There are a number of metrics we can use to get a snapshot of our health and well-being. From bodyweight to blood pressure and calories to heart rate, doctors and researchers are more interested in our biological statistics than ever before. As fitness wearables have grown more advanced, we’ve gained an opportunity to track these same metrics from the comfort of our own home.
Yet, there is one marker for resilience and well-being that researchers only began to utilize over the past two decades. It’s called heart rate variability, or HRV.
“Heart rate variability is a very broad term,” says Doctor Phyllis Stein, Associate Professor of Medicine and Director of the Heart Rate Variability Laboratory at Washington University School of Medicine. “What we actually measure is not heart rate, but an interval.”
This simple metric, once measured primarily in athletes and those with abnormal heart rhythms, has since become a key piece of data for individuals wanting insight into their bodies and current state of health. It can predict how well we may perform during a workout, just as it can determine how stressful the average workday was on our body.
So what exactly is heart rate variability? How do we measure it? And what can it tell us about our overall health? Let’s break down the intricacies of this emerging physiological measurement.
What Is Heart Rate Variability?
Before we can discuss why HRV is so important, first we must define exactly what it is.
Heart rate variability, or HRV for short, is a measure of the time between each heartbeat. Controlled by a fundamental part of our nervous system called the autonomic nervous system (ANS), HRV is one of many functions that occurs without us monitoring or even thinking about it.
Functions such as our heart rate, breathing, blood pressure, and digestion are also controlled by the ANS — they are regulated internally whether we like it or not. “The ANS is the global name for the parts of our physiology that include the brain, the sympathetic, and the parasympathetic nervous system,” notes Stein. The sympathetic and parasympathetic nervous system are also known as the fight-or-flight mechanism and relaxation response, respectively.
The primary part of our brain that works in tandem with the ANS is called the hypothalamus. It’s in this specific region that information is processed and signals are sent to our body in return. Some of these signals attempt to stimulate bodily functions, and other signals attempt to relax them.
For instance, a stressful day at work may cause our heart rate to increase and our blood pressure to rise. A delicious meal, on the other hand, may cause our breathing to relax and our heart to beat casually. “The autonomic nervous system is not only responding to physical demands, but emotional ones as well,” says Stein.
Normally, our body will handle these situations with ease and life will go on as always. But other times we may encounter stress, poor sleep, bad eating habits, unsettling relationships, or a lack of exercise — all of which can throw these systems off, forcing us into the fight-or-flight response.
Why Is HRV Important?
“Heart rate variability is a huge family of insights into how the body and the heart are talking to each other,” says Stein. “The autonomic nervous system is responding not only to physical demands, but emotional ones too.”
Determining when our body is in the emotional fight-or-flight response is a crucial part of our welfare. When our internal system is responding negatively to a stimulus, the variation between beats of the heart will be lower — a low HRV. If we’re calm and collected, this variation will instead be higher — a high HRV.
While we associate life’s stressful moments with an elevated heart rate, this doesn’t always translate to an elevated HRV. A high HRV signifies that your nervous system is balanced, and your body is capable of adapting to whatever comes next.
That is to say, a healthier ANS will be able to switch gears quickly and show more resilience or flexibility. We’ll be capable of adapting to stressful situations without entering the fight-or-flight response.
So why is our ability to adapt and remain calm so important? And why do we want our heart rate variability to be high?
We’d like this to be the case because recent research has found a relationship between low HRV and poor health. Those who suffer from persistent depression, anxiety, or mental stress tend to have a low HRV. And a low HRV can indicate an increased risk of heart disease or even death.
Those who have a higher HRV, on the other hand, tend to be more resilient and commonly engage in physical activity. HRV can act as a snapshot of our current lifestyle, and analyzing HRV can motivate those who are in poor health to seek improvement.
When we are more mindful, restful, and physically active, often our HRV will drastically improve. In a world of data, HRV acts as a figure that indicates how we react to our environment — be it healthy or otherwise.
How Do We Measure Heart Rate Variability?
Measuring heart rate variability is a task not easily performed at home, though that reality is slowly changing.
To measure HRV we need a piece of equipment known as an electrocardiogram, or ECG. An ECG is a machine that measures our heart’s voltage over a specific period of time. By utilizing electrodes placed on the skin, doctors and researchers can monitor the electrical activity of our heart and reach specific conclusions about our health.
As technology has advanced and we’ve found new ways to measure our body’s biological components, so too have we found ways to measure HRV at home. By utilizing wearables that contain heart rate monitors, we can evaluate HRV data and sync this data to our phone via apps provided by Google or Apple.
“We have a world of mathematicians that have incredibly complicated and wonderful ideas about how to measure heart rate variability at home,” says Stein. While such technology may not be as accurate as professional equipment, it is both cheaper and easier to use on a consistent basis.
What Do HRV Values Mean?
Measuring HRV on a machine and understanding what these HRV measurements mean are two entirely different tasks. With that in mind, let’s break down the intricacies of this measurement in more detail.
