While the maths behind these numbers is complex, you can think of them as approximations of the average fluctuation in the time intervals between heartbeats in milliseconds.
Shooting for a high heart rate variability may seem counterintuitive, since a low resting heart rate suggests someone has good cardiovascular fitness. High heart rate variability, though, is a way to measure how well your nervous system is cycling between its “fight-or-flight” stress response and its “rest-and-digest” relaxation response.
Here’s how it works. If you need to outrun a predator – or just go out for a jog – your nervous system triggers a range of physiologic responses that give you energy and acuity. Among other effects, your heart rate rises. When it does, your heart rate variability drops because the heart has to keep beating at a fast and steady pace to sustain you.
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