Jane is an endurance athlete who has been experiencing a training “funk” in the past two weeks. She is reporting fatigue during her training, with heavy or “dead” muscles, along with an inability to hit her training targets. As her coach, I begin to ask a series of questions to determine how we might pull Jane out of this funk.
We review the usual suspects: fueling and hydration directly before, during and after workouts; daily eating intake; sleep patterns; signs of illness; and stress levels. Perhaps you’ve been asked these sorts of questions by your coach. Our goal is to figure out if there is something impeding an athlete’s recovery, and any of these factors could be at the root of it.
What if there was an additional training metric that could add to this discussion and give us insight into how well a body is recovering from training load?
Well – there is! Heart rate variability (HRV) provides data-driven insights into recovery and adaptation. It doesn’t always answer the why, but it can help us to determine if continuing to train is smart, or if a rest day in time will help us save nine.
I was skeptical about the value of HRV at first. However, over the past several years, I’ve had a series of experiences in using it with athletes, and with myself, that have convinced me to add this metric to the data review. HRV has helped to monitor athletes’ return to health from illness, to track patterns of recovery after races and big training days, and to get a general sense of how an athlete responds to various types of training.
In this article, I’ll cover the basics of HRV to help you determine if tracking HRV is something that can add benefit to your programming. We’ll discuss:
- What HRV is
- Some examples of how I’ve used it, and
- How to measure HRV
What is HRV?
Heart rate variability (HRV) is a measure of the variability between heart beats. For example, if your resting HR is 60 bpms, your heart doesn’t beat at regular intervals, once per second. Rather, it changes its cadence, perhaps once every 1 sec, .9 sec, 1.1 sec and so on.
In general, more variability is often a good sign, and indicates that the body is recovering and adapting well. A higher HRV is correlated with a well-functioning parasympathetic nervous system (PNS), which is the part of the autonomic nervous system that regulates “rest and digest”. The autonomic nervous system (ANS) controls all of our automatic responses that are not intentionally directed, such as breathing, heart rate, and digestion.
The other part of the ANS is the sympathetic nervous system (SNS), which regulates “fight or flight.” When this system is overactive, we may find ourselves on the doorstep of under-recovery/over-training, and in turn our HRV will drop. For example, in research, HRV tended to drop in the day or two after intense or long training sessions for cyclists and runners. But, if we allow for recovery, it bounces back to baseline.
Training weakens us. Recovery strengthens us. HRV can help us see patterns of recovery in ways that allow us to rest when needed, and to push onward when the body is ready.
What can it tell us?
Before HRV can tell us anything, we have to determine our baseline. HRV numbers are not comparable from person to person, or from measuring platform to platform. You will need to track HRV for a minimum of 7 days in order to set a baseline. However, in my experience using HRV both personally and for my coached athletes, the data is much richer once we have at least a month of information.
I also like to ensure I have a collection of data that reflects various types of training – intense, long, easy, etc. For example, my HRV will respond differently after long endurance days versus higher intensity days. And, my HRV has been different since I’ve moved to 9200 feet (compared to my previous home at 5400 feet). All the details matter here!
But, like any single piece of data, HRV doesn’t speak for itself. It makes most sense in terms of its trends, and in combination with other data, such as training metrics, resting heart rate, sleep volume, and post-activity comments.
For example, the research has found research to support the following as patterns:
- A rising HRV with a consistent or decreasing resting HR (RHR) indicates the body is recovering and adapting to training.
- A decreasing HRV with an increasing RHR tells us that the fatigue is accumulating, and deeper recovery will be required soon.
- A decreasing HRV with a decreasing RHR may be a sign of overtraining or under-recovery, and immediate, deeper recovery is required.
Note that any single day may not be indicative of anything substantial. It is the overall pattern of the information that matters. So, if your HRV drops one day – that may be no big deal. But, if you see several days of dropping HRV – it’s time to right the ship! None of the above research trends are hard and fast rules. We must connect the HRV data with other training data and information.
I’ll provide a few examples to demonstrate how I use HRV as a coach to give you a sense of its application in programming.
Sleep Impact
In the first chart below you can see the impact of one night of poor sleep. On March 6th into 7th, this athlete got 3 hours and 48 minutes of sleep – a steep departure from her usual 8 hours of sleep. This athlete tracks her sleep using the Whoop strap.
In the next chart, we see corresponding HRV and RHR (resting heart rate) values for that same day. As a result of one night of poor sleep, this athlete’s HRV dropped 32 points below baseline, while her RHR rose 13 beats above her average. Taken together, we can say that this athlete was not ready to have a strong performance.
Since this was only one night, the athlete rebounded quickly. But, imagine the impact of night after night of disrupted or shortened sleep. It is significant!
