The Definitive Guide To Heart Rate Variability (HRV) For Training: Part I

In this series of articles dedicated to heart rate variability, we will give you a complete guide discussing what exactly HRV is, how to measure it, and how to use it in your training.

This is the first article of the series where we will dive into what HRV is, its physiology, and its features. Below is a sneak peek at the rest of the series coming up soon:

1. What is Heart Rate Variability? (physiology, HRV features)
2. How Do We Measure Heart Rate Variability? (ECG vs PPG, morning vs night, lying down vs sitting, timing of the measurement i.e. the response)
3. How Do We Use Heart Rate Variability in Training? (monitor relative changes, normal range, reduce intensity when suppressed)
4. The Dos and Don’ts (or myths, e.g. optimize performance, not HRV, issues with arrhythmias, readiness scores, real-time stress)

Ready? Let’s get started!

What Is Heart Rate Variability?

If your heart rate is 60 beats per minute, your heart is not beating exactly every second. There is always some variation between consecutive heartbeats, it could be 0.9 seconds, 1.1 seconds, and so on.

Technically speaking, the amount of variation in the time between consecutive heartbeats collected over a certain amount of time (e.g. a minute) is what we call heart rate variability (HRV).

We will see in the next section why this is the case, but for now, I can add that in more practical terms, HRV is a marker of stress, and as such, it can be a useful parameter to track for athletes aiming to improve their performance. 

In this series of articles, I will provide an overview of the physiological underpinnings of HRV, the technologies we can use to measure it, how we can use and interpret the data in the context of adjusting our training, and some of the dos and don’ts of HRV, ideally busting some myths along the way. 

Let’s Start With The Physiology

We all experience different forms of stress in our lives. Some of these forms of stress can be physical, others can be psychological, but they all impact our physiology in a similar way.

In particular, when we face a stressor (that’s what we’ll call something that causes stress), our body experiences a response that typically involves changes in autonomic nervous system activity (e.g. an increase in sympathetic activity and a reduction in parasympathetic activity), and in hormones (e.g. increased cortisol levels). 

What does HRV have to do with this?

The link comes from the autonomic nervous system. This is the part of the nervous system that takes care of many functions of our body without our conscious control (e.g. breathing, not something we remember to do at every breath).

The autonomic nervous system has two main branches. The sympathetic branch, or fight or flight, is the one that is active when we need to mobilize resources, e.g. when we start exercising.

The parasympathetic branch is instead the one that takes cover in the context of rest and recovery and is the most important one when it comes to HRV. 

Autonomic nervous system activity impacts many organs, including the heart. In particular, when we face a stressor, we typically have an increase in sympathetic activity and a reduction in parasympathetic activity, which causes heart rate to increase, and HRV to decrease.

Eventually, HRV is just an indirect way to assess the response of the autonomic nervous system to stress – when assessed under certain conditions (we’ll discuss protocols for measurement in the next part of this series). 

Why HRV And Not Just Heart Rate?

In the previous paragraph, we learned the basics of the stress response: we face a stressor, the sympathetic branch of the autonomic nervous system gets more active, while the parasympathetic branch gets suppressed

As a result, we have changes in heart rhythm, such as higher heart rate and lower HRV. 

This means that both resting heart rate and HRV are markers of stress, or of the stress response.

The reason why we focus on HRV is that typically it is a more sensitive marker of stress, with respect to resting heart rate.

This is the case as the parasympathetic system influences heart rhythm to a greater extent during the exhale, therefore causing high-frequency changes that are captured by HRV, but not heart rate. 

A more sensitive marker of stress allows us to capture changes that might not be visible in resting heart rate, for example when we face more subtle stressors, or chronic forms of stress that might not impact heart rate the way sickness or other strong stressors do.

This is why we typically focus on HRV. This being said, resting heart rate is a great tool too, is simpler to measure, less impacted by measurement artifacts, and can capture similar changes in many – but not all – circumstances. 

Got It, But How Do We Quantify HRV?

We are used to considering heart rate as an instantaneous measure. Right now, as I type these words, my heart rate is 51 beats per minute. We cannot do the same with HRV, which can be confusing at times.

By definition, quantifying the variability in a measure requires to collect data for a certain amount of time.

Historically, HRV measurements were either 24 hours long (a full day for a single HRV number) or 5 minutes long. These days, research has shown that for some of the metrics of interest, 60 seconds are in fact sufficient.

Once we have collected instantaneous beat-to-beat differences over a certain period of time, we can compute HRV.

Here is where things can get even more confusing, as there are probably more than sixty different ways to go from a series of beat-to-beat differences to an HRV number.

Fortunately, in the past decade, most research studies and products have settled on a specific way to compute HRV, called rMSSD.

This value (or a simple transformation of this value) is what you see in most of today’s wearables and apps that track HRV (Oura, Whoop, Garmin, HRV4Training, Elite HRV, tec.).

Due to how the rMSSD value is computed, it tracks well with parasympathetic activity, i.e. with the part of the autonomic nervous system in charge of rest and recovery.

This means that when we face a stressor, we have a reduction in parasympathetic activity and a resulting reduction in HRV. 

Keep in mind that HRV is just a generic marker of stress, and as such, is sensitive to most stressors (e.g. training, but also environmental stressors, sickness, the menstrual cycle, psychological stressors, etc.), but not specific to any in particular, which means that when we see a reduction, we need the right context to be able to interpret it.

But we are getting a bit ahead of ourselves here. 

In the next article in this series, we will look at how to acquire HRV data using today’s technologies, before moving on to how to interpret and use the data, in part three.