Lub-dub. Lub-dub. Lub-dub. That’s the rhythmic sound your heart is (hopefully) making during the approximately 100,000 times it beats per day. Many people think the heart beats at a regular tempo like a metronome, but the reality is quite different. The time between each beat, known as the interbeat interval of the heart, is fluid, not fixed. This variation is a remarkable aspect of the heart’s functionality as it adapts to the body’s moment-to-moment needs, impacted by factors such as health status, fitness, physical activity, psychological stress, medication, sleep, and the environment, to name a few.
Doctors have been aware of the so-called Heart Rate Variability (HRV) for at least a century, but only recently has HRV been applied to athletes. In this article, we will discuss what heart rate variability is, how it’s measured, how-to improve it, and how it can be used in exercise.
Podcast Discussion
Drs. Baraki and Feigenbaum took a deep dive into Heart Rate Variability on Episode 337 of the Barbell Medicine Podcast. Listen below:
What is Heart Rate Variability?
Heart rate variability (HRV) is the fluctuation in time intervals between successive heart beats. Heart rate is a separate measurement of how many times the heart beats per minute. A normal resting heart rate is somewhere between 60 to 100 beats per minute (bpm).1 There are special terms for heart rates above (tachycardia) and below this range (bradycardia). Jargon aside, the time between heart beats gets faster and slower based on a number of different inputs such as the brain, blood pressure, and even your breathing rate. Among the most important inputs to HRV is from the autonomic nervous system.
HRV and the Parasympathetic and Sympathetic Nervous System
Heart rate variability is an indirect assessment of autonomic nervous system functioning.2 The autonomic nervous system has two major divisions: 1) the parasympathetic nervous system (rest and digest) and, 2) the sympathetic nervous system (fight or flight). The parasympathetic nervous system’s main functions at the heart is to slow the heart rate, while sympathetic activity increases heart rate.
Parasympathetic input predominates at rest, typically resulting in a resting heart rate around 70 bpm. When greater blood flow is required, such as during exercise, the sympathetic system’s input goes up in order to increase cardiac output, which is how much blood the heart pumps out per beat. Greater sympathetic activity subsequently increases heart rate, blood pressure, and how much blood is pumped out during each beat.
In a healthy human heart, there is a dynamic relationship between the two branches, where increased activity in one branch can be associated with increased, decreased, or no change in activity of the other branch. In other words, changes in activity in one branch are not always associated with reciprocal changes in the other branch. After exercise for example, sympathetic activity remains increased while parasympathetic nervous system activity also rises. 3 The net result is a return towards resting heart rate after exercise, e.g. heart rate recovery.
Overall, it’s best to think of HRV as an assessment of the beat-to-beat variations in heart rate, which serves to fine tune the heart rate to the current needs of the individual. Further, heart rate variability is an indirect, non-invasive measure of autonomic nervous system functioning.
What is a Good Heart Rate Variability?
Higher levels of heart rate variability (HRV) are generally associated with greater health and physical fitness performance compared to lower HRV. For fitness performance, “higher” and “lower” are relative to an individual’s baseline HRV prior to exercise. For health, higher and lower HRV are relative to other individuals in the same demographic.
How Does HRV Correlate to Health?
Higher HRV (from baseline) is associated with increased levels of cardiorespiratory fitness (e.g. VO2 max) 4,5, weight loss in individuals with obesity 6, and subjective feelings of recovery in athletes. 7,8 Higher HRV is not always better however. For example, higher HRV levels are associated with atrial fibrillation, which is the most commonly treated heart arrhythmia.9
In contrast, reduced HRV is associated with increased risk of medical conditions like heart disease, metabolic syndrome, diabetes 10, and multiple sclerosis 11, among other diseases. Lower HRV is also associated with aging 12, low cardiorespiratory fitness 13, poor diet 14, lower muscle mass 15, psychological stress, 16 and alcohol intake. 17
Is Heart Rate Variability Like An Arrythmia?
Heart rate variability is the fluctuation in time intervals between successive heart beats, which is not an arrhythmia. Arrhythmias refer to an abnormal cardiac physiology where there are problems with the heart’s rhythm that may also produce an issue with the heart rate. The heart’s normal rhythm is called normal sinus rhythm, where the electrical signal travels through a specific route in the heart. Any deviation from this is an arrhythmia. For example, atrial fibrillation (a-fib) is when the muscles in the top chambers of the heart (atria) twitch in an uncoordinated fashion (fibrillate) due to irregular electrical activity. While heart rate variability tends to be higher in individuals with a-fib, it is not an arrhythmia or irregular heart beat. Rather, heart rate variability is an assessment of the interval between successive heart beats. No more. No less.
Can HRV Be Improved?
Heart rate variability seems to be improved by most things that increase aerobic fitness, muscular function, and/or health.
