According to the definition, heart rate variability refers to the time difference between the individual heartbeats or a way to quantify those differences in an understandable number. Now we probably think that our heartbeats at a constant rate, right? 60 beats per minute. Well, that could be the average of the beats our heart makes in one minute, but in between, we can see small but measurable differences.
HRV – short for Heart Rate Variability – is a non-invasive and practical method for measuring the activity of the autonomic nervous system (ANS) or at least part of it. Our body is constantly in need of being in a balanced state in order to maintain the so-called homeostasis. Everything, from our heart rate, hormone levels and blood pressure, is influenced by numerous stressors that are part of our everyday life. This is where our autonomic nervous system, being a good guy, keeps everything in balance without us knowing it, just so we can function normally.
The rate at which our heart rate beats is controlled by the parasympathetic branch of this autonomic nervous system, which is generally responsible for the rest and recovery processes in our body. And when we record the activity of the parasympathetic nervous system – HRV – we can observe how our body copes with everyday stress factors such as work or training.
How do we measure IT?
In the beginning, the only way to measure differences in heartbeats was with the ECG, which was impractical as most of us did not have access to the ECG device. So you would need to go to a hospital to get the measurement done. But in recent years, technology has evolved and with more sensitive heart rate belts that measure the electrical activity of the heart, we can measure these differences too. Most new heart rate belts do this nowadays. The most commonly used are H7 and H10 from Polar. Besides the heart rate belts, we can also measure HRV using photoplethysmography (PPG) and optical sensors. In some cases, this technique can also be used with the help of mobile devices with a mobile phone camera. Currently, the only validated app that can effectively measure HRV is HRV4Training (More about validation).
Now we know how to measure it and a lot of people actually measure it. But for the results to be meaningful and useful for interpretation we need to make sure that certain criteria are met.
- Time of the measurement
- Measurement frequency
A first and most important one is the time of measurement. I see many people taking HRV measurements at different points of the day. This gives you a lot of variability and the data itself is basically useless. The only two well established and reliable methods of HRV sampling are morning and night measurements.
Morning measurement: Most of the apps like HRV4Training or EliteHRV allow you to take measurements in the morning. You take your phone and/or heart rate belt and take measurements for 1 to 2 minutes. This is good enough to get an accurate reading. When reading in the morning, it is necessary to understand that every morning we try to take measurements under the same conditions as the day before. So as soon as we wake up, we try to relax a little and take the measurement while still in bed, lying down or sitting (can also be done in combination with readings in sitting or standing position). Occasionally you may need to go to the toilet first thing in the morning. This is not a problem. Then go back to bed, take a minute or two to relax and read the reading there. But if you are able to do the reading right after waking up in the morning, so that you do not get distractions that could lead to an increased heart rate (work mail, text, social media).
Night measurement: Is usually done with wearables like Oura ring, Whoop strap that you wear while sleeping. Here the apps usually take a few short data samples over the whole night and average the result. Here we have to be a bit careful, as HRV can vary depending on the stage of sleep, so we might get slightly more distorted results. The same could happen if we exercise in the morning and in the evening on the same day. This could lead to a reduction in HRV in the first part of the night, and with a sample from this part, we would get a lower HRV score. Something to keep in mind not only for the night measurement but also for the morning measurement.
With the standard measurement time, we also need a fairly consistent measurement routine. The minimum weekly measurement frequency is about 3 times per week. But in this case, a lot of data can be lost. If you can try to measure at least 5 times a week, but to measure how your body reacts to stress every day, then a daily measurement is the right thing. You only need a minute or two in the morning, and it’s no problem if you miss it occasionally. The more data you have, the easier it will be to understand what is happening to your body.
Make it a habit (the same as with training).
What are we measuring and what does that mean?
As we said that HRV computes the time difference between each heartbeat. For that, we need a sufficient amount of data. For clinical practice, it is recommended a 5-minute measurement period but we have quite a few studies that show that short 60 second or 120-second samples are long enough to get accurate data.
Most commonly used marker for parasympathetic activity is rMSSD (root mean square of the successive differences). This is usually expressed in milliseconds but numerous apps use different scorings of this result just for easier understanding. HRV4Training uses that system but you can always access the rMSSD data, EliteHRV uses a default value of rMSSD.
But why is rMSSD most used parameter? It tells us about how our physiology works. Vagus nerve which is a representation of the parasympathetic nervous system affects our pulse.
With most of the apps, we also get other markers like SDNN (Standard deviation of the NN (R-R) intervals), pNN50 (The proportion of NN50 divided by the total number of NN (R-R) intervals.), LF (Low-frequency power) and HF (High-frequency power).
