A little over 20 years ago, I was first introduced to heart rate variability. It absolutely blew my mind.
At that time, there was no fitness tech.
No Fitbit. No Apple Watch. No mobile apps.
I had never seen anything that even pretended to give you a window into how much stress your body was under and how well it was recovering–much less a technology that was actually doing it.
Not only that, but I learned that HRV was based on 40+ years of scientific research and had been tested on the first human in space back in 1961.
The more I dug into HRV and started using it, on myself and with everyone I trained, the more I was convinced this type of technology was the future of both fitness and health itself.
For the first time, it gave me the ability to eliminate so much of the guesswork of training. I had real data to help me make better training decisions.
As a coach, this was truly game-changing.
Fast forward to today and HRV is almost everywhere in fitness.
There are more devices purporting to measure HRV than ever before, but the problem is that like a lot of fitness tech, it often just ends up being another number on a screen instead of the powerful tool that it can be.
That’s because there’s still a lot of confusion about what HRV even is, let alone how it should be used to get better results from your training.
A lot of people don’t even have a clear understanding of the difference between heart rate and heart rate variability.
The truth is that if you don’t know what a number like HRV, or heart rate, really means, then it’s pretty much guaranteed that it’s not going to help you improve.
So today, I want to help end the confusion and misinformation and dive into the basics of both heart rate and HRV.
I’m going to untangle what both of these metrics are, what they can tell you about your fitness, and how you can use them to guide your training.
Let’s start with the easy one…
What is heart rate?
Your heart rate is simply the number of times your heart beats per minute (bpm).
Since heart rate is a standard measure, you can easily compare it from one device to another.
For example, you can see if your heart rate on your Apple watch matches your heart rate in your Morpheus training app.
But what about when these numbers don’t match up?
Unfortunately, not all heart rate monitors are highly accurate. These differences in accuracy most often come down to the type of sensor the monitor uses.
There are two primary sensors for measuring heart rate: optical and electrical.
Optical sensors measure the amount of light passing through your skin to track your pulse. This method requires consistent contact with your skin and can be subject to movement disruptions–especially at higher intensities.
Electrical sensors, such as chest straps, measure the electrical signals of your heart as it beats, making it the more accurate of the two methods.
What does heart rate tell you
You can use heart rate measures two different ways when it comes to your fitness:
- At rest (first thing in the morning) as a general marker of aerobic fitness
- During workouts
Generally speaking, the lower your resting heart rate is, the greater your aerobic fitness is.
If your average resting heart rate decreases over time, that’s a good sign that your aerobic fitness is improving.
When it comes to training, the best use of heart rate is to ensure you’re training at the right intensity.
For example, your heart rate monitor can help you avoid pushing too hard during a recovery workout.
You can gauge the intensity of a workout by how high your heart rate is relative to your maximum.
The key here is to use it as a tool to dial your intensity in, not just use it as a way to see how hard you can push yourself each and every workout or earn points.
I highly recommend using Morpheus for this kind of training, since it gives you personalized heart rate zones for different intensities based on your daily recovery.
The other way to use heart rate during training is to monitor what’s called heart rate recovery.
Heart rate recovery is how quickly your heart rate recovers after a period of exertion. Example: how many bpm your heart rate drops after a 20-second max effort interval.
This gives you insight into whether you’re producing energy aerobically versus anaerobically as you train:
- If your heart rate takes longer to drop, you’re more anaerobic and will fatigue faster
- If your heart rate drops faster, you’re more aerobic and can sustain energy output longer
You can monitor heart rate recovery throughout workouts to see if your conditioning is improving over time.
What is HRV?
HRV measures the average variability between heart beats over time, rather than the number of beats per minute (like heart rate). While it’s tempting to think of the heart beating steadily like a metronome, that’s not the case when you’re at rest.
To make things more complicated, there are multiple ways to measure HRV. There are different calculations for generating HRV scores and values are often plotted on different scales–in either the time domain or frequency domain.
These differences in the way HRV values are generated make it meaningless to compare numbers from one system to another.
What does HRV actually tell you?
HRV shows you the balance between your sympathetic (stress response) system and parasympathetic (recovery response) system.
The higher your HRV number, the more energy your body is devoting toward recovery.
So, on a daily basis, HRV is primarily an estimation of your recovery because it’s looking at the total cost of the stress you’re putting your body under.
Bigger changes in HRV, both up and down, reflect that your body has been under a high level of stress lately and thus your recovery is likely to be lower.
Smaller changes in HRV from day to day, however, mean your body is relatively stable and hasn’t been under much stress lately. Therefore, your recovery is likely higher.
It’s important to understand this concept.
HRV is not directly measuring recovery.
Instead, it’s measuring the signals from your body that happen as it goes through the processes of stress and recovery/repair.
From these signals, you can get a window into where your body is at at any given time.
Average HRV over time, on the other hand, is a powerful marker of aerobic fitness, similar to average resting heart rate.
But whereas a lower average resting heart rate correlates to greater aerobic fitness, higher average HRV is associated with greater aerobic fitness.
So if your average HRV increases over time, it’s a good sign that your conditioning is improving.
This makes HRV useful for evaluating whether your aerobic fitness is increasing.
But aside from athletic performance, aerobic fitness is strongly correlated with greater lifespan and healthspan (Teramoto and Bungum, 2010; Mandsager et al, 2018).
