Heart Rate Variability (HRV)
Heart Rate Variability (HRV) is simply the variance in time between the beats of your heart.
Suppose your heart rate is measured at 60 bpm (beats per minute). Well, that doesn't mean that your heart beats exactly once every second, rather within that minute there may be 0.95 seconds between 2 successive beats and 1.18 seconds between 2 other successive beats. The greater this variability is then the more your body is actually able to perform under strain.
The periods of time between successive heart beats are called RR intervals and are the spikes you would see on an ECG, measured in milliseconds.
So, how does this all come about? Well, it is actually controlled from your nervous system. The autonomic nervous system controls your involuntary physiology - heart, breathing etc. It has two aspects called parasympathetic (deactivating) and sympathetic (activating).
Parasympathetic activities cause a decrease in your heart-rate and handles things like digestion, nail-growing etc.
Sympathetic activities cause an increase in your heart-rate and handles things like exercise and activities that stress the body - 'fight or flight' activities.
The heart is constantly receiving (via the autonomous nervous system) inputs to increase and decrease heart-rate and this causes the fluctuations in your heart rate - Heart Rate Variability.
A high HRV indicates that your body is responsive to both parasympathetic and sympathetic inputs and is a sign that your nervous system is balanced.
A balanced autonomous nervous system indicates that your body is capable of adapting to it's environment, can handle strain and can perform at it's best.
On the other hand, a low HRV indicates that either the parasympathetic or sympathetic aspect is dominating- this can be a good thing, like when you are exercising or running the 100M! However, if you are not doing something active, then a low HRV is an indication that your body is working hard for some other reason - e.g. stress, illness, fatigue, dehydration. Your body is not primed to perform at it's best.
Many fitness devices (e.g. Fitbit, Whoop) calculate your HRV and can guide you as to whether your body is primed to perform and take on strenuous activity. It is a highly individualised indicator driven by sex, age, fitness, lifesytle etc. People who monitor their HRV need to do so over time and examine the trends the see and what aspects of their life are impacting those trends - classic ones are alcohol, caffeine, diet, fatigue, stress, dehydration, sugar consumption, workout intensities, general health.
Obviously there are biological factors (gender, age, genetics) that you can do little about, but you do have lots of control over most of the other factors to keep an optimum high HRV.