For example, both aerobic exercise and weight training, as well as combining the two in a single exercise program, increase heart rate variability by increasing physical fitness. 17 Weight loss in individuals with obesity also increases HRV 6, as does smoking cessation 19, improved sleep 20, and biofeedback training 21, to name a few.
There are plenty of other conditions, environmental factors, and even medications that can affect HRV. Generally speaking however, greater HRV is better than lower HRV, though there is no single value for heart rate value that can be characterized as good or bad in the context of health or performance. Instead, it’s likely better to view changes in an individual’s HRV trend over a longer period of time as reflective of an individual’s autonomic flexibility.
We’ll talk about some potential use cases for HRV monitoring in an upcoming section, but first, let’s talk about how HRV is measured.
How To Measure Heart Rate Variability
Heart rate variability is measured by using instruments that can detect small changes in time between heartbeats, e.g. an electrocardiogram (EKG) in the clinical setting, and photoplethysmography for wearables like smartwatches. There are many other ways to do this, each having their own pros and cons with respect to access, reliability, validation, and so on. A complete review of all available methods is beyond the scope of this article, but interested readers can review heart rate variability testing guidelines here and here.
Measuring Heart Rate Variability using an EKG
Time- and frequency-domain are the two standard ways of measuring heart rate variability in research and clinical settings using an electrocardiogram (EKG). Time domain uses statistics to analyze the time intervals between heart beats, whereas frequency domain identifies how often specific variations in heart rate occur. Generally speaking, three main frequencies in heart rate variation are detected, i.e. high (HF), low (LF), and very-low (VLF). The significance of each frequency is complex and not fully understood at this time, though many associate the HF domain with parasympathetic nervous system activity, and the LF domain with activity of the sympathetic system. The significance of the VLF component is unclear. 22,23
There are many variations of both time- and frequency-domain methods for measuring HRV, each leading to different results and making comparisons of HRV between different methods very challenging.
How To Measure Heart Rate Variability At Home
Heart rate variability can be measured at home using wearable technology like a smartwatch . Most people don’t have access to an EKG at home, but many do have a smartwatch, many of which have the ability to assess heart rate variability via a built-in heart rate monitor. Save for these options, HRV cannot be reliably assessed at home.
How Do Smartwatches Measure Heart Rate Variability?
Most wearables like smartwatches use photoplethysmography to assess heart rate variability. Photoplethysmography uses a light source directed at a tissue, and then measures how much light is reflected back, absorbed, and/or scattered. For HRV, most wearables like the Apple watch and WHOOP use LED light directed at arteries in the wrist. With each heart beat, blood volume in the arteries fluctuate and a different amount of light is reflected back to the wearable, which can be used to assess fluctuations in the interval between beats.24
Most devices have native applications that measure HRV in the background, which is then processed through a mathematical “filter” to remove suspected artifacts and generate a result that is displayed to the user. Many devices also provide raw, unfiltered data that can be accessed by third-party applications. The validity of HRV and other biometric outputs generated from consumer wearables has long been questioned. Most wearables track multiple things, e.g. heart rate, VO2max, and HRV, to name a few. A recent study of all systematic reviews and meta-analyses found that of the biometric parameters reported from wearables, less than 5% of the data generated by available wearables has been validated.25
For heart rate variability, a fair amount of research has been dedicated towards comparing wearables to gold standard measurements using an EKG. For example, the Polar H10 band has been validated in this manner, whereas Apple and WHOOP devices tend to underestimate HRV. 26, 27 Issues with HRV assessment using photoplethysmography, the software used to filter the raw data, and when the data is captured (e.g. sleep vs waking hours) have been raised. 28 There’s even been an investigation into using the Apple watch’s EKG function to assess HRV. 29
With further study and technological advances, it is possible that biometric data from wearables will be validated for use in both clinical and performance settings. For now, the application of wearable-generated HRV is largely speculative.
Heart Rate Variability and Exercise
Heart rate variability is promoted as a tool that can help an individual tailor their exercise training more precisely to their needs, thereby increasing their results. Individuals vary wildly when it comes to the results seen from a given training program. Some will get much stronger, gain a lot of muscle, and/or dramatically increase cardiorespiratory fitness (hyperresponders), some won’t see any results at all (non-responders), and others will fall somewhere in between.30
Why individuals respond so differently to exercise is an interesting topic. Most studies have found that sex, age, and ethnicity do not predict differences in training response. Instead, most research points to individual factors like genetics,31,32, nutrition 33, skeletal muscle androgen receptor content 34, as well as various training-related factors such as length of exercise intervention and what specific outcome(s) is/are being measured.34
To summarize, an individual is likely to get the best results from an exercise program if it is well-suited to them. Unfortunately, many of these factors are unknown when either initiating exercise or switching exercise programs. Instead, the efficacy of a training program can only be identified after the fact.
