HRV: what is it, why should you care?
High intensity training only has a positive effect if your body is in a condition to benefit from it. It is therefore of considerable value to find ways of measuring your readiness to take on a high intensity session.
This is why you should care about Heart Rate Variability (HRV): it can help you significantly increase the effectiveness of your training. It does this by guiding you to do your hard training sessions on days when your body is primed to accept hard training, and to recover on the days when your body needs recovery and will not benefit from hard training.
This is a big step forward on either following a fixed plan, which by its nature cannot take into account your body’s constantly varying ability to benefit from an intense session, or deciding purely on feel, which is too subjective to be effective.
“If you’re not listening to your body (and coach!) & adjusting your training accordingly, it’s certainly possible to put in 700 hours of training & not improve one bit!” – Alan Couzens (Exercise Physiologist, triathlon coach)
The supporting evidence for the use of HRV includes not only studies that show HRV-guided athletes improving more than those following a standard plan, but also that the phenomenon of “non-responders” (which is often seen in research studies) can disappear. Non-responders are a sub-set of the athletes under study, who show no statistically-significant change as a result of the training intervention. In one recent study, a group of Finnish researchers led by Olli-Pekka Nuuttila have shown that this phenomenon can even disappear completely when the training load is adjusted based on three measures: HRV, perceived fatigue & muscle soreness, and an index based on heart rate/running speed (HR/RS).
The equivalent index for cyclists to HR/RS would be heart rate at a set power output, or the Submaximal Fatigue Test (SFT). More on this in the appendix below.
Measures of readiness to take on intense training
Three measures taken together provide insights into the individual’s day-to-day state of recovery and thus ability to benefit from high intensity training:
- HRV indicates the state of the autonomic nervous system;
- Perceived fatigue and muscle soreness provide an indication of progress in muscle tissue repair;
- A Submaximal Fatigue Test (SFT) provides an indication of accumulated fatigue and the ability to take on load linked to glycogen depletion in the muscles.
In the remainder of this blog post, we will look mostly at HRV before concluding with our recommendations for how to use these three measures for the most effective training.
Heart Rate Variability
The use of Heart Rate Variability (HRV) has grown significantly over the past decade, and it is being used by more and more amateurs as well as professional athletes across a wide range of sports. As always, we at Alpine Cols learn as much as possible and test ideas before writing about them on the blog, so the following is the result of almost three years of experience. Based on this, I believe that any serious athlete – by which I mean anyone who trains hard for challenging objectives and cares about their performance – should consider using HRV to inform their training.
Why? Well, we have any number of precise measures to monitor our training load (e.g. hours, kilometres, power, TSS, CTL; …) but far fewer to monitor how we are responding to that load. In fact, most of the measures used to monitor response to training load are largely subjective, such as perceptions of soreness, fatigue, stress, sleep quality, overall feeling, etc. So how do I know whether I should train today at high intensity, at low intensity or not at all? How fatigued is too fatigued? What if I’m feeling a bit ‘meh’? Or if I’m stressed by something?
Until the advent of HRV, there was no objective way of knowing whether it would be better to stick with the plan, to change the workout or to cut the workout altogether. Most plans were (and still are) built on the theory of periodisation, often with three load weeks followed by one recovery week. The basic idea of load followed by recovery is of course valid, but the rigid framework often leads to disappointing outcomes and may explain why some people get labelled “non-responders”.
Without appropriate measures, it’s not possible to predict accurately either the ability to take on load productively or the need for recovery, and indeed these may vary for the same person from one week to the next. On occasion, I may be able to load for three weeks straight, on others I need to stop loading after just ten days. Equally, sometimes I might need eight days to recover, while other times three days are sufficient.
Enter the notion of HRV. Where Heart Rate (HR) is simply a measure of the number of beats per minute, Heart Rate Variability is a measure of the regularity of your heart rate, or, the extent to which the time between each beat varies. In practice the time between beats varies slightly, and the variation provides useful data.
To understand what’s going on we need to know that both HR and HRV are modulated by the autonomic nervous system in response to stress. The autonomic nervous system has two branches, the sympathetic branch and the parasympathetic branch. The sympathetic branch is activated by stress and assists in the famous ‘fight or flight’ response. If you need to be able to fight or run as hard as possible, your heart must beat powerfully. This increases your HR while reducing the variability between beats: your HRV is therefore low. On the other hand, the parasympathetic branch is responsible for the body’s resting functions, and is dominant under conditions of rest and low stress. In this case your HRV is high.
