I have been exploring heart rate variability for around 7 years. What began as curiosity has transformed into a constant yearning for understanding of our physiology, how to measure it, and what effects it. I have been measuring heart rate variability (HRV) for the past 5-6 years thanks to the accessibility and reliability of the app HRV4training. You can read more about the app at the link I provided.
In the time that I have been measuring my HRV, I have gained a great deal of insight and understanding into how my body responds to different metrics including but not limited to: training methods, how I eat, how I sleep, and weight loss vs weight gain.
Before I explore those insights lets cover what HRV is (as short and sweet as I can).
Homeostasis is defined as is the ability or tendency to maintain internal stability in an organism to compensate for environmental changes. Maintaining a constant temperature of 98.6 degrees, blood pressure around 120/80, and blood sugar around 60 are all examples of your body constantly working to maintain a stable internal environment (homeostasis).
The autonomic nervous system is primarily in charge of regulating these responses in order to maintain this balance. The nervous system is constantly monitoring changes in the internal and external environment and delegating physiologic responses to adapt to changes in the environment. For example, lets say the kids are running late in the morning and you have to get them to the bus stop to avoid driving them to school for the fifth time this month. You hear the bus down the street so you grab your kids hand and sprint with them to the bus stop. In order to do this, your heart rate will have had to increase to transport blood to your legs for running, your respiration (breathing rate) will have to increase to transport additional oxygen to the extremities to allow you to run, and depending on the temperature you may even have started sweating to keep your body from overheating. All of these events seem to happen independently, however it is the nervous system reacting to the stress and coordinating these adaptations.
The autonomic nervous system is comprised of two primary divisions: sympathetic (fight or flight) and parasympathetic (rest and digest). These two systems work together (yet somewhat antagonistically) to maintain homeostasis. Each of these systems exerts different effects on the body depending on different situations. In a stressful situation such as a near miss car accident, kitchen fire, traffic jam, angry spouse, or worthless boss and middle management, the sympathetic nervous system will take over. The sympathetic system will increase heart rate, increase blood pressure, increase respiration, cease digestion (leaving you feeling nauseous), and shunt circulation from the core to the extremities. 10,000 years ago these were very necessary responses in harsh environment we lived in. Think about it, if you encountered a lion you would need to fight the lion or run away. This is necessary for survival. Today these responses are less essential. You don’t necessarily need to fight your boss or run away from them if they shorten your deadline. However, this is still a stressful situation and your body will respond accordingly. The primitive part of your brain will have the same response to the lion entering your village as it would have to the driver that pulled out in front of you dangerously close. The sympathetic nervous system is also predominantly at work when you are exercising.
The parasympathetic system has quite different effects on the body. The parasympathetic nervous system will lower your resting heart rate, shunt circulation to the core away from the extremities, and lower your respiration rate. That is why the PSNS is referred to as “rest and digest.” When at rest this should be the division of the nervous system that is primarily working. This parasympathetic system (along with insulin) is responsible for that lethargic feeling we have after stuffing ourselves on thanksgiving (rest and digest). Parasympathetic nervous system activity is what allows us to recover from exercise in the hours and days following exercise.
So, what is heart rate variability?
Heart rate variability gives us a means of measuring activity of the autonomic nervous system and gives us an idea of whether or not we are in a sympathetic or parasympathetic state by measuring the difference in beats of the heart.
The following definition of HRV and what it represents was reproduced with permission from www.hrv4training.com and the app developer Marco Altini.
Practically speaking, our heart does not beat at a constant frequency. So even if we measure our pulse, and get a 60 beats per minute reading, it doesn’t mean we have a beat every second. The time differences between beats are slightly different, they can be 0.9 seconds, 1.2 seconds, and so on. When we talk about HRV, we talk about ways to quantify this variation between heart beats.
This explains also why HRV is not a single number, and there is sometimes a bit of confusion on different metrics to measure HRV since we can quantify these beat to beat differences in different ways (called features). Especially in the context of using HRV to monitor physiological stress, like training load and recovery, the community settled on one specific feature which is called rMSSD. It’s a time domain feature, easy to compute. So HRV4Training provides you with a transformation of rMSSD to make the value a bit easier to interpret, i.e. the Recovery Points. If you are an advanced user, you can also display actual features by selecting HRV View - Advanced from Settings.
What does HRV represent?
Here we need to take a step back and talk a bit about the autonomic nervous system. The autonomic nervous system regulates many body functions, mainly unconsciously, such as respiration, the heart beating and so on, and consists of two branches, the sympathetic and parasympathetic branches.
The sympathetic branch, is in charge of the fight or flight response, while the parasympathetic branch promotes a rest and recovery. Making a few simplifications, since the autonomic nervous system maintains an adaptive state of balance in our body, we can understand how we react to stressors, by analyzing autonomic function.
