Heart Rate Variability: What It Is And How To Use It Correctly

Is HRV (Heart Rate Variability) a crucial metric in assessing our overall health and fitness? In this article, we delve into the intricate details of HRV, its correlation with the autonomic nervous system, and how it mirrors the delicate equilibrium between our body’s parasympathetic and sympathetic activity.
Plus, we’ll take a look at all the different factors that can influence HRV, like our mood and training status, to give you some valuable insights on how to optimize your performance levels. Join us as we explore the intriguing concept of HRV and how it can greatly influence our well-being and athletic potential.

WHAT IS HEART RATE VARIABILITY?

Heart rate variability (HRV) is the fluctuation in the amount of time between heartbeats. In other words, your heart is not a metronome²⁶; each heartbeat varies from the next. If you imagine a heart rate at 60 beats per minute (BPM), your heart rate won’t actually beat every second; there will be a small variation in time, measured in milliseconds (ms), between each beat. Generally, the higher this variability is (i.e. the more HRV one has), the more relaxed, rested, and recovered an athlete^{3, 18, 34} can be. Lower HRV (i.e., less variability between heartbeats) is associated with more inflammation, increased disease risk (e.g., increased morbidity and all-cause mortality risk), and overall lower health status (e.g., worse biomarker results)⁷. Several factors influence these fluctuations in heartbeats. Heart rate and HRV respond within the incredibly dynamic system that is the human body. Its complexity allows it to quickly react to external changes (e.g., changes in temperature) and internal changes (e.g., mood and cognitive processes such as attention) so that the cardiovascular system can maintain its balance. Nonlinear systems are no strangers to variability, granting them robustness in unpredictable and changing environments²⁵.

Heart rate variability (HRV) is an important indicator of overall health and fitness as it reflects the balance between your body's parasympathetic (i.e., “rest and digest”) nervous system and sympathetic (i.e., “fight or flight”) nervous system.

An athlete’s HRV can be affected by their training status, mood, stress levels, and sleep quality, among other factors. Although many trainers claim that measuring HRV can provide valuable insight for athletes in terms of tracking progress over time with regard to optimizing performance levels, this metric cannot always accurately reflect adjustments made from one training session to another due to its sensitivity to many different factors. As such, measuring HRV accurately outside of a tightly controlled laboratory or medical setting is often unrealistic and produces unreliable and inaccurate results.

WHAT DOES HRV MEASURE?

HRV reflects the balance between sympathetic and parasympathetic activity in the body. The parasympathetic and sympathetic nervous systems are the two main branches of the autonomic nervous system (ANS). The ANS is responsible for controlling many functions. For example, this nervous system regulates functions we don’t think about, such as heart function, digestion, and breathing rate. The parasympathetic nervous system (PNS) – a branch of the ANS – is important for maintaining an overall balance (i.e., homeostasis within the body). Terms such as “rest and digest” and “feed and breed” are used to describe the PNS27. The sympathetic nervous system (SNS) puts the body in a state to act (e.g., “fight or flight”)²⁷. Generally, high HRV – more variability between heartbeats – is associated with more parasympathetic nervous system activity. On the other hand, low HRV – less variability between heartbeats – is associated with more sympathetic nervous system activity. In other words, more variability between heartbeats signals to us that we are in a more relaxed and rested state, and less variability between heartbeats signals to us that we are in a more stressed and alert state.

FACTORS THAT INFLUENCE HRV

The beat-to-beat changes in heart rate, otherwise known as HRV, are heavily influenced by various factors.

MOOD AND MOOD DISORDERS

Mood disorders (e.g., anxiety and depression) are significantly associated with lower HRV^6,8. This is due to the neural connection between the brain and the heart. To understand this better, we can look at the fact that the heart has an intrinsic heart rate (around 100 BPM). This means that if the heart was left alone, was not influenced by other body demands (e.g., exercise), and had the energy to do so, it would beat at around 100 BPM. The brain, at rest, exerts an inhibitory (or suppressive) influence on heart rate. In other words, the brain actively works to lower heart rate. This inhibition is phasic, meaning that the brain suppresses heart rate in a time-varying manner such that heart rate varies over time with the brain’s influence¹². The result of this inhibitory (or suppressive) activity is more HRV. When we experience a mood disorder such as anxiety and depression, our brain’s ability to suppress heart rate is weakened, and that’s why people with mood disorders can experience a reduction in HRV.