So what do the numbers actually mean? We are going to keep it really simple here.
So in very simple terms, higher HRV would mean we are more rested, unstressed and ready to perform (not always) and on the other hand, lower HRV would mean we are under higher stress (not always). I would not go too deep into it since Marco from HRV4Training did a great job with this in his 4 part guide on Heart Rate Variability on Medium. Anyone wanting to get more into understanding HRV I strongly recommend this a read.
Real life examples
We are going to start with an amateur cyclist who has been tracking HRV for the last 2 years and a lot has happened in those years so we got some quite interesting data out of it. We will focus on a period from August 2019 through May 2020. So let’s look at the data.
Just to explain the charts a bit. We got the data on daily readings (doted connected data) with 7day average (thicker line) and in the background, we have a “normal values area” which represents 60day moving average with 7day SWC.
So what we see in most parts of the chart is the data that is quite stable in this green normal range. His HRV averaged about 115 ms and the RHR was about 45 bpm. Well, that was all until the end of December when we got a significant decrease in HRV (from ~115ms to ~85ms) with a significant increase in RHR (from ~45bpm to ~49bpm). When the decline occurred, the athlete was in the process of completing a larger training block. There was a slight fatigue, but that was to be expected. With the recovery block (a few days of cycling around Christmas and a few easy rides) we did not get HRV back up, and the athlete continued to complain of persistent fatigue as he was tired all the time. It was strange. So we started training again. The bike was put aside for a few more days, but he did not get better. Well, that was the time for a visit to the doctor. After another 2 weeks of crappy feelings, lots of blood tests and examinations, the diagnosis came. Mononucleosis.
Not to say that the decrease in HRV and the increase in RHR was caused by mononucleosis, but the initial decrease in HRV could have been masked by the larger training block, and we missed the actual symptoms. But it is clear that it took about 7-8 weeks for HRV to recover. In mid-January we started training again because he felt okay. We started slowly and easily, but occasional fatigue was still there and I would say that this also had an effect on the persistence of low HRV levels.
Interestingly, we see a nice kick in his parasympathetic activity in mid-February, which brings HRV back into these 115-120 ms ranges and keeps it there from then on.
This next athlete is an U19 rider and his story is a bit different than our athlete no.# 1. With school (finishing high school) and training we can expect to be tough on the rider and at this point it is good to be very careful about what and how they train. So let’s get to the data first.
When we look at HRV here, we do not see such clear patterns as in Athlete #1, but in this case we will not only look at HRV. First, we could focus on the chart with RHR. We see a steady RHR jumping up and down a little bit, around 60bpm. That was all the way from early December to early June. Then we see a big rise in RHR. Down to about 68bpm (14% increase). But in the meantime, when we look at HRV at this point, we see a lower trend, but nothing big like athlete #1, but enough to see that the 7-day average stays below normal.
What happened? As mentioned before, this athlete had just finished high school and at the end of May the school leaving examination and the last exams started. So we reduced the training volume to about 5 hours per week. He used the bike to relax and recover from all the stress. So why has RHR reached that high values? Well, given the short nights and many days with only a few hours of sleep, sitting behind books most of the time, and with less training volume, this is an expected response.
Interestingly enough, after completing the exams and returning to normal training volume, we noticed a rapid decline in RHR, which not only returned to the initial values, but also dropped a little further into the 57 beats per minute range. On the other hand, from that point on we see a nice steady increase in HRV. No stress, more training and a lot of fun on the bike.
HRV can be a great tool to manage, track and adjust your everyday life according to the stressor we are exposed every day. But here we need to note a few things.
Daily fluctuation: HRV could be very variable on a daily basis, so handle it with care. A low or high HRV value does not necessarily mean that you are overly stressed or fully recovered. Follow HRV changes on an acute (a few days) and chronic (weeks to months) basis to get a real picture of what you are going through.
Normal = Good: As we have already said, a higher HRV rudimentarily means that we are more relaxed or less stressed and willing to perform, while a lower HRV means the opposite. Well, this applies to a longer time frame and not on a daily basis, since we just mentioned that HRV can be highly variable on a daily basis. In the beginning we mentioned that our body is constantly trying to maintain homeostasis – a balanced state that we could interpret as our body trying to keep our HRV stable and not all over the place. So values around our baseline are a good example of this homeostasis.
Context: is everything. When you know what the potential stressors are, it is easier to interpret what is happening to your body. If you know that you have had a stressful week at work and have not been getting enough sleep, you may suspect that this could also affect your HRV. Training is therefore hardly the only stress factor we experience in our daily lives.