We all possess differing “adaptive reserves” when it comes to managing stress. However, this reserve or resilience does vary hour-by-hour, day-by-day, and week-by-week. So would it not be useful for us to know in real-time how much is in the tank? You betcha! So welcome to Heart Rate Variability or HRV for short. As you’ll find out in this blog HRV is a useful metric to help you determine how you can modulate variables such as diet, exercise, and recovery so you get the best outcome for that day.
Heart Rate Variability (HRV) is a measure of beat-to-beat intervals of your heart.
Variability of inter-beat time indicates greater adaptability and therefore better health.
HRV can be used as a predictor of progressive disease.
The primary source of variability in your heart rate is the autonomic nervous system (ANS).
Modern lifestyle has resulted in sympathetic (fight/flight) dominance.
Historically HRV measures were only the reserve of full-time athletes to improve performance by minimising ‘overtraining’. But now with the advent of inexpensive heart rate monitors and smartphones this metric is now available to everyone. And that is a really good thing for all of us because over recent years we have now discovered that HRV has been found to be a predictor of progressive diseases such as cardiovascular disease, cancer, autoimmune disorders, infections, trauma, depression, and PTSD (post-traumatic stress disorder). HRV is a very good metric of general health because a higher HRV score is a sign that you currently possess a high reserve and are more resilient to stress, whilst a low HRV score would indicate that you are currently low in reserves and have reduced stress capacity.
What is HRV?
For the average adult, the resting heart rate falls somewhere between 60 to 90 beats per minute, although for some highly trained individuals this can be as low as 45 beats per minute. It is generally accepted that a lower resting heart rate is normally associated with a more efficient cardiovascular system. A low resting heart rate is not always good news however because it can also be a consequence of genetic factors or medical conditions like bradycardia. But good health is not all just about your resting heart rate. You see, your heart doesn’t beat like a metronome. The intervals between each beat varies subtly in duration. So if your resting heart rate is 60, your heart will not be beating every second, but rather it might be something like 1.05...1.00…0.97...and so on. And it is this variability in beat-to-beat times that constitutes Heart Rate Variability.
HRV and the autonomic nervous system.
The primary source of variability in your heart rate is the autonomic nervous system (ANS). Your natural pacemaker within the heart (Sinoatrial Node or SA Node), receives messages from the two components of the ANS - namely the Sympathetic System (fight or flight) or the Parasympathetic System (rest and digest).
The sympathetic system is activated by the hormones catecholamines, epinephrine, and norepinephrine, these hormones will stimulate the heart to beat faster and more steadily.
The parasympathetic system is activated by the hormone acetylcholine, this hormone slows the heart rate and introduces more variability. In short, a dominant sympathetic system will lead to reduced heart rate variability, whilst a dominant parasympathetic system will increase heart rate variability.
What does this mean in terms of health?
Well, a person in good health should have a strong and balanced sympathetic and parasympathetic system. They should also be able to easily shift between the two as and when necessary. A weak sympathetic system would leave you being slow to act and lethargic, a weak parasympathetic system however would make you hyperactive and slow to calm down.
Why is variability good?
Although it seems counterintuitive a variable heart rate is seen as a sign of good health. This is because a high HRV is a sign of a flexible and adaptable autonomic nervous system. The positive aspects of variability are not just seen in the heart rate, but in also other wide areas of our physiology, such as gait, speech, and even our thinking.
As in all things we are always looking to maintain a balance. Today's modern lifestyle is very different from our ancestors, and it is clear that many of us do not react well to the levels of stimulus that we are bombarded with day-in, day-out. The balance for many of us has shifted to a state of sympathetic dominance. - that is, an overactive sympathetic system and underactive parasympathetic system. The problem is that a dominant sympathetic system - as identified earlier - is deleterious to both our mental and physical health.