OBESITY, HIGH BODY MASS INDEX (BMI), AND WEIGHT GAIN

Obesity is generally associated with lower HRV ³¹. This is because, especially in more extreme cases, obesity can be stressful for the body. In addition, this risk factor is associated with lifestyles (e.g., a sedentary lifestyle) that can be associated with lower HRV.

EXERCISE

Specifically, the number of hours exercised per week is associated with higher HRV. Accordingly, athletes and those who exercise regularly tend to have higher HRV values in the population¹⁹. Interestingly, athletes who recently exercised will temporarily have a lower HRV, and this HRV will increase back to baseline once they have fully recovered from the workout. In other words, changes in HRV can be used to measure recovery²⁴.

SLEEP

The lower the quality and quantity of sleep we experience, the more likely we will have lower HRV. The link between sleep and HRV can be so strong that some have suggested that low HRV can be used as a screening tool against sleep disorders in the laboratory setting²⁹.

SUBSTANCE USE

Common substances such as caffeine, alcohol, nicotine, and sleeping pills can affect HRV⁴. The more recent their use, the more they influence HRV. Interestingly, caffeine consumption prior to exercise has been associated with delayed return of HRV to baseline⁴.

STRESS

GENDER

Women, generally, tend to have higher HRV^1,23. This trend aligns with other risk factors, such as life expectancy. See the graph below for more details regarding differences in HRV between men and women. We want to make it clear that these factors are not all-inclusive. Many different physiological and psychological factors influence HRV. This is why controlling (or accounting) for many of these factors is imperative when using HRV to index a particular physiological state.

Many athletes may try to use HRV as an index of “readiness” and “recovery”, while they are not necessarily wrong to do so, they need to consider the many factors that influence HRV.

For athletes who track their HRV and heart rate, we recommend they take into account their qualitative experience. Don’t let HRV data replace your intuition.

WHAT IS GOOD HEART RATE VARIABILITY?

Higher HRV scores are generally considered a sign of a healthy and adaptable nervous system, and overall good physical and mental health. Research suggests that higher heart rate variability is related to the following:

IMPROVED COGNITIVE FUNCTIONING

It is your ability to flexibly adapt to changes in your environment and make decisions in order to reach your goals. A whole academic discipline, psychophysiology, is dedicated to the study of HRV and cognitive functions. Numerous studies have demonstrated that higher HRV is associated with better cognitive functioning. For example, people with higher HRV do better than those with lower HRV in tasks involving multiple options, tasks that require you to suppress your memory, and tasks that require you to switch strategies (this is known as cognitive flexibility. Additionally, people with higher HRV are able to focus better^{13, 21, 30 , 35, 36}.

BETTER EMOTIONAL REGULATION

It is the ability to manage and control our emotions, so they do not negatively impact our lives. In other words, emotional regulation is the ability to utilize coping strategies to effectively manage emotional experiences ^14,15. A recent meta-analysis revealed high HRV as a critical factor for a person’s ability to employ these coping strategies more effectively to help respond to stressful situations². For mountain athletes, improved cognitive functioning and emotional regulation are essential and beneficial assets. Especially in stressful and demanding situations that require “cool-heads” like route finding, climbing in inclement weather, and dealing with rope partners. Better cognitive skills can keep us safe. These challenges are, after all, much of the reason why we climb in the first place.

WHAT IS BAD HEART RATE VARIABILITY?

While the term “bad” is overused, generally speaking, lower HRV can indicate that the autonomic nervous system, which regulates the body’s functions, is less able to respond to internal and external demands.

Low HRV can result from stress, poor sleep, unhealthy lifestyle habits, and certain medical conditions.

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HOW TO MEASURE HEART RATE VARIABILITY?

HRV is a measure of the variation in time between successive heartbeats. Since the variances in an individual’s heart rate are rather small (measured in milliseconds), specialized equipment is necessary for an accurate measurement. In a medical setting, heart rate variability is measured using an electrocardiogram machine (EKG). An EKG possesses sensors that are attached to an individual’s skin. These sensors measure the electrical activity of the heart. HRV is the time in milliseconds between successive R-peaks, peaks in the QRS complex of an EKG, otherwise known as R-R intervals, and is used as an indicator of the health of the autonomic nervous system. Outside of a lab or medical setting, athletes generally use alternative methods for measuring HRV.