If a non-invasive parameter like HRV can be used to both monitor and adapt exercise training to an individual, they are likely to get better results. A number of different mechanisms related to recovery, monitoring training load, and subsequent program management have been suggested.
We’ll address each in this section.
HRV and Recovery
Heart rate variability is often correlated with recovery. In this view, a return of heart rate variability to baseline levels means that an individual is recovered and ready for additional exercise. To assess the validity of this claim, we must first define recovery.
Defining recovery in the context of exercise is challenging, as there are many different types of recovery depending on the variable of interest, time frame, and subsequent application. For example, it only takes minutes for a healthy individual’s heart rate and ATP levels to return to resting levels after exercise, whereas it can take days to weeks for maximal muscle strength to be restored. If recovery status is being used to manage training, e.g. “should I train?”, then “recovered” would be defined as the individual being ready for additional training stimulus. On the other hand, if recovery status is being used to predict performance, then “recovered” would be defined by objective tests of strength or endurance, among others.
Recovery is probably best conceptualized as the balance between positive and negative training-induced changes, where positive changes refer to fitness adaptations, and negative changes refer to aspects of fatigue. In order to assess recovery in the context of resistance training, we’ll define recovery as the return to baseline strength and/or the restoration of the physiological milieu that allows for further adaptation. While the elements of a “responsive physiological state” are not fully understood, this criteria can be assessed through data looking at exercise interventions guided by measurements that are assumed to be consistent with a responsive physiological state, e.g. HRV.
The idea of recovery being important for managing training hinges on the assumption that inappropriate recovery reduces training results. In this view, an individual that is not recovered enough will be unable to fully adapt to further exercise, whereas an individual that is over-recovered is not maximizing their results because they’re not training enough. Ideally, an individual would exercise as much as is physiologically and logistically possible, where the physiological limit is governed by recovery.
There’s ample support for the idea that greater amounts of exercise tend to drive greater results provided the individual can tolerate it. 35,36,37 However, it has been challenging to identify a tool that measures recovery in a way that prevents exceeding the physiological limit of recovery. For heart rate variability to fit the bill, it needs to reliably correlate with recovery and needs to produce better training outcomes when used.
Let’s see if heart rate variability lives up to the task.
Does HRV Measure Recovery/Can Overtraining Cause A Low HRV?
Heart rate variability has been correlated to training status in endurance athletes, where more trained and experienced athletes have higher and more stable HRV in response to training when compared to athletes with less experience. 38,39 There’s also some data showing that HRV decreases when endurance training volume is increased rapidly, such as during periods of overreaching, which then returns to normal with a few days of rest, while other data shows that experienced, highly trained endurance athletes’ heart rate variability may be preserved during periods of overreaching. 40,41,42,43
That said, none of these studies assess HRV with respect to recovery as defined by performance or ability to adapt to training in the endurance setting.
Does Lifting Weights Affect HRV?
In strength training, there is data showing that HRV decreases when sets are taken to failure compared to training at the same intensity, but staying further away from failure. 44,45 In both of these studies, the groups training to failure had lower HRV and a longer-lasting reduction in countermovement jump height, likely due to the increased training stress incurred by training to failure. It is unclear what, if any effect this had on low speed strength. Another study found that national- or international level Olympic Weightlifters’ 1RM performance in the back squat, deadlift, seated shoulder press, and front squat recovered to baseline levels in about half the time (~30 hours) as HRV (~60 hours) after an intense session.46
Finally, a study of competitive CrossFit athletes found a higher risk of overuse injuries in those who had a low 7-day average HRV. 47 It is currently thought that injury risk increases when training load goes up too quickly. In this study, higher workloads did reduce HRV averages in the CrossFit athletes who subsequently reported an injury, whereas the average HRV were relatively stable (or even increased) in those who did not report an injury.
Overall, it does not appear that heart rate variability reliably correlates with recovery as defined by training-specific performance. HRV does seem to increase when training load and stress goes up, though it is unclear how sensitive HRV is for monitoring training load and recovery. For example, well-trained individuals seem to have a greater tolerance for higher training loads as reflected by a stable HRV. Additionally, HRV recovery occurred long after restoration of performance in strength athletes. It may be that longer term averages of HRV could be useful for monitoring training load and recovery that may lead to reduced injury risk and increased physical fitness, but further study is needed.
Next, let’s see if HRV correlates well with training responsiveness, or how well people respond to exercise.
Can HRV Be Used To Manage Lifting?
