, resulting in a higher heart rate variability.

Why is heart rate variability interesting?

The interesting thing about HRV is that it is so strongly linked to the autonomic nervous system. The activity in the ANS is in turn a direct reaction to our lifestyle and the world we live in. By following HRV, we can therefore gain an understanding of how our body reacts to our environment. Practically, we can apply this knowledge in mainly two areas:

1. HRV as a way to understand your work load

The activity in ANS depends on your environment. HRV changes in real time depending on the activity of the ANS. So if we can measure HRV we can understand how different activities affect the body in real time. We usually talk about HRV acting like a window to the inside of the body. In Linkura’s application in particular, this means that we can create views like below:

By analyzing this view, you can see how different activities in your everyday life affect your body. Among our users, over 70% usually state that measuring HRV has given them new insights into their body. A common insight is e.g. that TV in the evening rarely provides an objective recovery, no matter how good it feels in the moment. It is also clear that a good night’s sleep often does wonders to keep the body calm and recovered through all the challenges of the day.

2. HRV as a way to understand your balance

If we look at how HRV behaves over longer periods, we can calculate a measure we call resting HRV. It is easiest to think of the measure as the balance between stress and recovery in the body. For those who like details, it’s about your flexibility in the autonomic nervous system. The cool thing about this very biological measure is that it is connected to cognitive and subjective experiences.

If you have a high resting HRV, a high flexibility in your autonomic nervous system, you will more often experience positive emotions such as motivation, drive and commitment. If you have a low resting HRV, a low flexibility in your autonomic nervous system, you will be unmotivated and disengaged in the short term and in the long term have an increased risk of stress-related ill health.

Vilo-HRV is thus a way to objectively measure and quantify changes that affect the way you experience the world. It is agreed in the literature that the healthy body in its basic position has a high resting HRV. People who find themselves in this “base mode” have a subjectively high motivation, a high self-assessed ability to perform and a good sense of well-being.

In terms of the modern life we live, however, “base mode” is not the same as normal mode. Although high resting HRV is the body’s basic state, it is rare for individuals to be there. This is because, through our modern lifestyle, we often overload the system with impressions.

Low resting HRV is unfortunately connected with low motivation, low performance, poor well-being and, for those with very low resting HRV, increased risks for diseases such as fatigue syndrome. By measuring HRV, we can thus get a direct measure of our ability to experience motivation, drive, performance and the risk of overload.

Do you want to know more about this? Read more about mental fitness here!

How do you measure heart rate variability?

The most reliable way to measure HRV is with the help of an ECG meter. ECG is short for electrocardiography and is based on recording the electrical signals that form in the heart’s tissue when it beats. The signal provides an enormous amount of information, and in healthcare, ECG is often used to diagnose or monitor the heart. One of all the parameters that can be calculated from an ECG is precisely HRV and thanks to the signal’s richness of detail, it can be done with very high precision.

You can also measure HRV with optical technology, for example on the wrist, so-called photoplethysmography (PPG). That method is based on measuring variations in how much blood flows through the skin, which correlates with the frequency with which the heart beats. The disadvantage of this technique is that you need to sit basically completely still because the slightest movement disturbs the weak variations in the wrist’s blood flow.

For the same time series the different measurements usually looks like this:

Which technique is the best?

In practice, PPG is more widely used because the technology is more accessible. A decent camera in a smartphone or an optical sensor in a smartwatch is enough. At Linkura, however, we decided early on to invest in ECG because it is the technology that provides the best precision. We have therefore developed a portable ECG device that was designed to be user-friendly without sacrificing signal quality.

For several years now, our patented ECG device has been used by users all over the world. What is completely unique is that you can listen to your heart with medical signal quality with a meter that can be worn in a regular heart rate strap around the chest. The best of both worlds, if you ask us!