COMMON HEART RATE VARIABILITY MONITORS

Alternatively, HRV can be measured with a chest strap. Chest straps are very common to many endurance athletes (most of us use a chest strap to measure our heart rate). These devices generally index heartbeats and not the entire cardiac cycle. They’re an operationally less intensive option²² since only two electrodes need to be placed on the sternum with a connected strap wrapped around the body. A chest strap is a lower-fidelity version of an EKG. And remember, always make sure that your chest and chest strap are as clean as possible for the best results. Recent research studies have found that some chest straps possess rather accurate EKG technology. Certain models can measure R-R intervals during varying exercise intensities. The advantage of the wearable chest strap is that many are waterproof and are less cumbersome than the elastic adhesive strap that wraps around the chest¹⁷.

HOW DO SMARTWATCHES AND RINGS MEASURE HRV?

Smartwatches and rings measure HRV through Photoplethysmography (PPG). PPG is a technique that measures blood volume changes in blood vessels with infrared light to track heart activity ^{11, 26}. When our hearts contract (systole), blood volume increases in our blood vessels, and when our hearts relax (diastole), the amount of blood reduces in our blood vessels. The PPG device measures this blood volume change^{11, 26}. Unfortunately, PPG is the least accurate method because it can be affected by arterial stiffness, age, or stress levels^{20, 25, 26}. However, PPG remains one of the most accessible ways to estimate HR accurately so long as individuals limit their motion ^{11, 20, 25, 26}. This technology is used in smartwatches and rings that measure HRV. Generally, they are the least accurate because they are very sensitive to movement, and they measure heart activity very indirectly. In other words, they measure blood movement and not heart activity.

HOW TO CALCULATE HRV?

To calculate HRV from heart rate data, most off-the-shelf HRV products (e.g., chest straps, smart watches, etc.) come with software that uses the root mean square of successive differences (RMSSD) between heartbeats. This first requires measuring the successive time difference between heartbeats for a given interval in milliseconds (ms). Next, each of the obtained values is squared. You then compute an average value and finally take the square root of that mean. This method is commonly used because many tools only measure heart rate (and not the heart’s electrical activity). RMSSD is a common measurement because it can accurately measure HRV without being affected by breathing rate¹⁶. And breathing rate, even at rest, affects heart rate and thus heart rate variability.

Best practices suggest that we need to consider mean heart rate before assessing HRV because higher heart rates are associated with lower HRV. In other words, if every other variable stays the same, a higher mean heart rate will naturally have a lower HRV (e.g., 60 BPM will have a lower HRV than 45 BPM)³².

BEST HRV MEASUREMENT PRACTICES

As we have discussed, HRV is subject to immense variability due to a multitude of factors. With this in mind, it is crucial that you account for several factors each time you take an HRV measurement for accurate and reliable data. Many people are familiar with circadian rhythms, which are psychological and physiological biological changes that occur in the body in 24-hour intervals. As you might have guessed, HRV is subject to that circadian rhythm. For this reason, it is crucial to measure HRV at the same time every day⁵. For accurate HRV measurements, be sure to avoid ingesting caffeine beforehand.

Caffeine is known to affect HRV measurements. Sleep quality and quantity are heavily associated with HRV measurements, so try to get consistently good sleep²³. The brain has a connection to the heart, which means your mood can alter HRV. Try to maintain a consistent mood for the most reliable results. At the very least, be mindful of your mood each day and consider that when measuring HRV.

Additionally, higher HRV can often result from consistent exercise. However, HRV is temporarily lower during and shortly after exercise²³. It is best to exercise after taking an HRV measurement so results aren’t skewed. Posture and small movements affect the cardiovascular and autonomic nervous systems. When taking an HRV measurement, be as still as possible while maintaining the same posture²³.

Each of the factors above can have an effect on some of the systems that contribute to an HRV measurement. Please note that this is not a limited list. If you choose to track your HRV, ensure consistency in your measurement process.

HOW TO IMPROVE HEART RATE VARIABILITY?

There are a few different ways to improve your heart rate variability. Generally, the most significant improvement you can make on HRV has to do with lifestyle factors. Getting consistently restful sleep will help be one of the most helpful changes you can make.

Lower HRV is consistently associated with stress.

Do your best to downregulate, or lower feelings of stress. Coping strategies that alleviate stress can vary for each person, but try to find what works best for you; e.g., a ski tour, a mellow climb, or a run can be very helpful. Sometimes, a nice walk or meditation is the right answer.

A sedentary lifestyle is associated with lower HRV, and while this is not a risk for athletes, overtraining is. Make sure to manage your training effectively, so you don’t overtrain. Remember, we get stronger when we recover, not when we train. Eat a well-balanced diet to help maintain a healthy body. Finally, avoid unhealthy habits such as smoking or excessive alcohol consumption. Review how much caffeine you’re drinking, many athletes love coffee, but too much can affect HRV.

