The Science of HRV
Heart Rate Variability (HRV) is the beat to beat variation in our heart rate. This is an indirect measure of our autonomic nervous system (ANS).
The autonimoic nervous system (ANS) is the system in our bodies that controls everything that happens automatically. Think of things such as; heart rate, blood pressure, breathing, posture, hormones, digestion and so much more.
The ANS has two sides; the sympathetic side, we will call it the gas pedal. And the parasympathetic side, we will call this the brake pedal. We want a perfect balance of the two sides so the body can react to a stressor when necessary (gas pedal) and go back to a growth & repair state after (brake pedal).
HRV gives us a measure of how balanced the ANS is or isn’t. To way oversimplify it; a high HRV means good balance and your ANS is working well. A low HRV may indicate an imbalance between the brake and gas pedals and or a poor functioning ANS overall. There is much more to consider, but that is a basic idea for now.
Stress - a different definition....
The word stress, when used in this context is referring to all the things that place strain on our nervous system. This can be good or bad strain; stressors break down to three basic categories: Traumas, Toxins and or Thoughts.
Traumas, in these terms refer to physical strains, something like falling off a ladder or being chased by a bear, but can also be sitting in a car everyday on a long commute. Exercising is also a trauma to our nervous system, but a positive trauma when done properly.
Toxins would be anything entering our bodies that must be overly processed such as; poor dietary choices, medications and or environmental toxins. Thoughts are more what we think of as traditional "stress", this is; work, family, relationship, financial and or social stressors that weigh on our minds and will eventually have a physical, mental and emotional manifestation.
Through out this site, please keep this definition of stress in mind. But if you forget, don't stress about it :)
Breaking the word down; Heart Rate Variability
Heart rate is literally just a measure of how often your heart is beating (contracting) in a given amount of time. This is usually measured over the course of a minute and given as beats per minute (BPM).
Heart rate variability (HRV) is the measure of the distance between each of those beats of the heart. When we say, "the distance between heart beats", we are referring to the distance between R-R intervals. An R-R interval is the peak of a QRS complex that would be seen on an electrocardiograph (ECG). A QRS complex is seen on an ECG as the depolarization (electrical contraction) of the ventricles of the heart. I know, this seems complex. Just worry about the first sentence.
Here's where variance comes in;
Each beat of a human heart IS NOT and should not be in perfect rhythm. If a heart beat is 60 beats per minute (BPM), the heart did not beat once every second over the course of a minute. Rather, the heart beat at un-even intervals (eg. .78 seconds, 1.2 seconds, .9 seconds, etc.) and the sum of those beats was 60 in that minute.
This variance is very healthy. And without it, we would have no flexibility in our nervous systems and no adaptability in our physiology to adapt to stressors. As a result, we would die.