That is not how HRV works…

HRV, or Heart Rate Variability, is one of the most misunderstood health metrics out there. To make a long story short, it’s not a metric that can be used to determine how healthy someone is; it’s a metric that can be used to determine how much physiological stress someone is under. Let me explain…

HRV, or Heart Rate Variability, is a measurement of the variance in time between individual heartbeats. For example, say we wanted to determine the variation between three individual heartbeats. We measure the time between heartbeat one and heartbeat two and get a value of 1.1 seconds. We then measure the time between heartbeat two and heartbeat three and get a value of 1.0 second. If we take the difference between both measurements, we find that there was a 0.1 second difference, or variance, between the two measurements. That is the essence of what heart rate variability is. It’s the variation in time between individual heartbeats. One thing to note here is that there are several different ways for this variation to be measured. For instance, Apple uses a method called SDNN while Oura, Garmin, and Whoop use a method called rMSSD. While each method is accurate, they are designed to highlight different things thus they will come to different values. When reading various HRV studies online, it is important to know which method the study is using before using its data for comparison. So how is this a useful metric?

It’s because a person's HRV can be influenced by their sympathetic and parasympathetic nervous system responses. To make a long story short, the sympathetic response is the body's ‘fight or flight’ response and the parasympathetic response is the body’s ‘calm and collected’ response. The interaction between these two responses can be used as an analog that signals how much physiological stress the body is under. If the body is under more stress, the ‘fight or flight’ response becomes more dominant and tends to lower the heart’s HRV; if the body is under less stress, the ‘calm and collected’ response becomes more dominant and tends to raise the heart’s HRV. Keep in mind, physiological stress just deals with the physical stress experienced by the body. It’s not the same thing as general stress or mental stress, though both can contribute to physiological stress and thus affect your HRV. However, it should be noted that this is not a perfect metric…

While the Sympathetic and Parasympathetic responses are one of the stronger influences on HRV, they are not the only things that can affect it. Simply put, anything which can affect one’s heartbeat can also affect one’s HRV. This includes everything from mental stress, sickness, surgery, giving birth, severe injury, cardiovascular disease, menstruation, watching an exciting sporting event, a poor night's sleep, and any number of other things. Depending on how strong these other factors are, it’s quite possible for them to “drown out” any effect the Sympathetic and Parasympathetic nervous system has. This could cause the heart’s HRV value to change in unexpected ways. For instance, it's been reported that people who have gone through surgery see spikes in their HRV during recovery even though the surgical recovery should clearly be activating their sympathetic (fight or flight) response thus lowering their HRV. Also, the sympathetic and parasympathetic response tends to be more sensitive to cardiovascular stress than it is to muscular stress. This means that weight lifters and strength trainers might find limited use with this metric. This can make HRV a tricky metric to use. So how do we use it?

First, a rolling baseline needs to be established for each individual. To do this, each individual needs to measure their HRV (preferably using the rMSSD measurement method) under the exact same conditions, every day, for around 60 days or so (Oura, Whoop, and Garmin do this during sleep). Then each individual needs to average all their measurements together to calculate the rolling baseline (Oura, Whoop, and Garmin do this for you). You can think of this baseline as the “average” level of physiological stress each individual's body is under. After a baseline is determined, each successive value can be used to signal if the body is under more physiological stress than normal (HRV reading below baseline) or less physiological stress than normal (HRV reading above baseline). – assuming some other factor isn’t “drowning out” the physiological stress effect. Using this metric, along with other metrics (such as RHR, body temp, etc), a “readiness” or “recovery” metric can roughly be determined. For example…

Let’s say you hike for 6 miles, up a 2000 ft high mountain. This hike will put your body under a lot of physiological stress. Your body, reacting to this stress, activates your sympathetic nervous system and begins the recovery process. This causes your HRV to lower, and your RHR to increase, until this recovery process is sufficiently complete. Now, if you happen to be an avid hiker, you may feel sufficiently recovered the next day to resume intense physical activity, but your body may still be in recovery. Let’s say you resume intense activity while your body is in recovery, once done with this activity, your body will then need that much more recovery before it is back to normal. If you continue to do this over and over, you run the risk of overtraining and causing injury to yourself. However, since your HRV tends to stay lower than your baseline and your RHR tends to be elevated while your body is in recovery mode, you can simply wait until your HRV and RHR return to normal before resuming physical activity – thus avoiding overtraining. Also…

Let’s say, as an avid hiker, you notice over a period of a few weeks, that your rolling HRV baseline is slowly declining. Since your baseline can be used as an analog of your average level of physiological stress, you can then make a reasonable assumption that for some reason your average level of stress has been increasing. You are not overtraining per se, but you are getting more and more fatigued as time goes on. Perhaps, the weather has slowly been getting hotter. Perhaps you haven’t been getting enough sleep. Perhaps your work/life balance has been off. Etc. This could show up in your rolling baseline letting you know that you may need to change something. Since your HRV baseline can be used as an analog of your average level of stress, you can use long-term trends in your HRV baseline to help manage said stress. In fact, there are many ways to use HRV for training. Oura, Whoop, and Garmin all use HRV values (among other metrics) to create their readiness/recovery/body battery scores, all of which have anecdotally proven to be useful for a lot of people. I personally use HRV and RHR in my training. However, there is one thing that needs to be mentioned…

There is really no such thing as a healthy, or unhealthy, HRV. This means that there is no such thing as “improving” one’s HRV. You will often see people mention how they were able to “improve” their HRV by raising it, but there is no such concept in medical science. Think about it for a second, when was the last time you had your HRV taken at the doctor’s office? Probably never. That should TELL you how important, or really, unimportant it is for medical health. While it IS true that getting healthier will often raise one’s HRV, raising the HRV value alone is not what is making them healthier. In fact, an upward trend in HRV can signal that there is room to increase one’s training load. If someone were to increase their training load until their HRV stabilized, they would be even healthier than if they were to keep their previous load and let their HRV trend upward. Something else that is important to understand is that HRV baselines are very individualized. There can be two people, of the same age, sex, health, and fitness, that have vastly different baselines. One will not be healthier than the other. The typical range for HRV in humans is 20 – 120ms (rMSSD), but there are some people with baselines in the 200s and some with baselines in the single digits. Chances are, if someone has a medical problem, they will experience acute cardiovascular symptoms well before they ever notice a change in their HRV. A Cardiologist may occasionally use HRV to help diagnose issues, but they will do so in far more sophisticated ways with far more sophisticated equipment than is available to consumers. So while tracking HRV can be useful for training and wellness, it is NOT, and never HAS BEEN, used to determine one's level of health. “Improving” HRV is simply not a thing. However, as always, if you have any concerns about your health, contact a medical professional.

So in short, HRV can be a very useful metric if it is correctly used and correctly interpreted; but it is also a tricky metric that can give off false signals. It does not indicate one’s level of health and outside of fitness training, or wellness, there is very little use for HRV as a metric (unless you are a Cardiologist). So unless you are into either of the aforementioned, you can safely ignore this metric. It is certainly not a metric to stress over.