A heart rate-related metric that can be useful for runners and athletes of any other sport is heart rate variability or HRV.
If you wear a fitness tracker, Garmin running watch, or sleep tracker like Oura ring, you have most likely seen data for your heart rate variability.
But what exactly is HRV, and what does it tell you about your heart? What is a good HRV? And what factors affect heart rate variability in general?
In this article, we will answer these questions and more regarding heart rate variability, discussing the importance of this helpful metric.
We will cover:
- What Is HRV?
- What Causes Heart Rate Variability?
- How Is Heart Rate Variability Useful For Athletes?
- Benefits of Heart Rate Variability Tracking
- How to Measure Heart Rate Variability
Let’s jump in.
What Is HRV?
Heart rate variability is a measure of the differences or fluctuations in the length of time between each successive heartbeat.
A low heart rate variability number means that your heart is beating at a very steady and consistent interval, whereas high heart rate variability means that there is more variability or fluctuations in the timing of the intervals between each heartbeat.
When we discuss heart rate, we are only considering the number of times your heart is beating per minute on average. So, for example, if your resting heart rate is 60 bpm, that means that your heart will beat 60 times over the course of one minute.
However, unlike a metronome in which every single beat will occur exactly 1 second apart when set at 60 bpm, there is some variability in the length of time between each heartbeat.For example, with a heart rate of 60 bpm, instead of each heartbeat occurring exactly 1 second apart, you might have an interval of 0.9 seconds between two heartbeats, and then there is a 1.2-second gap between the next heartbeat, then a 0.8-second lapse before the next heartbeat, and so on.
The differences in the number of milliseconds between each heartbeat is your heart rate variability. Unlike many cardiovascular-related metrics, such as resting heart rate and blood pressure, wherein a lower number is ideal, with heart rate variability, you actually want to have a higher number.
The higher your heart rate variability, the more variety there is in the spacing between each heartbeat or, said another way, the less consistently steady your heart rate is.
What Causes Heart Rate Variability?
It certainly sounds counterintuitive that this would be optimal because most people would assume that having a very steady, consistent heart rate would indicate a healthy heart that is not prone to experiencing some sort of arrhythmia.
However, research demonstrates that a high heart rate variability indicates that your heart is “ready” to handle stressors, such as vigorous exercise or just everyday stress.
To understand why a high HRV is a good thing, it is helpful to understand why we have heart rate variability in the first place. In other words, why doesn’t the heart always beat at a perfectly steady rhythm? What causes heart rate variability?
Your heart rate is controlled by the autonomic nervous system. This is the branch of the nervous system that automatically regulates important physiological processes under your unconscious control, such as your breathing, heart rate, blood pressure, and digestion.
There are two major subdivisions of the autonomic nervous system, which work in opposition to help regulate these various functions, including your heart rate.
One of the branches of the autonomic nervous system is the sympathetic nervous system. This is the “fight or flight“ nervous system, which responds to the presence of physical or emotional stressors by having a stimulatory effect on your heart rate.
The sympathetic nervous system increases your heart rate in order to prepare your body to “fight“ or “flee“perceived stress.
The other subdivision of the autonomic nervous system, the parasympathetic nervous system, has a relaxing effect on the body, slowing the heart rate down.
This branch of the nervous system is sometimes called the “rest and digest nervous system because it governs the processes that facilitate rest, relaxation, and digestion in the absence of a perceived threat or stress.
Your heart rate at any given moment, whether during a hard workout, while you are sleeping, as you sit at your desk doing work, or when you are nervously trying to calm your jitters before a first date, is determined by the dynamic interplay between the sympathetic and parasympathetic nervous systems.
The autonomic nervous system, as a unit, responds to the internal and external stimuli that are either encountering some type of physical or emotional stress or allowing your body and mind to relax and calm down.
Your heart rate variability is ultimately a result of the instantaneous differences in the autonomic control of your heart rate.
The parasympathetic nervous system is associated with a higher heart rate variability because fluctuations in the spacing between heartbeats increase more dramatically when the body and brain are in a relaxed state.
However, when your body is being predominantly controlled by the sympathetic nervous system, your heart rate variability is low. Control by this branch of the autonomic nervous system leads to a more steady and consistent heart rhythm.
