Heart Rate Variability (HRV)
Heart rate variability (HRV) is the variation in time between consecutive heartbeats, measured in milliseconds. Higher HRV generally reflects a more recovered, parasympathetically-driven state; lower HRV reflects accumulated fatigue or stress.
Why it matters
HRV is one of the most-used objective signals for training readiness. Because the gap between heartbeats is controlled by the autonomic nervous system, a drop in HRV often shows up before you consciously feel tired — making it an early signal that a hard session may need to become an easy one. Tracking HRV trends (not single days) helps everyday athletes balance training stress against recovery.
How it's measured
HRV is calculated from the RR intervals (the time between successive heartbeats) captured by a chest strap, optical wrist sensor, or finger sensor, usually first thing in the morning. The most common metric is RMSSD — the root mean square of successive differences between intervals. Many apps report ln(RMSSD) to make the day-to-day numbers easier to compare.
HRV Calculator — compute RMSSD from RR intervals
Frequently asked questions
What is a good HRV?
There is no universal 'good' number — HRV varies widely by age, genetics, and measurement method. What matters is your own baseline and trend over time. A reading well below your personal 7-day rolling average suggests you are carrying more fatigue than usual.
Why does HRV drop after a hard workout?
Intense exercise shifts the autonomic nervous system toward sympathetic ('fight or flight') dominance and suppresses parasympathetic activity while the body recovers. This temporarily lowers HRV. As you recover, HRV typically rebounds toward or above baseline.
How do I measure my HRV?
Take a short measurement in a consistent position (usually lying down) at the same time each day, ideally right after waking. Consistency of timing and posture matters more than the device — changing either will move the number independent of your actual recovery.
Educational, performance-oriented content for athletes — not medical advice. Thresholds and reference ranges come from group data and vary between individuals.