Recovering from Illness
In September 2020, one of my athletes contracted COVID. Two days prior to feeling any symptoms, he did his usual long ride. However, I noticed that his HRV (which he tracks daily using a Whoop strap) had dropped by nearly 30 points. Even though he felt no symptoms and his ride went fine, his HRV was already identifying the early signs of the immune system fighting. In the next 4 days, his HRV continued to drop, and the symptoms worsened. He sat at a plateau, with his HRV about 45 points below baseline for over a week. It was 12 days before it finally began to shift back up toward baseline, and almost 2 months before he returned to the actual baseline.
Four weeks after his initial diagnosis with COVID, we could still see the impact on his training data. In the chart below, you see the data from a standard steady-effort bike ride. The athlete is asked to maintain a specific HR and then we see what we get in the watts.
On September 2, perhaps a day or two before he got COVID, he held 175 watts for 138 bpms. EF (efficiency factor) is 1.33. On 10/5, 4 weeks after diagnosis, he held 153 watts for 140 bpms, while the EF dropped to 1.09 (this is a significant drop for this metric).
In this case, when we combine the HRV data with the training data, we knew that 4 weeks out, the athlete was not ready for a full return to routine training. But, we were okay to do shorter, aerobic work.
Once we did return to routine training, we used HRV to determine how much recovery was necessary. At first, this meant more easy days between the hard days. Whereas, pre-COVID, he may have needed only 1-2 easier days between hard or longer workouts, post-COVID, he was needing up to 4 easy days between any long or intense efforts. In November, this athlete returned to normal training, as his HRV values stabilized and his training metrics and subjective reports made clear he was feeling like his “old self” again.
For athletes suffering from long COVID or other lingering viruses like Mono, HRV can be very useful to ensure you aren’t overloading your body when it needs more recovery. But, again: this information should not be used without context from the entire collection of training data and your subjective reports of how you feel. Some days, we can train just fine with a low battery. But, when I observe a pattern of data, all of which is pointing toward recovery, it’s time to take it!
How can I measure it?
Regardless of the device or platform you use to track HRV, the research is clear that consistent, daily tracking is key to obtaining useful information. Additionally, taking HRV in the morning has been found to get the most reliable and accurate results.
You can measure HRV from a few different devices or platforms.
HRV4 Training
This app uses either your phone’s camera/flash (holding your finger to it), or a bluetooth compatible HR monitor (like a chest strap). No fancy equipment required! You will need to remember to take your HRV each morning, preferably before you get out of bed. The data collected by the HRV4 app can be synced directly to TrainingPeaks.
The app is a $9.99 one time fee. It tracks HRV, and offers a journal option to report other subjective measurements.
Wearables
There are a variety of wearable technologies that will track a full panel of recovery-related information: HRV, resting HR, and sleep information. Wearables that have this ability include:
- Some models of Garmin and Suunto watches
- There is no additional charge to get HRV data from your Garmin (or similar) watch. However, this data will not transfer into TrainingPeaks. You will need to share manually with your coach.
- Can use a wrist or HR strap. A Bluetooth, chest HR strap will get the most accurate information. The further you get from the heart, the less reliable the information.
- Evidence suggests that the sleep data from these devices is not accurate.
- Whoop
- Subscription-based service; there is no extra fee for the device itself. Monthly memberships start at $30/month. If you prepay a year, it drops to $24/month. You can get a free month if you sign up via this link: https://join.whoop.com/F1241F. (Full disclosure: if you use this link, I will also get a free month added to my subscription.)
- Whoop syncs directly with TrainingPeaks through its app.
- Wrist-based tracking. Not reliable for activities.
- My caveat:
- I find the strain metric to be utter nonsense, as the wrist-based sensor simply doesn’t work for motion. So, your usual training device plus a HR strap will account for your TSS and CTL.
- On the flip side, I have found the sensor to work functionally for sleep and rest, which supports a reliable HRV (as this is based on resting – not activities). According to DC Rainmaker, this sensor is improved in the 4.0.
- Oura
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- Device purchase is $300-$400, plus a $5.99/month fee for access to all of the app features. Oura does not sync directly with TrainingPeaks. However, there is a workaround if you use a third party app, such as Strava.
- DC Rainmaker has a review on this one as well. As far as the tech goes, he seems to like the Oura over the Whoop.
Should you track HRV?
Like all training questions, there isn’t a one size fits all answer to this. HRV works best for athletes who will take it consistently, and who are willing to use the information as an additional input for decision making about training. But, HRV – like any metric – is not useful without consistency in tracking, or when used without the context of other metrics.
If nothing else, HRV reminds us that recovery is a part of training. It is not a break from training. Further, HRV doesn’t replace your sense of feel and RPE. Nothing is more important or useful than strong body awareness, which is the athlete’s best tool for success.
Tune into you!