How Is Heart Rate Variability Useful For Athletes?
Measuring your HRV can provide insight into which branch of the autonomic nervous system is primarily controlling your body, which, in turn, can inform you as to the degree of stress that you are under.
Because the sympathetic nervous system is associated with real or perceived stress, when your HRV is low, your body and brain are operating in a stressed state. On the other hand, if your HRV is high, your physical and mental stress levels are low, and your body is, therefore, able to handle a higher amount of stress.
In this way, monitoring your HRV can help provide guidance as to your body’s readiness to train or take on a hard workout.
For example, if you wear a sleep tracker that measures your heart rate variability overnight and your HRV is normally around 48 ms, and you notice that it drops to only 24 ms the following night, it’s a good idea to push off your hard workout for another day or so and swap in a less physically taxing workout.
Your body is already stressed, and adding the rigorous workout on top of whatever stress you are already contending with can be counterproductive.
High levels of acute stress can potentially compromise your performance in the workout itself and can increase the risk of injury or illness. It’s much like adding more water to an already overflowing bucket. You are just adding more stress onto a body that is at its capacity to handle the current stress.
Even more concerning than acute stress is chronic stress, which can have implications for your overall health and serious consequences with your training.
Essentially, you can use your HRV data as a measurable, observable biomarker for something that’s otherwise not easy to measure or quantify—physiological stress.
When you have good heart rate variability, your nervous system is healthy and well-balanced. Your parasympathetic nervous system is helping to bring down your heart rate and dismantle any physiological or emotional stress as soon as your heart rate increases from such a perceived stress.
High HRV essentially indicates that your body is coping well with any stress that you are facing. As your HRV drops, your body is showing signs of increased stress, poor nervous system regulation, and a sympathetic-dominant nervous system.
Benefits of Tracking Your Heart Rate Variability
Given the factors that determine your HRV, there are several benefits to measuring and monitoring your heart rate variability for athletes, including the following:
- Providing a noninvasive way to measure physiological stress and nervous system health.
- Providing information about your degree of recovery from workouts.
- Helping give insight into your body’s “readiness to train.”
- Serving as a sign of acute fatigue and a potential forewarning of illness.
- Helping prevent overtraining syndrome.
How to Measure Heart Rate Variability
It is not possible to manually measure your HRV as you can by measuring your heart rate by taking your pulse. Unfortunately, you need some type of technology that can more precisely measure your heart rhythm and the milliseconds of difference between successive heartbeats.
The most accurate way to measure HRV is through an electrocardiogram (ECG), but higher-end heart rate monitors and wearable fitness trackers can now measure heart rate variability.
Depending on the device, this biometric is typically assessed by either measuring the time between R intervals on an ECG (RR intervals) or the time between beats using optical wrist-based heart rate monitoring.
HRV is not a particularly useful biometric to concern yourself with during exercise.
For one, your heart rate is beating so fast during cardio exercise that the number of milliseconds of difference between any two beats is automatically going to be much shorter than when your heart rate is slow, such as it is at your resting heart rate.
More importantly, if you recall that the whole purpose of measuring heart rate variability primarily is to assess the balance of your nervous system, it doesn’t really make sense to measure HRV during exercise since the sympathetic nervous system, by nature, should be dominating during exercise.
Not only is the sympathetic nervous system associated with a low HRV in the first place, but you also don’t expect there to be a good balance of the two divisions of the autonomic nervous system when you are physically stressed in your body.
That is when the sympathetic nervous system is designed to be commanding your heartbeat control.
On the other hand, your HRV while you rest, particularly overnight as you sleep, is valuable.
Here, not only is your heart rate naturally going to be much slower, allowing for the detection of minute differences between the interval of time in between each heartbeat, but also you want your parasympathetic nervous system to be dominating your heart rate control during rest.
If you find that your HRV is still low while you are sleeping, red flags should go up since this would be abnormal and indicative of high stress.
Across the population, heart rate variability tends to decline with age and be slightly lower in females.
WHOOP, a company that designs and manufactures wearable fitness trackers, aggregated HRV data from their users and put together what constitutes a good heart rate variability here.