In a previous post, resting heart rate was shown to reflect physical and mental condition. A low resting heart rate correlates with good health compared to a high resting heart rate.
It becomes complex when you consider age. Older people have lower resting heart rate. And individuals with the same age, nutrition and overall fitness level can have vastly different heart rates. For instance, my maximum heart rate when running is around 172bpm while a friend has 200bpm, with the same heart rate sensor. It does not mean however that I am more or less fit than others.
Heart Rate Variability aims to bring more actionable data to assess your current physical condition.
Understanding Heart Rate Variability
While heart rate measures the average over time, heart rate variability measures the variation in time between heartbeats. As you can see in the illustration above, the first two intervals are spaced by 0.859 sec, then the followings 0.793 sec, then 0.726sec. Heart rate variability data are calculated from those measures.
Heart rate variability (HRV) is a natural physiological phenomena and reflects the condition of the autonomous nervous system. Factors that affect HRV are hormones, sleep, nutrition, physical and respiratory activity, stress, diseases, thermoregulation.
A healthy and relaxed individual has high heart rate variability. The heart is at rest, and responds quickly to inputs from the nervous system. If heart rate needs to be increased, it is increased quickly and easily. Inversely, a sick, anxious, hungry individual who has done strenuous exercise will have low heart rate variability. In this case, the heart is in a constant elevated stress, might have impaired left ventricular function and does not respond well to inputs.
In the example above, HRV follows the respiration in a healthy and well-rested individual.
Daily HRV can reflect recent physical activity, nutrition, sickness and stress. If you run a marathon, your HRV will significantly decrease the following morning. If you develop a tooth cavity or get infected, your HRV will also decrease. A few days of rest will increase HRV over time.
As you can guess, heart rate variability has interesting applications for athletes and those interested in performance. Soviet coaches and doctors were the first to use HRV to train russian athletes and optimize their training schedule. HRV is now used by elite athletes worldwide to optimize their training plan.
How To Use HRV to Optimize your Performance
The first step is to measure on a regular basis your heart rate variability. I have used HRV4Training, an iOS app that uses the optical camera and the LED. Other similar apps such as EliteHRV offer the same functionality for similar prices.
If you are serious, you can also look at Hexoskin’s wearable shirt, which measures HRV with its advanced sensors. Sports watches also measure HRV with a chest heart monitor, although they give it a different name such as “Stress Score” in the case of Garmin.
Since the goal is to compare variations, it is important to measure HRV at the same time every day, in the same body position. I measure HRV when waking up. I recommend you do the same.
After measuring, HRV4Training will show HRV, a metric between 0 and 100. You can also add information such as training data, nutrition, sleep, mental energy. Since HRV providers use different metrics, it is good to see how they compare to each other:
The data is different. What is clear though is that males have usually higher HRV than females. Younger individuals and those who exercise also have higher HRV than those who are older and are sedentary. It is fair to say that HRV correlates with overall fitness.
As you can see, the first advantage of HRV measurement is its accessibility to anyone. You can measure lactate clearance in a lab, with expensive and invasive devices, and within an protocol that can last 3 hours. HRV takes 2 minutes and cheap by comparison to measure fatigue. It outputs a simple number, understandable without a medical degree, that takes into account the cardiovascular, respiratory and nervous systems
A higher than usual HRV means you will feel great today, and you will most likely have good performance.
A lower than usual HRV means you didn’t sleep well, haven’t recovered fully. Or perhaps a worsening trend in your health?
The second advantage of HRV measurement is being able to optimize your day. You can increase intensity, duration or goal if HRV is higher than usual. It takes away the second guessing if you are “okay” or if there’s something wrong.
If HRV is low, you can dial down intensity and lower expectations. You can also plan rest days, knowing that this will increase your HRV in the following days.
The third advantage of HRV measurement is being able to program your peak day. Let us assume you have a race, a competition or an important keynote to deliver on a given date. You want to perform, be at your best, and decrease chances of sickness. By reviewing history of HRV, you can see how HRV increases following rest, and then calculate how many rest days are required. You will know that you came to the competition at your best, and hopefully beat the competition.
Can you rest forever and expect HRV to increase as well? No at all. HRV only increases during the rest period after strenuous exercise, assuming you have good sleep and good nutrition. Exercise will have an impact as follows on HRV:
The fourth advantage of HRV measurement is knowing how much stress or unknown diseases you have. Often, we can measure exactly how many hours we work and how much exercise we had. What is difficult to measure is mental stress or diseases you might have, and their impact on your health. HRV measurement can definitively help. If you have the same exact nutrition, sleep and exercise, and find a lower HRV, look into possible mental stress (see the questionnaire here). It is also possible you have an infection, a toothache or maybe a small indigestion problem. Medical teams for example can see changes of HRV in cardiovascular diseases, diabetic complications, sepsis infections, or changes in medication.
In this case, it’s a sign you might want to “eat clean” today, have an extra nap, or even consult your health professional for a check-up.
If you are also planning life style choices, HRV tracking can also let you know how you are coping with it. For example, you can measure if a new diet increases or decreases over time your HRV. At equal parameters, a decrease means it’s not working out and you should look for another diet.
Is HRV the Magic Bullet?
Like resting heart rate, HRV is a limited metric.
Today, there are many tools available, but unfortunately, HRV has not been standardized. Every commercial provider seems to use their own calculation, their own metric and use a different name. You will also likely have to test different tools before finding one that’s appropriate and accurate for you.
HRV is also difficult, if not impossible to measure while moving. It is best measured when waking up. Running, walking, standing up will change the metric. This means you cannot measure HRV accurately just before your key performance. This limits its usage.
And while HRV can be very useful to optimize your performance, it’s not a magical solution. It cannot be used as a routine clinical tool. It is difficult to estimate precisely its response to various stressors, medication or physical exercise. What you can remember is that it’s not exact science since there are so many variables at play (stress, nutrition, sleep, physical activity, health all influence HRV). More clinical research and more standardization is needed before widespread use of HRV.
However, knowing the advantages and limitations, I can guarantee HRV is a great tool for understanding overall health, resilience, and ability; and how your lifestyle choices affect your health and performance.