“Heart rate variability is based on a set of numbers generated from the times between heartbeats” says Stein, or the periods of time that occur between two beats. These are also known as RR intervals and are measured in milliseconds. The RR interval refers to the time between two peaks on a traditional ECG heartbeat waveform.
Additionally, NN intervals refer to the intervals between two normal R-peaks. In practice, RR intervals and NN intervals are the same, and the use of NN intervals merely emphasizes that normal R peaks, like the one pictured below, were used.
Another term worth remembering during HRV analysis is the root mean square of standard deviation, or RMSSD for short. The root mean square of standard deviation is simply the successive differences between neighboring RR intervals. According to researchers Fred Shaffer and J. P. Ginsberg, we can calculate this “by first calculating each successive time difference between heartbeats in milliseconds. Then, each of the values is squared and the result is averaged before the square root of the total is obtained.”
Methods by which we can analyze HRV data and extract this information exist in either the time-domain or frequency-domain. Each method of analysis is different, but both contain a plethora of information.
Time-domain analysis, for example, is often used in clinical applications of HRV and is the simplest method of HRV analysis. Time-domain analysis utilizes interbeat intervals to measure NN intervals and RMSSD.
Frequency-domain techniques, on the other hand, “are performed on the interbeat interval signal, a plot of the RR intervals versus time or beat number. It is very important with frequency-domain techniques that the data points be equidistant,” according to Harvard Bioscience.
Is Heart Rate Variability a Sign of Fitness?
Now that we know high HRV is a sign of good health, let’s determine what this benchmark means in terms of our fitness.
When we have a high HRV, our body is capable of remaining resilient in times of stress or pressure. It lets us know that our nervous system is balanced and our body is capable of adapting to its environment. As Stein notes, “heart rate variability is dependent upon not only our health, but the relationships we have with our world too.”
If we have low HRV, one branch of the autonomic nervous system is sending stronger signals to our heart than the other. There are times when we welcome this reality — say, if we’re running a marathon and want our body to focus on providing energy to our legs (sympathetic nervous system) as opposed to processing food (parasympathetic nervous system).
If we’re not doing something active, however, low HRV may instead indicate that our body is working harder for a different reason. Perhaps we’re dehydrated, stressed, fatigued, or sick.
To look at it in a different way, the less one branch of the autonomic nervous system is dominating the other, the more room becomes available for our sympathetic nervous system to take control. This is why high HRV indicates that we’re fit and ready to go.
What Is a Normal Heart Rate Variability?
Heart rate variability is different for everyone, and therefore, highly individualized. Those who are 20-25 years old will usually have an HRV somewhere between 55-105 milliseconds, while people who are 60-65 years old will have an HRV between 25-45 milliseconds.
As Stein notes, HRV will change over the course of a single day and over the course of our lifetime. “Sleep is different from awake. Being relaxed is different from being stressed out. You can have someone come in multiple days in a row to measure their HRV, and it’s not going to be exactly the same,” she says. All we can say with certainty is that HRV decreases as we age, but again, it will do so differently in everyone.
Focusing On the Trends
More important than heart rate variability itself is the trends we can see when measuring HRV over a prolonged period of time. When using an HRV monitor, it becomes immediately apparent that HRV will vary greatly, from morning to night and day to day. These fluctuations can be attributed to many factors, but they are perfectly normal.
A more practical approach to HRV is to measure it over a period of time such as a month or so. “We can measure heart rate variability every five minutes and gain a lot of information,” says Stein. “But we can learn a lot more if we measure it and monitor it over a longer period of time.”
For example, if we’re incorporating exercise into our daily routine, HRV should steadily increase. A downward trend, on the other hand, may be indicative of overtraining, poor sleep, illness, bad eating habits, or failure to hydrate.
What Factors Influence Heart Rate Variability?
Heart rate variability can be influenced by training, lifestyle, and biological factors.
Training factors that influence heart rate variability include the intensity of a workout, exposure to unfamiliar stimuli, training load, and proper balance between rest days and training days. As we have seen, training is a great way to improve heart rate variability, but it must be executed in a healthy manner.
Lifestyle factors that influence heart rate variability include diet and nutrition, stress, sleep habits, and alcohol consumption. “Anything that makes us more able to use oxygen in a good way,” says Stein, “would improve heart rate variability. Even things that make us feel safer can influence this.” Leading a healthy lifestyle that focuses on proper diet and physical fitness, while paying mind to mental health, is a valuable means of improving HRV.
Finally, biological factors such as age, gender, genetics, and health conditions can influence HRV as well. As we age, our HRV tends to decline, and men often have higher HRV than women. Genetics and health conditions such as cardiovascular disease are additional factors that will influence our heart’s ability to operate normally.
Should We Focus on Heart Rate Variability?
Measuring and tracking heart rate variability is one of many forms of analysis we can use to monitor healthy individuals or to identify those who should seek improvement.
A healthy heart is made up of more than our resting heart rate and the time between beats. It is the sum of other vitals that include blood pressure, heart rate, oxygen saturation, and more. All are worth examining and taking into consideration. As wearable fitness technology improves, HRV tracking will only grow more accessible for people looking to understand their health.