Follow your heart but take your brain with you
Most underused marker of general health
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.
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.