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What is Heart Rate Variability?
Heart rate variability is literally the variance in time between the beats of your heart. So, if your heart rate is 60 beats per minute, it's not actually beating once every second. Within that minute there may be 0.9 seconds between two beats, for example, and 1.15 seconds between two others. The greater this variability is, the more "ready" your body is to execute at a high level.
RR intervals show heart rate variability.
Heart rate variability is determined by the time between heartbeats, known as RR intervals.
These periods of time between successive heartbeats are known as RR intervals (named for the heartbeat's R-phase, the spikes you see on an EKG), measured in milliseconds. WHOOP calculates HRV using RMSSD, the root mean square of successive differences between heartbeats.
HRV And The Autonomic Nervous System
Although HRV manifests as a function of your heart rate, it actually originates from your nervous system. Your autonomic nervous system, which controls the involuntary aspects of your physiology, has two branches, parasympathetic (deactivating) and sympathetic (activating).
The parasympathetic nervous system (often referred to as "rest and digest") handles inputs from internal organs, like digestion or your fingernails and hair growing. It causes a decrease in heart rate.
The sympathetic nervous system (often called "fight or flight") reflects responses to things like stress and exercise and increases your heart rate and blood pressure.
Heart rate variability comes from these two competing branches simultaneously sending signals to your heart. If your nervous system is balanced, your heart is constantly being told to beat slower by your parasympathetic system and beat faster by your sympathetic system. This causes a fluctuation in your heart rate: HRV.
sympathetic vs parasympathetic
HRV is caused by two competing branches of the autonomic nervous system, sympathetic and parasympathetic.