Heart Rate Variability (HRV) is the millisecond variation between successive heartbeats. A heart at rest beats slightly irregularly, and the size of that irregularity reflects autonomic nervous system balance: high HRV usually indicates parasympathetic dominance (rested, recovered); low HRV often indicates sympathetic dominance (stressed, fatigued, or sick).
The most-used HRV metrics for athletes are RMSSD (root mean square of successive differences) and LnRMSSD (log-transformed). Measured first thing in the morning over 1–5 minutes for 4–6 weeks gives the athlete's individual baseline; daily deviations below that baseline correlate with elevated injury and illness risk and prompt training-load adjustments.
HRV is highly individual — absolute numbers don't compare cleanly between athletes, but a single athlete's deviation from their own rolling average is genuinely informative.
Where it's used
Pre-training readiness screening, overtraining-syndrome monitoring, post-competition recovery tracking.
References
- Plews DJ, Laursen PB, Stanley J, et al. (2013). Training adaptation and heart rate variability in elite endurance athletes. Sports Med.
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