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NORMAL HEART VARIABILITY RANGE

Unfortunately, there is not a specific HRV score that would be considered healthy for everyone. It is challenging to interpret HRV measurements, and these scores are subject to variability, given the multitude of lifestyle and psychological factors. If you think your heart rate variability is low (or are concerned about your HRV values), you should see a physician.

However, there can be some range fluctuations in HRV scores regarding gender and age; see the chart below.

The Heart Rate Variability chart above shows HRV by age and gender³³. It visually depicts the average HRV measurement and range for each gender and age group. The error bars (lines that go through each bar above) represent one standard deviation. This means that every age group has a wide and acceptable spread. Everyone included in this pool of data was considered to be a healthy individual. The study excluded people if they had chronic diseases (e.g., cardiovascular disease, hypertension, type 2 diabetes, obesity) and if they used certain pharmaceuticals (e.g., antidepressants, beta-blockers, epilepsy medications). The study included 84,772 people. 60% of the individuals in the study were women with an average age of 40 years old. The data collection method was with an EKG, and data were processed using RMSSD.

HEART RATE VARIABILITY BY GENDER

Looking back at the Heart Rate Variability Chart above, you will notice that women possess higher HRV compared to men after the age of 20.

Prior to menopause, women generally have higher HRV compared to men. Research has found that estrogen helps increase the parasympathetic nervous system's influence on heart activity²⁸.

Equally, estrogen appears to inhibit the effects of the sympathetic nervous system. Similar to female sex hormones, androgens (male sex hormones) can also impact the autonomic nervous system’s influence on heart activity. In particular, there seems to be a significant connection between these hormones and the autonomic nervous system. As such, it’s understandable why testosterone levels in men and estrogen levels in women may contribute to distinguishing differences between genders when examining HRV²⁸.

HEART RATE VARIABILITY BY AGE

Generally, a reduction in HRV is typical throughout aging. This decline in HRV may be due to decreased parasympathetic nervous system activity, which is responsible for slowing the heart rate down and promoting relaxation. For example, if you look at the Heart Rate Variability Chart above, HRV is generally higher in healthy children and young adults than in middle-aged and older adults. Some age-related HRV decline is associated with Individuals’ lower cardiovascular fitness levels¹⁰. However, many athletes, especially endurance athletes who focus on aerobic training (running, ski mountaineering, mountaineering, etc.), have lower HRV values due to better overall fitness than their sedentary peers. Interestingly, one study found that middle-aged master athletes have similar HRV values as youth but only in certain HRV metrics⁹. However, this study did not use RMSSD as a way to calculate HRV and instead used a less accurate method, so we should approach these results carefully and with some skepticism. Overall, HRV can be useful in assessing autonomic nervous system function and general health, and age definitely needs to be considered when interpreting HRV data.

WHAT SHOULD YOU DO WITH HRV DATA?

If you choose to measure HRV on your own, make sure you follow a strict protocol (outlined above). This includes measuring HRV at the same time each day.

We don’t recommend using HRV to track your fatigue, recovery state, and fitness. Instead, we urge you to pay attention to other things affecting your training and overall health. For instance, be mindful of your sleep patterns, food, stress, and mental health. Are you sleeping enough? Is your sleep quality good? Are you regularly using caffeine and alcohol? How about other recreational substances? How are those affecting your sleep and training? Are you eating a balanced diet? Are you stressed at work? At home? Do you have stress management tools that you can use? Are you feeling recovered after recovery workouts? Do you have a regular workout that you can use to index your fatigue (e.g., daily fingerboard routine runs, etc.)? These, and many more, are excellent ways to measure your training and overall health.

Make sure you don’t overtrain. This is a tough one, especially for endurance athletes. We all want to crush our goals. One of the easiest ways to prevent overtraining is to measure your training load. For this, you can use training applications or even a spreadsheet. Keeping track of how you feel after each workout can give you a signal when you’re overtraining. When increasing load, do it slowly and progressively, or have a coach direct your training. Make sure to recover, rest as needed, and seek expert guidance when you’re not sure you’re resting enough.

CONCLUSION

While HRV can be a useful tool for measuring the balance of the sympathetic and parasympathetic nervous systems, it is influenced by many factors and can be challenging to measure accurately.

There are many tools, indices, and procedures to index training state, recovery state, and overall health, but make sure you take care of the basics first, and the rest will fall into place.

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