We’ve all felt our hearts race during tense situations or exciting moments, and we’ve all used our thumping chests as proof of a good workout. So we know firsthand that our heart is tuned in to our mental, emotional, and physical states.
These states are communicated to our heart by our autonomic nervous system which regulates many organs and unconscious processes in our body. It can be divided into two branches that have opposite effects on the heart: your sympathetic nervous system kicks in during stress (like exercise) and ramps your heart rate up. Your parasympathetic nervous system slows your heart down when it’s time to relax and share a meal with the family.
In addition to the heart’s intrinsic pacemaker, these two systems are the major inputs that control heart rate (1). However, it is difficult to measure the individual contribution of these at a given time.
Thus, researchers take advantage of the heart’s poor sense of rhythm to indirectly estimate parasympathetic activity. The heart would make an awful drummer because it’s constantly changing the length of time between each beat! It speeds up with each breath due to decreased parasympathetic activity, and slows down with each exhalation due to increased parasympathetic activity (2).
This inconsistency is called heart rate variability (HRV). The more variable the timing between each beat, the higher the HRV. Because increased parasympathetic activity leads to increased HRV during breathing, researchers often use HRV as an indicator for parasympathetic activity. However, because heart rate is sensitive to many variables, it is safer to view HRV as an indicator of general autonomic nervous system function (2).
Example of an electrocardiogram, or recording of the heart’s electrical activity, to show HRV. Each spike is a heartbeat and the time intervals are shown in seconds.
HRV and Physical Health
The relationship between HRV and health has been studied in unborn babies (3) through to senior citizens (4). Low HRV is predictive of increased risk of mortality from all causes in middle-aged men, and this likely extends to the general population as well (5). It is also associated with increased blood pressure and can be used to project how well patients will recover from heart attacks. Moreover, reduced HRV has been seen in people with gastrointestinal disorders and is associated with the progression of diabetes symptoms (2).
Correlation Not Causation
It is important to remember that HRV is not the cause of disease; it is simply an indicator of general autonomic nervous system function. In these cases, it provides a clue about how well the body can prevent and recover from disease states. Because the nervous system is involved with so many important functions, from breathing and digestion to heart rate and immune response, it is unsurprising that it is associated with such a variety of disorders.
HRV and Mental Health
Considering that the brain is a major part of the nervous system, it follows that changes in HRV are also associated with mental health issues. Decreased HRV is related to mood disorders (6), depression (7), anxiety (8), and post-traumatic stress disorder (PTSD) (9). Veterans with PTSD in this last study were given HRV training that relieved PTSD symptoms and increased HRV.
Likewise, increased HRV is associated with improved self-control, ability to deal with negative emotions and situations, and more social engagement (6).HRV and Exercise
Endurance exercise leads to lower resting heart rates. The heart grows, increases in strength, and beats less frequently. This reduced heart rate is often attributed to increased parasympathetic activity (although this is still debated) and increased HRV is seen in these athletes (1). High baseline HRV is a good predictor for success in training programs for both amateur athletes (10) and special forces recruits in the military (11). On the other end, it may be a useful tool for preventing overtraining in elite athletes (12).
Heart Rate Variability (HRV) Training
Given its relationship with all of these important processes, researchers have been studying how to harness HRV for physical and mental health applications. Generally referred to as HRV training, they often involve breathing exercises (13) and relaxation or meditation techniques(14).
These are often paired with HRV biofeedback programs. These systems give instant visual or auditory information about your HRV as you work on moving it towards a target value. The idea is that with practice, you’ll learn how to regulate HRV without help (13). Practically, these programs often advise people to:
HeartMath is one of the better known companies selling HRV biofeedback products for mental health support. They provide a variety of devices and apps that claim to combat fatigue, depression, and anxiety. Much of their research appears to be reasonably solid, but given the limited measurements the devices make, some of their claims may over-reach. If you want a visual reminder that you’re stressed out and need to relax or exercise, these should work. Beyond that, you’ll have to judge for yourself.
The Pip is another stress relief app and device. Sold as a personal stress trainer, it gives instant feedback to help you retrain your reaction to stress. It works with several apps that give tips and monitor performance and progress. However, it doesn’t measure heart rate or HRV. It essentially measures how much sweat is on your fingertip. While sweating is controlled by the nervous system, it tells you very little about your mental or physical state.
There are dozens of smartphone applications for recording HRV for mental health, exercise, and sleep. Many of them use smartphone cameras to measure HRV. The recordings are surprisingly accurate, but only give you info when your finger is on the phone’s camera (15). Thus, they are only suitable for acute HRV recordings a minute or two in length. Some of these apps sync with more accurate HRV recording devices like the wristwatch and chest strap shown below.
Heart Rate Variability (HRV) Recordings and Exercise
Exercise companies sell wearable HRV monitors for athletes to record during the day and workouts to optimize training and performance. Most of them are worn on the wrist and have other sports watch functions. They can give fairly accurate and reliable recordings.
Some fitness devices such as the FitBit do not record heart rate precisely enough to calculate accurate HRV values. Thus, they can give rudimentary HRV numbers but they shouldn’t be trusted.
For the most accurate recordings, chest strap monitors like the Polar H7 are ideal. With good contact with your body, they can record throughout the day and training sessions to give solid measurements.
BioforceHRV sells an HRV exercise system that includes their app, training book, and a Polar chest strap. The book informs the athlete how to interpret the HRV recordings from the app to determine the optimal intensity and duration for the day’s workout. This system and others like it may be beneficial for elite athletes willing to study and learn the intricacies of their HRV patterns. The large number of factors involved means this task is not trivial.
So here’s the bottom line...
The processes related to HRV are complex so it is difficult to extract meaningful information from these data. Thus, most of us should not concern ourselves with HRV measurements. For elite athletes looking for an extra edge or those who want a tool to help manage stress levels, HRV training may be helpful. For the rest of us, as long as we eat right, exercise, and watch our stress, our body will do its best to keep us healthy.
Mitch Simon is PhD student at the University of California, Davis. The lab he works in studies the heart.