That’s why it’s a worthy goal to increase your average HRV regardless of why you train.
How should you measure your HRV?
This is a hugely important topic and what you need to know is that since HRV measurement is so sensitive to stress, it’s absolutely crucial that you measure it at rest.
And there are two common ways of doing that:
- Active measurement: you initiate a 2.5-5 minutes test and remain still while your device calculates your HRV. This is often with an HRV-specific device, like Morpheus.
- Passive measurement: your device monitors HRV in the background without your knowing participation. This is often with a watch-like device, such as an Apple Watch.
Since passive measurements don’t require any effort or attention, they can be more convenient. However, they’re not gathered in standardized conditions (such as lying down, at rest).
It’s like trying to compare your body weight yesterday morning to tonight after dinner. You have no way of knowing if the differences are meaningful.
What about devices that measure overnight when you’re sleeping?
The problem is that these devices all take only short snapshots of HRV periodically while you’re asleep. They don’t measure it continuously throughout the entire night because it would very quickly drain the battery.
Active measurements, on the other hand, are the equivalent of stepping on a scale at the same time each morning.
This is why passive measurements are much less accurate gauges of your HRV, both on a daily basis and over time, than active measurements.
The unfortunate and often untold truth is that a lot of HRV apps and devices out there are using passive measurements simply because they assume that most people won’t put in the effort to measure HRV actively.
That’s why they never tell you that virtually all of the now 60+ years of research on HRV is done using active measurements and there is next to none using passive.
They’d rather sell you a device and/or app that’s easier and doesn’t require you to do anything, even if that means it’s much less accurate.
The problem with this, of course, is that accuracy is everything if you want to use HRV as a tool rather than just another number on a tracking screen.
Should you measure HRV during exercise?
This is a big difference between heart rate and HRV.
While heart rate is a useful training tool for measuring aerobic fitness, relative intensity, and improvement over time, HRV during training is, well… meaningless.
That’s because once your heart rate climbs over 100 bpm, the variability in your heart rate drops to zero.
The sympathetic (stress) system is driving all the work, and there’s very little parasympathetic (recovery) activity to measure.
In other words, there is no real value in looking at HRV during a workout, or while doing anything other than resting really.
What do changes in HRV mean?
As I talked about earliest, HRV is primarily a tool to help gauge the balance between stress and recovery within your body each day. But unlike what a lot of people have been told, just because your HRV increases, that doesn’t necessarily mean you’re more recovered.
When you initially increase your stress load through training, your sympathetic system cranks up and drives your HRV down.
As you recover, your parasympathetic system ramps up and overshoots pre-training levels before returning to baseline.
Where you are on the parasympathetic cycle of recovery depends on when you measure your HRV and how quickly you recover.
So, if you wake up to a huge jump in HRV, you’re probably not fully recovered. You may be in the middle of the recovery process, and your HRV hasn’t returned to baseline yet.
As you interpret changes in your daily HRV measurements, you have to consider the context of your stress-recovery cycle.
Apps like Morpheus do the hard work for you by turning your HRV into a recovery score.
Does HRV predict how hard you can work out?
This is a common point of confusion: daily HRV does not gauge what your body is capable of. It does not predict performance.
You can have a low recovery score and still set a PR at the gym.
So what is it telling you?
Your HRV is predicting the cost of expending energy and creating additional stress.
Here’s what I mean by that:
If you do a high-stress load workout when your recovery is low, it will take you much longer to recover to baseline than if your recovery is higher.
Your energy reserves for exertion, regeneration, and repair are depleted when your recovery is low. Spending more energy will deplete them further.
Initially, this means it will take your body longer to recover, adapt, and improve.
If you keep extending yourself beyond what you can recover from, your body will eventually enter a recovery deficit. This is where symptoms like chronic fatigue, injury, and suppressed immunity rear their ugly heads.
Long story short: your HRV doesn’t tell you what you can do.
It does, however, help guide you towards what you should do.
It can help you make better training choices based on objective feedback from your body.
What people most often learn quickly is that while you can push yourself on low-recovery days (and sometimes it’s unavoidable)… you’ll pay a cost for doing it.
Seeing the impacts of all your decisions around training, nutrition, sleep, etc., in real-time, is the single most important way to get better at dialing in your volume, intensity, and recovery in a way that keeps you moving forward towards your goals.
Which should I use, HRV or heart rate?
Hopefully by this point in the article, you know that HRV and heart rate are both valuable because they have different uses:
- Daily HRV can help you estimate recovery
- Heart rate during training can help you measure intensity and the relative stress of the workout
- Heart rate recovery during training can help you track whether your fitness is improving
Both average HRV and average resting heart rate can help you gauge aerobic fitness.
Ultimately, you should use both heart rate and HRV as tools to make better decisions about your training and lifestyle.
They’re easy to monitor and reliable ways to get an inside-out view of what your body is trying to tell you.
If you aren’t measuring heart rate and HRV yet, make sure to check out Morpheus. It gives you everything you need to track recovery and training in a complete system. Our next shipment of HRV bands will be in stock soon.
If you’re a coach and want to dig into exactly how to use fitness tech like HRV, heart rate, sleep trackers, and more to help people reach their conditioning goals, make sure to click here to get on the insider’s list for my BioForce Conditioning Certification.