This means we would expect higher parasympathetic activity under conditions of rest, when we are well recovered. Since the autonomic system regulates the heart beating, we can use HRV as a proxy to autonomic function, and therefore use HRV as a way to measure how we react to stressors like a workout for example. This is where collecting HRV data can become very interesting, because we can, for example, better understand how much time our body needs to get back to normal after an intense workout.
Thank you Marco, back to me.
How do I measure HRV?
I use the app HRV4training. This app does not require a chest strap (though you could use one if you wish), it has been reliably validated numerous times, and is commonly used in HRV studies on different populations of athletes that are published in scientific literature.
I measure first thing in the morning within a couple minutes of my alarm going off (I usually allow a few minutes to go by while laying in bed before measuring because, let’s face it, the alarm can sometimes be stressful). Consistency is incredibly important in measuring HRV. First thing in the morning is the best time to measure. When you get out of bed, make coffee, let the dog out, and then measure, there is a lot of “noise” in the reading and makes the reading far less accurate. Measuring at the end of the day or middle of the day is practically useless, in my opinion. Did you drink coffee? Get stuck in traffic? Fight with your boss or spouse? These are all stressful situations that will change your HRV reading. Be consistent.
So, what have I learned from measuring my HRV?
There is a marked difference in my heart rate variability depending on whether or not I’m primarily training aerobic endurance or more anaerobic conditioning and strength. My military fitness requirements necessitate that I possess a great deal of aerobic endurance to be able to run or ruck long distances as well as possess the ability to run short sprints, exert high amounts of work in a short time, and be able to lift and carry a great deal of weight. These differing requirements force me to periodize the way I train throughout the year. Part of the year I will predominantly train aerobic fitness and endurance while maintaining a baseline of strength. Other parts of the year I will prioritize anaerobic conditioning and strength while trying to maintain a baseline of aerobic fitness. During times when I train aerobic fitness my resting HR is very low and my rMSSD increases. These are both signs of parasympathetic dominance. When I’m training more strength and anaerobic conditioning my resting HR goes up and rMSSD goes down, indicating more sympathetic stress. Is this significant? To me it is. Remember that HRV is a very individualized metric. However, this does fall in line with what some of the scientific literature says.
Sleep is important. Very, very important. On days where I don’t get much sleep (less than 6 hours) I exhibit more signs of sympathetic nervous system activity. Not exactly conducive for training. When I am away with the National Guard, I generally don’t get a great deal of sleep and my HRV reflects this. If you are not sleeping, you are not recovering adequately. Measuring your HRV gives objective insight into this, as opposed to gauging your recovery subjectively based on how you feel.
When I gain weight, my HRV exhibits more sympathetic activity versus when I have lost weight. When I came home from JRTC (Army training rotation in Louisiana) I weighed 198 pounds. The lowest my weight has been since my senior year of high school. As a result my HRV was high and resting heart rate was low. All parasympathetic signs. At the end of this winter, I was around 210 pounds and my HRV went the opposite direction and was showing a great deal of sympathetic activity. Again, I stress that this is an objective individual response that I have noticed in myself, but I have made a few individual inferences about this response. Speaking generally (very generally), less sympathetic activity is a good thing in regards to overall health. Parasympathetic activity is a better indicator of overall health than sympathetic (though not too much parasympathetic activity, but I don’t have time in this post to dig too deep). I don’t think there are too many people that will argue that carrying around an extra ten pounds is necessarily a good thing for overall health.
I’ve learned the importance of being able to measure objective metrics such as HRV. Technology is fantastic, but yes, I do think we are TOO plugged in these days. While it may not be necessary to monitor every aspect of your physiology at all times with fancy watches and rings, having the means to measure ANS activity in the morning can be incredibly helpful. All of these aforementioned insights I have gained have helped to guide me in my training, eating, and lifestyle to maximize my response to what I’m doing. I’ve learned what works for me in training and what doesn’t work based on HRV and its correlations to performance and my subjective feelings of recovery. Sure, I’ll concede that subjective responses have been adequate for athletes for generations, but we have the means and availability to monitor an important metric from the convenience of our own bedroom, and the best part is that it is all backed by evidence.
Last, the day after I get adjusted by my chiropractor (my lovely wife), I notice an increase in parasympathetic activity. While i haven’t taken the time to really quantify the changes, it is merely an observation. I am first and foremost a chiropractor. Yes, I train people and yes I am furthering my career in the reserve component of the military, but this initial inquiry into HRV began as an exploration into a possible objective outcome measure for care in our office. Inconsistencies in measuring patient’s HRV in an office setting posed a very real challenge to making this an effective outcome measure (and getting people to download an app and measure each morning proved even more difficult), but I have spoken with many chiropractors who are utilizing the app and they all say the same thing. While this is all anecdotal, there is some preliminary evidence that chiropractic (and other modalities referred to as “manual therapy”) can affect HRV. Someday I hope to be involved in more in depth research to understand and quantify what these changes mean and how reproducible they are.
Whether you are an athlete or not, I recommend downloading the HRV4training app and measuring your own HRV. The science has been around for a long time, but has garnered a lot of recent attention and is showing a lot of promising applications.