Measuring HRV thus gives you an insight into the state of your autonomic nervous system and therefore the level of physiological stress you are currently under. This is the combination of acute stressors, such as hard training, a long flight, illness, or an evening of heavy drinking; and chronic stressors, such as chronic disease, long term challenges, financial worries or relationship difficulties at work or at home. HRV can’t tell the difference between these various stressors; it effectively sums them up. This doesn’t matter because your ability to benefit from hard training is affected by the total stress, not the training stress alone.
In the words of Andrew Flatt (Associate Professor in Sports Medicine, specialist in HRV research):
“When tasked with recovering from training & coping with excess stress, recovery loses every time. Energy is diverted from muscle growth/repair to sustaining the alarm response by shifting from anabolic to catabolic processes, & from parasympathetic to sympathetic activation.”
If you are already under excess stress, adding training stress will only make things worse!
How to measure HRV?
More and more wearables provide a measure of HRV taken automatically during the night. At first sight seductive, this may not be the best approach since during sleep you are still recovering from the stress of the previous day. Much better to take the measurement under your own control in the exact same conditions every morning, immediately after waking up. At this point you can best evaluate the extent of your overnight recovery.
Use an app on your mobile phone. For a number of reasons we recommend HRV4Training as the best app currently available. (Beyond using the app ourselves, we have no links with HRV4Training). You can either use the camera and flash together as an optical sensor, or, pair the phone with a top quality chest strap such as the Polar H10. The two are equally valid. Most people find using the camera slightly quicker and easier.
The measurement takes one minute. Both RHR (Resting Heart Rate) and HRV are collected at the same time. You are then asked to reply to several questions (including sleep quality, muscle soreness, travel and alcohol consumption) which can help in interpreting the measure and making correlations. You can link the app to Strava so that it will pull in your training data from the previous day, and to Training Peaks so it will upload the results.
I measured my HRV, now what?
The measurement is useless in isolation and there is no value in comparing your data to anyone else’s. It is only useful when compared to your own data, established over the longest time period possible (at the very least one week). HRV is not something you measure occasionally: either you get in the habit of doing it every day, or it is pointless. The value is only meaningful in context and in comparison to your own normal values.
When you begin measuring your HRV on a systematic basis you will soon see that it varies quite a bit from day to day. Some of this variation is completely normal. The question is, therefore, when should we be concerned, or what level of change is “not normal”?
One of the principle reasons we recommend the HRV4Training app is that it calculates and displays your normal values as a range. These are based on the previous 60 days of data, which has been shown to provide the best balance between acute and chronic responses. It is these normal values which make the data actionable, by identifying whether your responses are showing meaningful changes or just small variations that you should ignore.
The way to interpret the data changes depending on whether you are interested in short-term acute changes or in long-term trends.
First, look at your daily reading. So long as this is within the normal range, all is good. Stick to the training plan, and do exactly what it says (whether high intensity, medium intensity, low intensity, short or long, or even a rest day: do whatever the plan says).
On the other hand, if the daily measurement falls outside your normal range, and your plan calls for either a high intensity ride or for a very long ride, you should seriously consider changing the plan. You don’t necessarily have to take a rest day but it’s advisable to stick to a very easy pace and limit the duration.
Once a week or so, zoom out to look at the long-term trends. Changes in the normal range over time indicate how your body is adapting to chronic physiological stress. If the trend is to a higher value for HRV, this is a sign that you are coping well with the current level of stress. On the other hand, if the trend is negative, this is a warning sign that the stress is having a detrimental effect. As noted above, HRV alone can’t discern between the different types of stress (training, illness, travel, etc.) so it is important to interpret the data in your personal context. Look for correlations between your HRV and the known sources of stress you are under.
1. High training load resulting in short-term acute stress.
In October 2022 I rode the Cent Cols Challenge. This consisted of riding 1,850km and climbing 38,000m in just ten days (with a rest day in the middle). The average day was thus 185km and 3,800m. Needless to say this is rather more than I normally ride in one day, let alone ten days in a row. Here are my RHR and HRV responses (the ten-day period is indicated by the double arrow):
In this case, the response was equally marked in RHR and HRV, which is to be expected when the stress is entirely related to a big increase in training load. Note that the short-term effect is to increase RHR and decrease HRV, whereas the long-term effect should be the opposite. Both RHR and HRV were clearly outside the normal zone from the first day to the sixth, at which point they started to stabilise, signifying my body was becoming better accustomed to the load. Both recovered to be within the normal range the day after completing the Challenge, suggesting that my autonomous nervous system was well on the way back to its normal condition.
