With the explosion of wearable devices now available to monitor athletic recovery, the use of heart rate variability (HRV) as a recovery marker has well and truly hit the mainstream. Previously limited to the domain of elite athletes with ready access to the tools needed to measure HRV, these days anyone with an interest in assessing the recovery state of the nervous system can open an app on their phone. However, implementing HRV successfully to guide your training does require some understanding of what it takes to obtain accurate, trustworthy numbers, as well as what practical significance those numbers have. In this article we’ll provide you with some guidelines on how you can implement a HRV monitoring practice to help guide your training plan. But before we do that, let’s start at the beginning.

What is HRV?

HRV refers to a number of formulae that seek to quantify the natural variability between your heartbeats. When we think about somebody with a resting heart rate of 60 bpm, we might assume that their heart is beating regularly, metronomically, at one second intervals. In reality, there is quite a lot of variability between the beats. There might be 1.2s between two beats, followed by 0.8s between the next two, etc. As it turns out, this variability between beats is a very good thing and is indicative of a sensitive, responsive autonomic nervous system (ANS).

The ANS is the part of the nervous system responsible for control of our unconscious body processes, like breathing, digestion, and heart rate. There are two branches to the autonomic nervous system: the sympathetic branch that controls our “fight and flight” processes in response to stress, e.g., the dry mouth, muscle tension, and quickening of heart rate we might feel before public speaking. The other branch of the autonomic nervous system—the parasympathetic branch—is responsible for the opposite processes, that is, switching the body over to rest and repair mode after we are done facing our stressor. A good way to look at these opposing forces is as your gas pedal and your brake pedal. It goes without saying that if you have a car with a lot of acceleration on the gas, you’re going to want an equally powerful braking system. Monitoring your HRV can give you some real time “onboard diagnostics” as to the current strength of your gas and your brakes.

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The link between HRV and training adaptation

In the simplest terms, a higher HRV typically means you’re more ready to train and is what we’re looking for, while a lower score signals there’s some fatigue in the system. This isn’t always the case, though—there can be an inverse relationship at the elite level—and we’ll get into that later on. Remember, too, that everyone’s HRV is as unique as they are, that is, there is no universally applicable “good” score. Instead, it is applicable to you and your baseline (which you can establish after logging sufficient data). Keeping in mind that athlete adaptation happens during the periods when our body is in recovery mode, it should come as no surprise that science has found a relationship between strong HRV numbers and a positive response to training load, as well as poor HRV numbers and a failing adaptation. For example, in a 2012 case study on two elite athletes (one who adapted successfully to the training and one who overtrained) by Dan Plews and colleagues, the overtrained athlete exhibited a massive 25% drop in HRV during his period of overtraining versus the successful triathlete whose HRV only shifted within a narrow range of only ~3% through the season. Leveraging this sensitivity of HRV as a marker of potential failing adaptation, many coaches are now monitoring their athletes’ HRVs on the lookout for potential changes so that they can intervene before reaching the state of overtraining indicated by large drops in HRV.

So how do we do this? How do we know when HRV is sufficiently low that we should be raising the red flag and adjusting training?

While the 25% drop in HRV above represents an extreme case, even smaller changes can indicate a significantly blunted training response. For example, Ida Heikura, in her 2015 Master’s thesis showed a significant difference in the improvement in 3,000m time trial performance when low intensity training was planned whenever the seven-day rolling average of HRV fell below 0.5 SD (standard deviation) from the athlete’s mean—on average, a drop of ~6%. This helps to illustrate that much smaller drops, while not indicative of overtraining, should be listened to if the objective is to get the best training response.

But, fear not! You don’t need to keep the calculator at the ready and run standard deviation calculations each morning to figure out what training you should do today. Most of the major apps will keep track of your normal range and will show you when your average falls outside of this range. For example, HRV4Training has a highlighted section that will show users a range of 1 standard deviation from their 60-day mean.

This view provides athletes with two key levels of “action”:

1. When your seven-day baseline (the white line in the picture) falls outside of this range, it is definitely a “red light” and a good signal that training should be adjusted (reduced), i.e., it is good timing for a recovery week.
2. When the daily value (the individual bars in the picture) falls outside of this range, it is more of a “yellow light” to assess how the athlete is feeling and, if the schedule permits, a good day to consider moving a key workout if one was planned for the day.

In the picture above, we see a nice stable pattern of HRV values, with the seven-day baseline staying within the limits of the 60-day long term average and only one solitary day falling outside it. However, this isn’t always the case. Sometimes we will see a lot more variability with many values falling outside the normal range. So what should we do we do when we see an athlete with these patterns?

How to improve a “bad” HRV profile

The advantage of HRV monitoring for the athlete is that it is a global measure of total stress. If it only reflected training stress, it would be relatively worthless, as it would always track with training load. However, in the real world of real life athletes with real jobs, real mortgages, and real life stressors, it’s quite common for HRV not to track with the training load and is more representative of the impact of these outside stressors. This provides the coach with a window into the athlete’s greater life that they may not always have access to. This additional context can be very helpful when planning how much/what type of training load to prescribe.

Depending on the type/intensity of training, it can offer either a disruptive or stabilizing force to the athlete’s HRV. When the athlete’s life is providing a lot of stress and a lot of instability in the HRV levels, it makes sense to not add to this with the training, but instead to provide a boosting, or at least, a stabilizing force.