The best use case for heart rate variability in resistance training may be to increase training load and subsequent training volume in eligible individuals. Increasing training volume -provided it is well-tolerated- could have many benefits in that it would likely reduce the proportion of non-responders and increase lifter’s results. 36,37,48
In one study, 21 older women were randomized to two groups, one that used heart rate variability to tell them whether or not they should train (up to 5-days a week), and another that trained 3x/wk on a fixed schedule. The training program was the same in both groups, including typical exercise science stuff like leg press, leg extension, bench press, lat pull-down, and so on for 3 sets of 9-12 repetitions to failure. Over the 7-week study period, the HRV group trained an average of 27 times out of a possible 35, while the fixed training group trained 21 times. Of note, the average age in this study was about 66-years old, yet they were able to tolerate a considerable amount of training volume. In the HRV group for example, the lowest number of sessions completed was 19 and the highest was 31 out of a possible 35 sessions. One participant in the fixed group trained 21 times, despite HRV indicating they were only recovered for 9 of them. In the end, there were no differences between groups in strength, muscular hypertrophy, or functional test scores.49
In a second study, a similar study protocol was used in 22 young men. This time around however, it was a race to 20 sessions. The HRV group would train whenever their HRV was back to baseline (up to 5-days a week) and the fixed group would only train 3x/wk. As predicted by math, the fixed group took 7-weeks to get to 20 sessions, whereas the HRV group took just over 5-weeks. Again, there were no differences in strength, muscular hypertrophy, or amount of non-reponders between groups.50
While there were no differences in training outcomes between those using HRV and those adhering to a fixed schedule, it should be noted that those using heart rate variability had a much higher training volume comparatively. It’s not surprising that no differences in training outcomes were noted, as 7-weeks is a relatively short time to see large differences in the outcomes tested. However, one might speculate that the group training with a higher training volume would see better results in the long-term, especially if HRV is only used to increase training volume, and not decrease it by avoiding exercise.
How-To Use HRV For Exercise
Most health conditions in adults would not benefit from HRV monitoring since it does not change clinical management in patients. For example, heart rate variability is lower in individuals who have recently suffered a heart attack, but the knowledge of reduced heart rate variability does not warrant any changes to how they are cared for.
The use of HRV in the setting of sport and training may be different, particularly in endurance training. However, more research is needed for strength training. For performance-minded individuals, we recommend the following for using heart rate variability in the context of strength training:
- For convenience, HRV values in sport should be obtained and averaged over 5- to 7-days using a wearable device under consistent conditions, e.g. sleeping vs. waking hours, standing vs. sitting vs. sleeping, and so on. Baseline HRV averages should be established during a period of light activity, e.g. a deload or low stress week in one of our programs.
- An additional training session may be performed if the individual’s current average HRV is at or above their baseline average.
- Training should generally proceed as planned for those with an average HRV below their baseline average. Caveats to this guidance include the assumption someone is following an autoregulated program, they have not suffered an acute injury, and/or have another issue unrelated to HRV monitoring that would preclude training.
- For programming purposes, we recommend eliminating the nearest rest day and “moving up” the next scheduled session if opting to train by HRV and logistics allow.
This type of training is experimental. Therefore, we recommend individuals monitor their response to this approach and adjust as needed to best suit their needs. The Carl Sagan quote is applicable:
“It pays to keep an open mind, but not so open your brains fall out.”
If you decide to try this approach, let us know how it goes by logging your training on our forum or contacting us at support@barbellmedicine.com.
Take-Home Message
Heart rate variability or “HRV” is the change in the time intervals between adjacent heartbeats, as the result of a number of regulatory systems involved in adapting the body to its current environment.
One of the most important systems involved in regulating heart rate variability is the autonomic nervous system, specifically the parasympathetic and sympathetic nervous system branches. Heart rate variability can be viewed as the resulting balance between these two branches, where an increase in parasympathetic nervous system activity tends to increase HRV, and increased sympathetic nervous system activity tends to decrease HRV.
Heart rate variability can be increased through pretty much all types of exercise, especially endurance training, lifting weights, and exercise training programs that combine the two. HRV also increases with healthy behaviors such as reducing in alcohol intake, smoking cessation, eating a healthy diet, maintaining a healthy body composition, using biofeedback training, and improving both sleep quality and quantity.
Heart rate variability has been put forth as a proxy for recovery status. When an athlete’s HRV returns to the baseline, they are said to be recovered and therefore ready to train. If an athlete can manage their recovery better by using HRV parameters, they may be able to tailor their training more appropriately and increase their fitness, likely through increasing training volume.
Based on the available data however, HRV does not seem to accurately identify recovery from resistance training as measured by performance. Additionally, the existing data does not show an improvement in resistance training outcomes like strength or muscle hypertrophy when using HRV compared to a fixed training schedule.
Future research may further refine the use of heart rate variability as an instrument that can be used to predict performance and manage training, which could improve the results people get from lifting weights, especially if it increases the amount of exercise someone does. At present however, there is currently little use for HRV as a stand-alone measurement in managing resistance training.