2. Heavy cold resulting in short-term acute stress.
Just before Christmas I caught a heavy cold, with a visible response on both RHR and HRV:
Note that the day-to-day response is more sensitive in HRV than in RHR, with the exception of 24/12. I didn’t do any training at all during this period. On 28/12 I did a 2h turbo session with a few high-intensity intervals at the end (6x 40/20, 6×30/30 and 6×20/40). The effect was immediately visible in a below-normal HRV the following morning, which recovered the day after. This indicates that I wasn’t yet in good condition to benefit from the HIT. Under normal conditions, HRV should not be affected by a single day of HIT. If it is, it implies that either the session was too hard for your current level, or HRV is being affected by some other stressor.
3. Long-term adaptation to training load.
The data here is for the range March 1st to May 15th, a period when I was training very consistently, including cycling, strength training and swimming, averaging 16 hours per week:
RHR has a slight decreasing trend, while HRV has a clear increase throughout the period. This indicates that, viewed over the long term, my body was tolerating the training load very well.
4. Effects of an accident.
The accident occurred on July 2nd. I broke my collarbone quite badly as well as 4 ribs. Since I take my morning readings using a chest strap, I was in no state to do so for the first few days. The effects are still extremely obvious 2 weeks later, with my RHR some 20 beats above the trend pre-accident, and my HRV showing an equally impressive drop. I took 5 measures and then left another two week gap without taking any. When I started again, on Aug 4, both RHR and HRV were close to the (new) normal range, but some way away from the pre-accident normal range:
Looking now at the data in the period from two to three months post-accident, it shows a slow positive trend in both RHR (reducing) and HRV (increasing). RHR is only a couple of beats off the pre-accident normal. HRV is still depressed. This confirms that I am still in the recovery process and my sympathetic nervous system is still quite active. I am therefore avoiding high-intensity training for the time being and concentrating on rebuilding my aerobic capacity.
If you are serious about improving the effectiveness of your training, you should consider using HRV.
Getting started is easy: install an app on your phone (we recommend HRV4Training, but there are alternatives) and begin measuring every morning. A one minute measurement is sufficient. You can’t do anything with the data until you have established a normal range, which requires 7 days of data.
From this point on, use your morning reading as an indicator of the daily state of your autonomic nervous system:
- If it is outside the normal range, whether too high or too low, avoid high-intensity training.
- If it is within the range, stick to the plan.
HRV alone is not sufficient, however. You should also take into account your perceptions of fatigue and muscle soreness: if both are high, even if HRV is normal, skip the high-intensity session.
Once a week, do a Submaximal Fatigue Test (SFT) as an added input to confirm whether or not your body is accepting the current training load.
As a final word, this brief article can only serve as an introduction to the subject. Most of what I have learned comes from experts such as Marco Altini, Andrew Flatt and Alan Couzens. I encourage you to read their articles and follow them on Twitter to learn more.
Appendix: Submaximal Fatigue Test (SFT)
This test was developed by Jeroen Swart, Performance Coordinator at UAE Team Emirates. It is very simple and can be done weekly:
- Warm up for 10 minutes
- Ride for 3 mins at your current FTP or Critical Power.
- Evaluate the effort on a Rate of Perceived Exertion (RPE) scale and estimate how long you could have continued at that same effort level (Time To Exhaustion, TTE)
The effort should feel like 17/20 on the standard RPE scale, which goes from 6 to 20.
If all is well, RPE should be down and HR either similar to the previous week, or if well rested it may be a little higher, showing a positive response to the stimulus. If, on the other hand, your RPE is up and your HR is down (for the same power), it’s a clear indication that you are fatigued. (The HR response to the SFT is thus the opposite to what you would expect to see when measuring your RHR first thing in the morning. RHR should be similar or slightly lower if well rested and recovered, but higher if fatigued).
Over the long term (two to three months) you would expect to see a slow decline in HR at current FTP, due to increased fitness, as well as an increased estimate for Time To Exhaustion.
DeBlauw, J., Crawford, D., Stein, J., Lewis, A., & Heinrich, K. (2021). Association of Heart Rate Variability and Simulated Cycling Time Trial Performance. Journal of Science and Cycling, 10(3), 25-33.
Nuuttila, Olli-Pekka & Nummela, Ari & Elisa, Korhonen & Häkkinen, Keijo & Heikki, Kyröläinen. (2022). Individualized Endurance Training Based on Recovery and Training Status in Recreational Runners. Medicine and Science in Sports and Exercise. 10.1249/MSS.0000000000002968.
Marco Altini, PhD. Founder of HRV4Training
Andrew Flatt, Associate Professor in Sports Medicine, specialist in HRV research
Alan Couzens, Exercise physiologist and triathlon coach
Outstanding article Marvin. You made what can be a complex topic simple and usable. Best of luck in your comeback from injury.
Thank you Gene, much appreciated!
Best wishes, Marvin