Stephen Seiler and colleagues researched the interplay of training intensity and HRV and found that “exercise below the first ventilatory threshold causes minimal disturbance to ANS balance.” In other words, easy aerobic training doesn’t negatively affect HRV and, in my experience, will often boost it. Similarly, activities like yoga and walking in nature have been shown to have a positive effect on the parasympathetic nervous system. This all points to the fact that when HRV is already low or ‘wiggly,’ training should be adjusted to include those activities that have been shown to stabilize or boost it. For athletes with a lot of life stress or a long history of high intensity training stress, this may amount to months of low intensity exercise focused on bringing the parasympathetic nervous system back up to the same level as the sympathetic. In other words, be sure to bring the strength of your brakes in line with the strength of your engine.

What else can low HRV be indicative of?

Because of the sensitivity of the measure, HRV can often indicate a looming illness before it appears. In a 2021 study by the Mt. Sinai group that looked at HRV in COVID-positive and COVID-negative individuals, a significant difference in HRV patterns was seen up to seven days before the individual tested positive. Differences were also evident seven days post-illness, showing that the ANS was still not fully recovered from the stress of the illness.

I’ve seen similar patterns, with respect to illness, in the athletes I work with, i.e., that HRV will often be depressed prior to any evidence of illness and will continue to be depressed even after the athlete is symptom-free and feels “back to normal.” For this reason, when HRV is significantly depressed below normal range, I recommend a “better safe than sorry” approach, even if the athlete is not yet showing symptoms of illness or excessive fatigue.

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Getting started using HRV

If you’re thinking that HRV might be a worthwhile addition to your training plan then the next obvious question is: How do I get started? When I first started monitoring HRV more than a decade ago, it was a time-intensive endeavor that involved downloading heart rate files and manually calculating HRV metrics. Fortunately, things have evolved significantly since then. These days, getting started is just a matter of downloading one of the HRV apps to your phone and getting into the habit of doing your morning tests.

While there are a number of apps available, I have had personal experience with (and would recommend) either ithlete or HRV4Training. I have tested both independently against raw heart rate data and both lead to consistent, reliable metrics. I currently use ithlete for my own readings, largely because I am fundamentally lazy and while both apps will sync your HRV data to TrainingPeaks, the pro version of ithlete (subscription costs ~$5/month) will also sync your subjective wellness data (soreness, mood, etc.) and I like having all my numbers in one place.

Once you have downloaded your app of choice, I would strongly recommend pairing it with a trusty Bluetooth heart rate strap. While some apps offer an option to use the phone camera or a finger sensor to take a reading, I’ve found considerably better reliability when the data is taken from a quality heart rate strap. The Polar H10 is a great option.

When you have your app and heart rate strap, testing is a simple matter of:

• Wake up in the morning, visit the restroom if needed and put on your heart rate strap
• Lie back down in bed and open the app
• Wait for your heart rate to stabilize and begin the test
• A simple one-minute resting test is long enough for most measures

Once you have a baseline established, you will be able to see how today’s value compares to your long-term average and will be in a better position to make decisions as to whether today should be a hard/loading day or an easy/recovery day.

What if you’re tired but your HRV is high?

If you’ve been monitoring for a while and you come across a day where you’re feeling kind of tired, but your HRV is super high, what should you do? Remember that as powerful as HRV is, it is still only one number and is much more useful/powerful when assessed within the context of other metrics. Here are a couple of real world examples:

1. After a big aerobic training load, e.g., a training camp, it is common to see unusually high HRV numbers coupled with low resting heart rate numbers and a sensation of general fatigue. This makes sense when we think about HRV as a metric of the strength of our recovery systems and, after big aerobic load, our recovery systems are activated to the fullest. Still, this obviously does not mean that we are ready for work. In cases like this, having the additional context of resting heart rate and subjective measures can be very helpful.
2. Another situation where we shouldn’t necessarily trust our HRV is when we’re approaching a race. Since HRV gives us a window into the relative balance of our “fight and flight” stress system and “rest and repair” recovery system, as we taper for a race, it’s very normal to be more in “fight or flight” than “rest and repair” mode. In cases like this, the stress that we are under could be interpreted as a positive—a summoning of the body’s resources getting ready for a big performance. Therefore, drops in HRV at these times shouldn’t be interpreted as a negative thing, especially for higher intensity events that benefit from being a little fired up.

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Where is HRV headed in the future?

All in all, once you have a sufficient baseline established, your HRV numbers are worthy of a significant weighting when determining training load, but for best effect, they become even more powerful when combined with other measures. This brings us to the final point: Where is HRV monitoring headed in the future?

We’ve discussed the importance of considering HRV in the context of other factors above. Wouldn’t it be great if a machine was able to take some of the cognitive load off us and weigh these factors together, appropriately, before arriving at a recommendation? After all, learning to listen to your body and knowing when to push—and when to hold back—can be one of the most crucial decisions an athlete makes.

This is where HRV monitoring is heading—into a super-powered version of HRV monitoring that uses machine learning techniques to appropriately weight all the various monitoring features to arrive at an appropriate recommendation.

We are already seeing this in the research where, for example, in this 2019 paper a combined model was used that included HRV, self-reported life stress and perceived exertion from the previous day’s training to arrive at a model that predicted the likelihood of injury with an incredible 97% accuracy! While these combined models are yet to reach the commercial market for your average age-group athlete, given the rate of advancement in the field, it’s only a matter of time.

After using HRV myself and with the athletes that I coach for more than a decade now, it is an absolutely vital element in my decision-making process, even more so when it comes to the typical age grouper for whom training stress is only a relatively small piece of the overall stress pie.

Whether you’re a long-term HRV user or somebody that is completely new to it, I hope that the above will offer some useful pointers on how HRV data can be applied to better guide your training, recovery, and race-day performance.

This story originally appeared on Triathlete.